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MAX1845EEI+-MAX1845EEI+T-MAX1845ETX+-MAX1845ETX+T
Dual, High-Efficiency, Step-Down Controller with Accurate Current Limit
General DescriptionThe MAX1845 is a dual PWM controller configured for
step-down (buck) topologies that provides high efficien-
cy, excellent transient response, and high DC output
accuracy necessary for stepping down high-voltage bat-
teries to generate low-voltage chipset and RAM power
supplies in notebook computers. The CS_ inputs can be
used with low-side sense resistors to provide accurate
current limits or can be connected to LX_, using low-side
MOSFETs as current-sense elements.
The on-demand PWM controllers are free running, con-
stant on-time with input feed-forward. This configuration
provides ultra-fast transient response, wide input-output
differential range, low supply current, and tight load-reg-
ulation characteristics. The MAX1845 is simple and easy
to compensate.
Single-stage buck conversion allows the MAX1845 to
directly step down high-voltage batteries for the highest
possible efficiency. Alternatively, two-stage conversion
(stepping down the 5V system supply instead of the bat-
tery at a higher switching frequency) allows the minimum
possible physical size.
The MAX1845 is intended for generating chipset, DRAM,
CPU I/O, or other low-voltage supplies down to 1V. For a
single-output version, refer to the MAX1844 data sheet.
The MAX1845 is available in 28-pin QSOP and 36-pin
thin QFN packages.
ApplicationsNotebook Computers
CPU Core Supplies
Chipset/RAM Supply as Low as 1V
1.8V and 2.5V I/O Supplies
FeaturesUltra-High EfficiencyAccurate Current-Limit OptionQuick-PWM™with 100ns Load-Step Response1% VOUTAccuracy over Line and LoadDual Mode™Fixed 1.8V/1.5V/Adj or 2.5V/Adj OutputsAdjustable 1V to 5.5V Output Range2V to 28V Battery Input Range200/300/420/540kHz Nominal Switching FrequencyAdjustable Overvoltage Protection1.7ms Digital Soft-StartDrives Large Synchronous-Rectifier FETsPower-Good Window Comparator2V ±1% Reference Output
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit19-1955; Rev 2; 1/03
Pin Configurations appear at end of data sheet.Quick-PWM and Dual Mode are trademarks of Maxim Integrated
Products.
EVALUATION KIT
AVAILABLEVCC
OUTPUT1
1.8V
BATTERY
4.5V TO 28V
ILIM1
ON2
DL1
TON
OUT1
LX1
DH1
FB1
GND
VDD
BST1
ILIM2
ON1
REF
DL2
CS2
OUT2
LX2
DH2
FB2
BST2
SKIP
5V INPUT
PGOOD
OUTPUT2
2.5V
MAX1845EEI
UVP
OVP
CS1
Minimal Operating Circuit
Ordering Information
PARTTEMP RANGEPIN-PACKAGEMAX1845EEI-40°C to +85°C28 QSOP
MAX1845ETX-40°C to +85°C36 Thin QFN
6mm ✕ 6mm
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
ABSOLUTE MAXIMUM RATINGS (Note 1)Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
V+ to AGND..............................................................-0.3 to +30V
VCCto AGND............................................................-0.3V to +6V
VDDto PGND............................................................-0.3V to +6V
AGND to PGND.....................................................-0.3V to +0.3V
PGOOD, OUT_ to AGND..........................................-0.3V to +6V
OVP, UVP, ILIM_, FB_, REF,
SKIP, TON, ON_ to AGND......................-0.3V to (VCC+ 0.3V)
DL_ to PGND..............................................-0.3V to (VDD+ 0.3V)
BST_ to AGND........................................................-0.3V to +36V
CS_ to AGND.............................................................-6V to +30V
DH1 to LX1..............................................-0.3V to (VBST1 + 0.3V)
LX_ to BST_..............................................................-6V to +0.3V
DH2 to LX2..............................................-0.3V to (VBST2 + 0.3V)
REF Short Circuit to GND...........................................Continuous
Continuous Power Dissipation (TA= +70°C)
28-Pin QSOP (derate 10.8mW/°C above +70°C)........860mW
36-Pin 6mm ✕6mm Thin QFN
(derate 26.3mW/°C above +70°C).............................2105mW
Operating Temperature Range...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
ELECTRICAL CHARACTERISTICS(Circuit of Figure 1, VDD= VCC = 5V, SKIP= AGND, V+ = 15V, TA
= 0°C to +85°C, typical values are at +25°C, unless otherwisenoted.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
PWM CONTROLLERSBattery voltage, V+228Input Voltage RangeVCC/VDDVCC, VDD4.55.5V
FB1 to AGND1.7821.81.818
FB1 to VCC1.4851.51.515
V+ = 2V to 28V, ILOAD
= 0 to 8A, SKIP = VCC,
+25°C to +85°CFB1 to OUT10.9911.01
FB1 to AGND1.7731.81.827
FB1 to VCC1.4771.51.523
DC Output Voltage OUT1
(Note 2)VOUT1
V+ = 2V to 28V, ILOAD
= 0 to 8A, SKIP = VCC,
0°C to +85°CFB1 to OUT10.98511.015
FB2 to AGND2.4752.52.525V+ = 4.5V to 28V,
ILOAD = 0 to 4A,
SKIP = VCC,
+25°C to +85°CFB2 to OUT20.9911.01
FB2 to AGND2.4632.52.537
DC Output Voltage OUT2
(Note 2)VOUT2V+ = 4.5V to 28V,
ILOAD = 0 to 4A,
SKIP = VCC,
0°C to +85°CFB2 to OUT20.98511.015
Output Voltage Adjust RangeOUT1, OUT215.5V
Dual-Mode Threshold, LowOVP, FB_0.050.10.15V
OVP, ILIM_VCC -
VCC -
0.4Dual-Mode Threshold, High
FB11.92.02.1
ROUT1VOUT1 = 1.5V75OUT_ Input ResistanceROUT2VOUT2 = 2.5V100kΩ
FB_ Input Bias CurrentIFB-0.10.1µA
Soft-Start Ramp TimeZero to full ILIM1700µs
Note 1:For the MAX1845EEI, AGND and PGND refer to a single pin designated GND.
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
ELECTRICAL CHARACTERISTICS (continued)(Circuit of Figure 1, VDD= VCC = 5V, SKIP= AGND, V+ = 15V, TA
= 0°C to +85°C, typical values are at +25°C, unless otherwisenoted.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSTON = AGND120137153
TON = REF153174195
TON = float222247272On-Time, Side 1 (Note 3)tON1V+ = 24V,
VOUT1 = 2V
TON = VCC316353390
TON = AGND160182204
TON = REF205234263
TON = float301336371On-Time, Side 2 (Note 3)tON2V+ = 24V,
VOUT2 = 2V
TON = VCC432483534
TON = AGND125135145
TON = REF125135145
TON = float125135145On-Time Tracking (Note 3)
On-time 2 with
respect to on-
time 1
TON = VCC125135145
Minimum Off-Time (Note 3)tOFF400500ns
Quiescent Supply Current (VCC)ICCFB_ forced above the regulation point11001500µA
Quiescent Supply Current (VDD)IDDFB_ forced above the regulation point<15µA
Quiescent Supply Current (V+)I+Measured at V+2570µA
ON1 = ON2 = AGND, OVP = VCC or AGND<15Shutdown Supply Current (VCC)ON1 = ON2 = AGND, VOVP = 1.8V15µA
Shutdown Supply Current (VDD)ON1 = ON2 = AGND<15µA
Shutdown Supply Current (V+)ON1 = ON2 = AGND, measured at V+,
VCC = AGND or 5V<15µA
Reference VoltageVREFVCC = 4.5V to 5.5V, no external REF load1.9822.02V
Reference Load RegulationIREF = 0 to 50µA0.01V
REF Sink CurrentREF in regulation10µA
REF Fault Lockout VoltageFalling edge, hysteresis = 40mV1.6V
Overvoltage Trip Threshold
(Fixed-Threshold Mode)
OVP = AGND, with respect to error-
comparator trip threshold112114117%
1V < VOVP < 1.8V, external feedback,
measured at FB_ with respect to VOVP-28028mV
Overvoltage Comparator Offset
(Adjustable-Threshold Mode)1V < VOVP < 1.8V, internal feedback,
measured at OUT_ with respect to OUT_
regulation point
-3.50+3.5%
OVP Input Leakage Current1V < VOVP < 1.8V-100<1100nA
Overvoltage Fault Propagation
DelayFB_ forced 2% above trip threshold1.5µs
Output Undervoltage ThresholdUVP = VCC, with respect to error-comparator
trip threshold657075%
Output Undervoltage Protection
Blanking TimeFrom ON_ signal going high1030ms
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
ELECTRICAL CHARACTERISTICS (continued)(Circuit of Figure 1, VDD= VCC = 5V, SKIP= AGND, V+ = 15V, TA
= 0°C to +85°C, typical values are at +25°C, unless otherwisenoted.)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSCurrent-Limit Threshold (Fixed)AGND - VCS_, ILIM_ = VCC405060mV
AGND - VCS_, ILIM_ = 0.5V405060Current-Limit Threshold
(Adjustable)AGND - VCS_, ILIM_ = 1V85100115mV
ILIM_ Adjustment RangeVILIM_0.32.5V
Negative Current-Limit Threshold
(Fixed)VCS_ - AGND, ILIM_ = VCC, TA = +25 oC-75-60-45mV
Thermal Shutdown ThresholdHysteresis = 15oC160oC
VCC Undervoltage Lockout
Threshold
Rising edge, hysteresis = 20mV, PWMs
disabled below this level4.054.4V
MAX1845EEI1.55ΩDH Gate-Driver On-Resistance
(Note 4)BST - LX forced to 5VMAX1845ETX1.56Ω
MAX1845EEI1.55ΩDL Gate-Driver On-Resistance
(Note 4)DL, high stateMAX1845ETX1.56Ω
MAX1845EEI0.51.7ΩDL Gate-Driver On-Resistance
(Note 4)DL, low stateMAX1845ETX0.52.7Ω
DH_ Gate Driver Source/Sink
CurrentVDH_ = 2.5V, VBST_ = VLX_ = 5V1A
DL_ Gate Driver Sink CurrentVDL_ = 2.5V3A
DL_ Gate Driver Source CurrentVDL_ = 2.5V1A
ON_, SKIP2.4
Logic Input High VoltageVIHUVPVCC -
ON_, SKIP0.8Logic Input Low VoltageVILUVP0.05V
VCC levelVCC -
Float level3.153.85
REF level1.652.35
TON Input Logic level
AGND level0.5
Logic Input CurrentTON (AGND or VCC)-33µA
Logic Input CurrentON_, SKIP, UVP-11µA
PGOOD Trip Threshold (Lower)With respect to error-comparator trip
threshold, falling edge-12.5-10-7.5%
PGOOD Trip Threshold (Upper)With respect to error-comparator trip
threshold, rising edge+7.5+10+12.5%
PGOOD Propagation DelayFalling edge, FB_ forced 2% below PGOOD
trip threshold1.5µs
PGOOD Output Low VoltageISINK = 1mA0.4V
PGOOD Leakage CurrentHigh state, forced to 5.5V1µA
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
ELECTRICAL CHARACTERISTICS(Circuit of Figure 1, VDD= VCC = 5V, SKIP= AGND, V+ = 15V, TA
= -40°C to +85°C, unless otherwise noted.) (Note 5)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
PWM CONTROLLERSBattery voltage, V+228Input Voltage RangeVCC/VDDVCC, VDD4.55.5V
FB1 to AGND1.7731.827
FB1 to VCC1.4771.523D C O utp ut V ol tag e, O U T1 ( N ote 2) VOUT1V+ = 2V to 28V, SKIP = VCC,
ILOAD = 0 to 10A
FB1 to OUT10.9851.015
FB2 to AGND2.4632.537D C O utp ut V ol tag e, O U T2 ( N ote 2) VOUT2V+ = 2V to 28V, SKIP = VCC,
ILOAD = 0 to 10AFB2 to OUT20.9851.015V
Output Voltage Adjust RangeOUT1, OUT215.5V
Dual-Mode Threshold (Low)OVP, FB_0.050.15V
OVP, ILIM_VCC -
VCC -
0.4Dual-Mode Threshold (High)
FB_1.92.1
ROUT1VOUT1 = 1.5V75OUT_ Input ResistanceROUT2VOUT2 = 2.5V100kΩ
FB_ Input Bias CurrentIFB-0.10.1µA
TON = AGND120153
TON = REF153195
TON = float217272On-Time, Side 1 (Note 3)tON1V+ = 24V, VOUT1 = 2V
TON = VCC308390
TON = AGND160204
TON = REF205263
TON = float295371On-Time, Side 2 (Note 3)tON2V+ = 24V, VOUT2 = 2V
TON = VCC422534
TON = AGND125145
TON = REF125145
TON = float125145On-Time Tracking (Note 3)On-time 2, with
respect to on-time 1
TON = VCC125145
Minimum Off-Time (Note 3)tOFF500ns
Quiescent Supply Current (VCC)ICCFB forced above the regulation point1500µA
Quiescent Supply Current (VDD)IDDFB forced above the regulation point5µA
Quiescent Supply Current (V+)I+Measured at V+70µA
Reference VoltageVREFVCC = 4.5V to 5.5V, no external REF load1.982.02V
Reference Load RegulationIREF = 0 to 50uA0.01V
Overvoltage Trip Threshold
(Fixed-Threshold Mode)
OVP = GND, with respect to FB_ regulation
point, no load112117%
Output Undervoltage ThresholdUVP = VCC, with respect to FB_ regulation
point, no load6575%
Current-Limit Threshold (Fixed)AGND - VCS_, ILIM_ = VCC3565mV
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current LimitMAX1845 toc01
LOAD CURRENT (A)
FREQUENCY (kHz)
FREQUENCY vs. LOAD CURRENTOUT1, SKIP = VCC
OUT2, SKIP = VCC
OUT1, SKIP = GND
OUT2, SKIP = GND
MAX1845 toc02
INPUT VOLTAGE (V)
FREQUENCY (kHz)
FREQUENCY vs. INPUT VOLTAGE
(TON = FLOAT, SKIP = VCC)OUT1
OUT2
IOUT1 = 8A
IOUT2 = 4A
__________________________________________Typical Operating Characteristics(Circuit of Figure 1, components from Table 1, VIN= 15V, SKIP= GND, TON= unconnected, TA= +25°C, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS (continued)(Circuit of Figure 1, VDD= VCC = 5V, SKIP= AGND, V+ = 15V, TA
= -40°C to +85°C, unless otherwise noted.) (Note 5)
Note 2:When the inductor is in continuous conduction, the output voltage will have a DC regulation level higher than the error compara-
tor threshold by 50% of the output voltage ripple. In discontinuous conduction (SKIP= AGND, light load), the output voltage will
have a DC regulation higher than the error-comparator threshold by approximately 1.5% due to slope compensation.
Note 3:On-time and off-time specifications are measured from 50% point to 50% point at DH_ with LX_ = GND, BST_ = 5V, and a
250pF capacitor connected from DH_ to LX_. Actual in-circuit times may differ due to MOSFET switching speeds.
Note 4:Production testing limitations due to package handling require relaxed maximum on-resistance specifications for the QFN
package. The MAX1845EEI and MAX1845ETX contain the same die, and the QFN package imposes no additional resis-
tance in-circuit.
Note 5:Specifications to -40°C are guaranteed by design, not production tested.
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSAGND - VCS_, ILIM_ = 0.5V3565Current-Limit Threshold
(Adjustable)AGND - VCS_, ILIM_ = 1V80120mV
VCC Undervoltage Lockout
Threshold
Rising edge, hysteresis = 20mV, PWMs
disabled below this level4.054.4V
ON_, SKIP2.4
Logic Input High VoltageVIHUVPVCC -
ON_, SKIP0.8Logic Input Low VoltageVILUVP0.05V
TON (AGND or VCC)-33Logic Input CurrentON_, SKIP, UVP-11µA
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current LimitMAX1845 toc03A
SUPPLY VOLTAGE V+ (V)
SUPPLY CURRENT (mA)
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE (SKIP = VCC)ICC
VCC = VDD = 5V
IDD
I+ (25μA TYP)
MAX1845 toc03B
SUPPLY VOLTAGE V+ (V)
SUPPLY CURRENT (1015202530
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE (SKIP = GND)VCC = VDD = 5V
IDD (600nA typ)
ICC
MAX1845 toc04a
LOAD CURRENT (A)
EFFICIENCY (%)
EFFICIENCY vs. LOAD CURRENT
(8A COMPONENTS, SKIP = VCC)V+ = 7V
V+ = 20V
V+ = 12V
OUT1 = 1.8V
MAX1845 toc04b
LOAD CURRENT (A)
EFFICIENCY (%)
EFFICIENCY vs. LOAD CURRENT
(8A COMPONENTS, SKIP = GND)V+ = 7V
V+ = 20V
OUT1 = 1.8V
V+ = 12V
CURRENT-LIMIT TRIP POINT
vs. ILIM VOLTAGE
MAX1845 toc05
ILIM VOLTAGE (V)
CURRENT-LIMIT TRIP POINT (mV)
MAX1845 toc04c
LOAD CURRENT (A)
EFFICIENCY (%)
EFFICIENCY vs. LOAD CURRENT
(4A COMPONENTS, SKIP = VCC)V+ = 7V
V+ = 20V
V+ = 12V
OUT2 = 2.5V
MAX1845 toc04d
LOAD CURRENT (A)
EFFICIENCY (%)
EFFICIENCY vs. LOAD CURRENT
(4A COMPONENTS, SKIP = GND)V+ = 7V
V+ = 12V
V+ = 20V
OUT2 = 2.5V
MAX1845 toc06
OVP VOLTAGE (V)
NORMALIZED THRESHOLD (V)
NORMALIZED OVERVOLTAGE PROTECTION
THRESHOLD vs. OVP VOLTAGEMAX1845 toc07a
IOUT2
2A/div
20µs/div
VOUT2
100mV/div
LOAD-TRANSIENT RESPONSE
(4A COMPONENTS, PWM MODE, VOUT2 = 2.5V)
Typical Operating Characteristics (continued)(Circuit of Figure 1, components from Table 1, VIN= 15V, SKIP= GND, TON= unconnected, TA= +25°C, unless otherwise noted.)
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current LimitMAX1845 toc09
400µs/div
VOUT2
1V/div
IOUT2
2A/div
STARTUP WAVEFORM
(4A COMPONENTS, SKIP = GND, VOUT2 = 2.5V)MAX1845 toc09
100μs/div
VOUT2
1V/div
IOUT2
5A/div
SHUTDOWN WAVEFORM
(4A COMPONENTS, SKIP = GND, VOUT2 = 2.5V)
Typical Operating Characteristics (continued)(Circuit of Figure 1, components from Table 1, VIN= 15V, SKIP= GND, TON= unconnected, TA= +25°C, unless otherwise noted.)
PIN
QSOPQFN
NAMEFUNCTION32OUT1
Output Voltage Connection for the OUT1 PWM. Connect directly to the junction of the external
inductor and output filter capacitors. OUT1 senses the output voltage to determine the on-time
and also serves as the feedback input in fixed-output modes.33FB1Feed b ack Inp ut for O U T1. C onnect to G N D for 1.8V fi xed outp ut or to V C C for 1.5V fi xed outp ut, or
connect to a r esi stor - d i vi d er netw or k fr om O U T1 for an ad j ustab l e outp ut b etw een 1V and 5.5V .34ILIM1
Current-Limit Threshold Adjustment for OUT1. The current-limit threshold at CS1 is 0.1 times the
voltage at ILIM1. Connect a resistor-divider network from REF to set the current-limit threshold
between 25mV and 250mV (with 0.25V to 2.5V at ILIM). Connect to VCC to assert 50mV default
current-limit threshold.
435V+Battery Voltage-Sense Connection. Connect to input power source. V+ is only used to adjust the
DH_ on-time for pseudofixed-frequency operation.
On-Time Selection Control Input. This four-level input pin sets the DH_ on-time to determine the
operating frequency.
TONFREQUENCY (OUT1) (kHz)FREQUENCY (OUT2) (kHz)AGND620460
REF485355
Open345255TON
VCC235170SKIPPulse-Skipping Control Input. Connect to VCC for low-noise forced-PWM mode. Connect to AGND
to enable pulse-skipping operation.
Pin DescriptionMAX1845 toc07b
IOUT1
5A/div
20μs/div
VOUT1
100mV/div
LOAD-TRANSIENT RESPONSE
(8A COMPONENTS, PWM MODE, VOUT1 = 1.8V)
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
Pin Description (continued)
PIN
QSOPQFN
NAMEFUNCTIONPGOODPower-Good Open-Drain Output. PGOOD is low when either output voltage is off or is more than
10% above or below the normal regulation point.4OVP
Overvoltage Protection Threshold. An overvoltage fault occurs if the voltage on FB1 or FB2 is
greater than the programmed overvoltage trip threshold. Adjustment range is 1V (100%) to 1.8V
(180%). Connect OVP to GND to set the default overvoltage threshold of 114% of nominal.
Connect to VCC to disable OVP and clear the OVP latch.5UVPnd er vol tag e P r otecti on Thr eshol d . An und er vol tag e faul t occur s i f the vol tag e on FB1 or FB2 i s l ess
than the und er vol tag e tr i p thr eshol d ( 70% of nom i nal ) . C onnect U V P to V C C to enab l e und er vol tag er otecti on. C onnect to GN D to d i sab l e und er vol tag e p r otecti on and cl ear the U V P l atch.7REF+2.0V Reference Voltage Output. Bypass to GND with 0.22µF (min) capacitor. Can supply 50µA
for external loads.8ON1OU T1 ON /OFF C ontr ol Inp ut. C onnect to AGN D to tur n OU T1 off. C onnect to V C C to tur n O U T1 on.11ON2OU T2 ON /OFF C ontr ol Inp ut. C onnect to AGN D to tur n OU T2 off. C onnect to V C C to tur n O U T2 on.12ILIM2
Current-Limit Threshold Adjustment for OUT2. The current-limit threshold at CS2 is 0.1 times the
voltage at ILIM2. Connect a resistor-divider network from REF to set the current-limit threshold
between 25mV and 250mV (with 0.25V to 2.5V at ILIM). Connect to VCC to assert 50mV default
current-limit threshold.13FB2Feedback Input for OUT2. Connect to GND for 2.5V fixed output, or connect to a resistor-divider
network from OUT2 for an adjustable output between 1V and 5.5V.14OUT2
Output Voltage Connection for the OUT2 PWM. Connect directly to the junction of the external
inductor and output filter capacitors. OUT2 senses the output voltage to determine the on-time
and also serves as the feedback input in fixed-output modes.15CS2
Current-Sense Input for OUT2. CS2 is the input to the current-limiting circuitry for valley current
limiting. For lowest cost and highest efficiency, connect to LX2. For highest accuracy, use a sense
resistor. See the Current-Limit Circuit (ILIM_) section.16LX2External Inductor Connection for OUT2. Connect to the switched side of the inductor. LX2 serves
as the internal lower supply voltage rail for the DH2 high-side gate driver.18DH2High-Side Gate Driver Output for OUT2. Swings from LX2 to BST2.19BST2
Boost Flying Capacitor Connection for OUT2. Connect to an external capacitor and diode
according to the standard application circuit in Figure 1. See MOSFET Gate Drivers (DH_, DL_)
section.20DL2Low-Side Gate-Driver Output for OUT2. DL2 swings from PGND to VDD.21VDDSupply Input for the DL Gate Drivers. Connect to system supply voltage, +4.5V to +5.5V. Bypass
to PGND with a low-ESR 4.7µF capacitor.22VCCAnalog Supply Input. Connect to system supply voltage, +4.5V to +5.5V, with a 20Ω series
resistor. Bypass to AGND with a 1µF capacitor.—GNDGround. Combined analog and power ground. Serves as negative input for CS_ amplifiers.
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
Standard Application CircuitThe standard application circuit (Figure 1) generates a
1.8V and a 2.5V rail for general-purpose use in note-
book computers.
See Table 1 for component selections. Table 2 lists
component manufacturers.
Detailed DescriptionThe MAX1845 buck controller is designed for low-volt-
age power supplies for notebook computers. Maxim’s
proprietary Quick-PWM pulse-width modulator in the
MAX1845 (Figure 2) is specifically designed for han-
dling fast load steps while maintaining a relatively con-
stant operating frequency and inductor operating point
over a wide range of input voltages. The Quick-PWM
architecture circumvents the poor load-transient timing
problems of fixed-frequency current-mode PWMs while
avoiding the problems caused by widely varying
switching frequencies in conventional constant-on-time
and constant-off-time PWM schemes.
5V Bias Supply (VCCand VDD)The MAX1845 requires an external 5V bias supply in
addition to the battery. Typically, this 5V bias supply is
the notebook’s 95% efficient 5V system supply.
Keeping the bias supply external to the IC improves
efficiency and eliminates the cost associated with the
5V linear regulator that would otherwise be needed to
supply the PWM circuit and gate drivers. If stand-alone
capability is needed, the 5V supply can be generated
with an external linear regulator such as the MAX1615.
The power input and 5V bias inputs can be connected
together if the input source is a fixed 4.5V to 5.5V sup-
ply. If the 5V bias supply is powered up prior to the bat-
tery supply, the enable signal (ON1, ON2) must be
delayed until the battery voltage is present to ensure
startup. The 5V bias supply must provide VCCand
gate-drive power, so the maximum current drawn is:
IBIAS= ICC+ f (QG1+ QG2) = 5mA to 30mA (typ)
where ICCis 1mA typical, f is the switching frequency,
and QG1and QG2are the MOSFET data sheet total
gate-charge specification limits at VGS= 5V.
Free-Running, Constant-On-Time PWM
Controller with Input Feed-ForwardThe Quick-PWM control architecture is a pseudo-fixed-
frequency, constant-on-time current-mode type with
voltage feed-forward (Figure 3). This architecture relies
on the output filter capacitor’s effective series resis-
tance (ESR) to act as a current-sense resistor, so the
output ripple voltage provides the PWM ramp signal.
The control algorithm is simple: the high-side switch on-
PIN
QSOPQFN
NAMEFUNCTION—23AGNDAnalog Ground. Serves as negative input for CS_ amplifiers. Connect backside pad to AGND.24PGNDPower Ground26DL1Low-Side Gate-Driver Output for OUT1. DL1 swings from PGND to VDD.27BST1
Boost Fl yi ng C ap aci tor C onnecti on for O U T1. C onnect to an exter nal cap aci tor and d i od e accor d i ng
to the stand ar d ap p l i cati on ci r cui t i n Fi g ur e 1. S ee the M OS FE T G ate D r i ver s ( D H _, D L_)
secti on.28DH1High-Side Gate Driver Output for OUT1. Swings from LX1 to BST1.30LX1External Inductor Connection for OUT1. Connect to the switched side of the inductor. LX1 serves
as the internal lower supply voltage rail for the DH1 high-side gate driver.31CS1
Current-Sense Input for OUT1. CS1 is the input to the current-limiting circuitry for valley current
limiting. For lowest cost and highest efficiency, connect to LX1. For highest accuracy, use a sense
resistor. See the Current-Limit Circuit (ILIM_) section.
6, 9, 10,
17, 25,
29, 36
N.C.No Connection
Pin Description (continued)
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limittime is determined solely by a one-shot whose pulse
width is inversely proportional to input voltage and
directly proportional to output voltage. Another one-shot
sets a minimum off-time (400ns typ). The on-time one-
shot is triggered if the error comparator is low, the low-
side switch current is below the current-limit threshold,
and the minimum off-time one-shot has timed out
(Table 3).
On-Time One-Shot (TON)The heart of the PWM core is the one-shot that sets the
high-side switch on-time for both controllers. This fast,
low-jitter, adjustable one-shot includes circuitry that
varies the on-time in response to battery and output
voltage. The high-side switch on-time is inversely pro-
portional to the battery voltage as measured by the V+
input, and proportional to the output voltage. This algo-
rithm results in a nearly constant switching frequency
despite the lack of a fixed-frequency clock generator.
The benefits of a constant switching frequency are
twofold: First, the frequency can be selected to avoid
noise-sensitive regions such as the 455kHz IF band;
second, the inductor ripple-current operating point
remains relatively constant, resulting in easy design
methodology and predictable output voltage ripple.
The on-times for side 1 are set 35% higher than the on-
times for side 2. This is done to prevent audio-frequen-
cy “beating” between the two sides, which switch
asynchronously for each side. The on-time is given by:
On-Time = K (VOUT+ 0.075V) / VIN
where K is set by the TON pin-strap connection (Table
4), and 0.075V is an approximation to accommodate
for the expected drop across the low-side MOSFET
switch. One-shot timing error increases for the shorter
on-time settings due to fixed propagation delays; it is
approximately ±12.5% at higher frequencies and ±10%
at lower frequencies. This translates to reduced switch-
ing-frequency accuracy at higher frequencies (Table
4). Switching frequency increases as a function of load
current due to the increasing drop across the low-side
MOSFET, which causes a faster inductor-current dis-
charge ramp. The on-times guaranteed in the Electrical
Characteristicstables are influenced by switching
delays in the external high-side power MOSFET.
VDD = 5V
BIAS SUPPLY
POWER-GOOD
INDICATOR
MAX1845EEI
VCC
OUTPUT1
1.8V, 8A
VIN
7V TO 24V
CMPSH-3A
ILIM1
DL1
TON
CS1
OUT1
GND
3 ✕ 470μF
470μFD1Q4Q1
LX1
DH1C5
0.1μF
0.1μF
0.22μFFB1
VDD
UVP
1μF
3 ✕ 10μF
2 ✕ 10μF
C11
1μF
2.2μH
4.7μH
BST1
ILIM2
REF
ON1
ON2
OVP
DL2
CS2
100kΩ
OUT2
LX2
DH2
FB2
PGOOD
BST2
SKIP
4.7μF
20Ω
5mΩ
OUTPUT2
2.5V, 4A
10mΩ
ON/OFF
CONTROLS
Figure 1. Standard Application Circuit
MAX1845Two external factors that influence switching-frequency
accuracy are resistive drops in the two conduction
loops (including inductor and PC board resistance) and
the dead-time effect. These effects are the largest con-
tributors to the change of frequency with changing load
current. The dead-time effect increases the effective
on-time, reducing the switching frequency as one or
both dead times. It occurs only in PWM mode (SKIP=
high) when the inductor current reverses at light or neg-
ative load currents. With reversed inductor current, the
inductor’s EMF causes LX to go high earlier than nor-
mal, extending the on-time by a period equal to the
low-to-high dead time.
For loads above the critical conduction point, the actual
switching frequency is:
where VDROP1is the sum of the parasitic voltage drops
in the inductor discharge path, including synchronous
rectifier, inductor, and PC board resistances; VDROP2 is
the sum of the resistances in the charging path; and
tONis the on-time calculated by the MAX1845.
Automatic Pulse-Skipping SwitchoverIn skip mode (SKIP= GND), an inherent automatic
switchover to PFM takes place at light loads. This
switchover is effected by a comparator that truncates
the low-side switch on-time at the inductor current’s
zero crossing. This mechanism causes the threshold
between pulse-skipping PFM and nonskipping PWM
operation to coincide with the boundary between con-
tinuous and discontinuous inductor-current operation
(also known as the critical conduction point). For a 7V
to 24V battery range, of this threshold is relatively con-
stant, with only a minor dependence on battery voltage: KV V-V LOAD(SKIP)OUT_INOUT_≈×⎛⎜⎞⎟VVV
OUTDROPINDROP=+ ()
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
Table 1. Component Selection for
Standard Applications
Table 2. Component Suppliers*Distributor
COMPONENTSIDE 1: 1.8V AT 8A/
SIDE 2: 2.5V AT 4AInput Range4.5V to 28V
Q1 High-Side MOSFET
Fairchild Semiconductor
FDS6612A or
International Rectifier
IRF7807
Q2 Low-Side MOSFET
Fairchild Semiconductor
FDS6670A or
International Rectifier
IRF7805
Q3, Q4 High/Low-Side
MOSFETs
Fairchild Semiconductor
FDS6982A
D1, D2 RectifierNihon EP10QY03
D3 RectifierCentral Semiconductor
CMPSH-3A
L1 Inductor
2.2µH
Panasonic ETQP6F2R2SFA
Sumida CDRH127-2R4
L2 Inductor4.7µH
Sumida CDRH124-4R7MC
C1 (3), C2 (2) Input
Capacitor
10µF, 25V
Taiyo Yuden
TMK432BJ106KM or
TDK C4532X5R1E106M
C3 (3), C4 Output Capacitor
470µF, 6V
Kemet T510X477M006AS or
Sanyo 6TPB330M
RSENSE1
5mΩ, ±1%, 1W
IRC LR2512-01-R005-F or
DALE WSL-2512-R005F
RSENSE2
10mΩ, ±1%, 0.5W
IRC LR2010-01-R010-F or
DALE WSL-2010-R010F
MANUFACTURERUSA PHONEFACTORY FAX
[Country Code]Central Semiconductor516-435-1110[1] 516-435-1824
Dale/Vishay203-452-5664[1] 203-452-5670
Fairchild Semiconductor408-822-2181[1] 408-721-1635
International Rectifier310-322-3331[1] 310-322-3332
IRC800-752-8708[1] 828-264-7204
Kemet408-986-0424[1] 408-986-1442
NIEC (Nihon)805-867-2555*[81] 3-3494-7414
Sanyo619-661-6835[81] 7-2070-1174
Siliconix408-988-8000
800-554-5565[1] 408-970-3950
Sumida847-956-0666[81] 3-3607-5144
Taiyo Yuden408-573-4150[1] 408-573-4159
TDK847-390-4461[1] 847-390-4405
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limitwhere K is the on-time scale factor (Table 4). The load-
current level at which PFM/PWM crossover occurs,
ILOAD(SKIP), is equal to 1/2 the peak-to-peak ripple cur-
rent, which is a function of the inductor value (Figure
4). For example, in the standard application circuit with
VOUT1= 2.5V, VIN= 15V, and K = 2.96µs (Table 4),
switchover to pulse-skipping operation occurs at ILOAD
= 0.7A or about 1/6 full load. The crossover point
occurs at an even lower value if a swinging (soft-satu-
ration) inductor is used.
The switching waveforms may appear noisy and asyn-
chronous when light loading causes pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM
noise vs. light-load efficiency are made by varying the
inductor value. Generally, low inductor values produce
a broader efficiency vs. load curve, while higher values
result in higher full-load efficiency (assuming that the
coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values
FB2
OUT 2
PWM
CONTROLLER
(FIGURE 3)
REF
AGND*
* IN THE MAX1845EEI, AGND AND PGND ARE INTERNALLY CONNECTED AND CALLED GND.
FAULT1
FAULT2
REF
20Ω
VDD
VCC
OUT1
UVP
OVP
FB1
SKIP
TON
ON1
ON2
5V INPUT
DL1
VDD
LX1
CS1
DH1
BST1
VDD
VDD
VDD
PWM
CONTROLLER
(FIGURE 3)
PGND*
MAX1845
PGOOD
VCC - 1V
0.5V
ILIM1ILIM2
DL2
VDD
LX2
CS2
DH2
BST2
VDD
VCC - 1V
0.5V
2V TO 28V
Figure 2. Functional Diagram
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current LimitFROM
OUT
REF
FROM ZERO-CROSSING
COMPARATOR
ERROR
AMP
TON
FEEDBACK
MUX
(SEE FIGURE 9)
TO DL DRIVER
SHUTDOWN
TO DH DRIVER
ON-TIME
COMPUTE
TON
1-SHOT
FROM ILIM
COMPARATOR
FROM
OPPOSITE
PWM
OPPOSITE
PWM
TOFF 1-SHOT
TRIG
TRIG
FAULT
TIMER
TON
TO PGOOD
OR-GATE
1.1V0.9V
0.7V
0.1V
1.14V
OVP
VCC - 1V
UVP
FB_
OUT_
Figure 3. PWM Controller (One Side Only)
include larger physical size and degraded load-tran-
sient response (especially at low input voltage levels).
DC output accuracy specifications refer to the threshold
of the error comparator. When the inductor is in continu-
ous conduction, the output voltage will have a DC regula-
tion higher than the trip level by 50% of the ripple. In
discontinuous conduction (SKIP= GND, light-load), the
output voltage will have a DC regulation higher than the
trip level by approximately 1.5% due to slope compensa-
tion.
Forced-PWM Mode (SKIP= High)The low-noise, forced-PWM mode (SKIP= high) dis-
ables the zero-crossing comparator, which controls the
low-side switch on-time. This causes the low-side gate-
drive waveform to become the complement of the high-
side gate-drive waveform. This in turn causes the
inductor current to reverse at light loads as the PWM
loop strives to maintain a duty ratio of VOUT/VIN. The
benefit of forced-PWM mode is to keep the switching
frequency fairly constant, but it comes at a cost: The
no-load battery current can be 10mA to 40mA, depend-
ing on the external MOSFETs.
Forced-PWM mode is most useful for reducing audio-
frequency noise, improving load-transient response,
providing sink-current capability for dynamic output
voltage adjustment, and improving the cross-regulation
