MAX1844ETP+T ,High-Speed Step-Down Controller with Accurate Current Limit for Notebook ComputersMAX184419-1993; Rev 3; 9/02High-Speed Step-Down Controller with Accurate Current Limit for Notebook ..
MAX1845 ,Dual, High-Efficiency, Step-Down Controller with Accurate Current LimitApplications Minimal Operating CircuitNotebook Computers5V INPUTBATTERY CPU Core Supplies4.5V TO 28 ..
MAX1845EEI ,Dual / High-Efficiency / Step-Down Controller with Accurate Current LimitMAX184519-1955; Rev 2; 1/03Dual, High-Efficiency, Step-DownController with Accurate Current Limit
MAX1845EEI ,Dual / High-Efficiency / Step-Down Controller with Accurate Current LimitApplications Minimal Operating CircuitNotebook Computers5V INPUTBATTERY CPU Core Supplies4.5V TO 28 ..
MAX1845EEI+ ,Dual, High-Efficiency, Step-Down Controller with Accurate Current LimitApplications Minimal Operating CircuitNotebook Computers5V INPUTBATTERY CPU Core Supplies4.5V TO 28 ..
MAX1845EEI+ ,Dual, High-Efficiency, Step-Down Controller with Accurate Current LimitELECTRICAL CHARACTERISTICS(Circuit of Figure 1, V = V = 5V, SKIP = AGND, V+ = 15V, T = 0°C to +85°C ..
MAX4741EUA ,0.8 / Low-Voltage / Single-Supply Dual SPST Analog Switches
MAX4741EUA ,0.8 / Low-Voltage / Single-Supply Dual SPST Analog Switches
MAX4741EUA ,0.8 / Low-Voltage / Single-Supply Dual SPST Analog Switches
MAX4741EUA+ ,0.8Ω, Low-Voltage, Single-Supply Dual SPST Analog Switches
MAX4743EKA+T ,0.8Ω, Low-Voltage, Single-Supply Dual SPST Analog Switches
MAX474CPA ,Single/Dual/Quad, 10MHz Single-Supply Op AmpsFeaturesThe single MAX473, dual MAX474, and quad MAX475' 15V/µs Min Slew Rateare single-supply (2.7 ..
MAX1844EEP+-MAX1844EEP+T-MAX1844EEP-T-MAX1844ETP-MAX1844ETP+-MAX1844ETP+T
High-Speed Step-Down Controller with Accurate Current Limit for Notebook Computers
General DescriptionThe MAX1844 pulse-width modulation (PWM) controller
provides high efficiency, excellent transient response,
and high DC output accuracy needed for stepping
down high-voltage batteries to generate low-voltage
CPU core or chipset/RAM supplies in notebook com-
puters.
Maxim’s proprietary Quick-PWM™ quick-response,
constant-on-time PWM control scheme handles wide
input/output voltage ratios with ease and provides
100ns “instant-on” response to load transients while
maintaining a relatively constant switching frequency.
Efficiency is enhanced by an ability to drive very large
synchronous-rectifier MOSFETs. Accurate current sens-
ing to ensure reliable overload protection is available
using an external current-sense resistor in series with
the synchronous rectifier. Alternatively, the synchronous
rectifier itself can be used for less accurate current
sensing at the lowest possible power dissipation.
Single-stage buck conversion allows the MAX1844 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
battery) at a higher switching frequency allows the mini-
mum possible physical size.
The MAX1844 is intended for CPU core, chipset,
DRAM, or other low-voltage supplies as low as 1V. It is
available in 20-pin QSOP, and QFN packages and
includes both adjustable overvoltage and undervoltage
protection.
For a dual step-down PWM controller with accurate cur-
rent limit, refer to the MAX1845. The MAX1714/ MAX1715
single/dual PWM controllers are similar to the MAX1844/
MAX1845 but do not use current-sense resistors.
ApplicationsNotebook Computers
CPU Core Supplies
Chipset/RAM Supplies as Low as 1V
1.8V and 2.5V Supplies
FeaturesUltra-High EfficiencyAccurate Current-Limit OptionQuick-PWM with 100ns Load-Step Response1% VOUTAccuracy Over Line and Load1.8V/2.5V Fixed or 1V to 5.5V Adjustable Output
Range2V to 28V Battery Input Range200/300/450/600kHz Switching FrequencyAdjustable Overvoltage Protection Adjustable Undervoltage Protection1.7ms Digital Soft-StartDrives Large Synchronous-Rectifier FETs2V ±1% Reference OutputPower-Good Window Comparator
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers19-1993; Rev 3; 9/02
EVALUATION KIT
AVAILABLE
Pin Configuration appears at end of data sheet.*Contact factory for availability.
Quick-PWM is a trademark of Maxim Integrated Products.
VCC
5V INPUT
BATTERY
4.5V TO 28V
OUTPUT
2.5V
SHDN
ILIM
BST
OUT
SKIP
VDD
MAX1844
UVP
REF
PGOOD
LATCH
OVP
GND
Minimal Operating Circuit
Ordering Information
PARTTEMP RANGEPIN-PACKAGEMAX1844EEP-40°C to +85°C20 QSOP
MAX1844EGP*-40°C to +85°C20 QFN
MAX1844ETP-40°C to +85°C20 Thin QFN
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
ABSOLUTE MAXIMUM RATINGSStresses 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 GND..............................................................-0.3V to +28V
VCC, VDDto GND.....................................................-0.3V to +6V
OUT, PGOOD, SHDNto GND..................................-0.3V to +6V
FB, ILIM, LATCH, OVP, REF, SKIP,
TON, UVP to GND..................................-0.3V to (VCC+ 0.3V)
BST to GND............................................................-0.3V to +34V
CS to GND.................................................................-6V to +30V
DL to GND..................................................-0.3V to (VDD+ 0.3V)
DH to LX.....................................................-0.3V to (BST + 0.3V)
LX to BST..................................................................-6V to +0.3V
REF Short Circuit to GND...........................................Continuous
Continuous Power Dissipation (TA= +70°C)
20-Pin QSOP (derate 9.1mW/°C above +70°C)...........727mW
20-Pin 5mm ✕ 5mm QFN (derate 20.0mW/°C
above +70°C).................................................................1.60W
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, V+ = 15V, VCC= VDD= 5V, SKIP= GND, TA
= 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITSBattery voltage, V+ 2 28 Input Voltage Range VCC , VDD 4.5 5.5 VFB = OUT 0.99 1.01FB = GND 2.475 2.5 2.525 Error Comparator Threshold (DC Output Voltage Accuracy) (Note 1)V+ = 4.5V to 28V, SKIP = VCC FB = VCC 1.782 1.8 1.818VLoad Regulation Error ILOAD = 0 to 3A, SKIP = VCC 9 mVLine Regulation Error VCC = 4.5V to 5.5V, V+ = 4.5V to 28V 5 mVFB Input Bias Current -0.1 +0.1 µAOutput Adjustment Range 1 5.5 VFB = GND 90 190 350 OUT Input Resistance FB = VCC or adjustable feedback mode 70 145 270 kΩSoft-Start Ramp Time Rising edge of SHDN to full current limit 1.7 msTON = GND (600kHz) 140 160 180TON = REF (450kHz) 175 200 225TON = unconnected (300kHz) 260 290 320 On-Time V+ = 24V, VOUT = 2V (Note 2) TON = VCC (200kHz) 380 425 470nsMinimum Off-Time (Note 2) 400 500 nsQuiescent Supply Current (VCC) FB forced above the regulation point 550 800 µAQuiescent Supply Current (VDD) FB forced above the regulation point <1 5 µAQuiescent Supply Current (V+) 25 40 µAShutdown Supply Current (VCC) SHDN = GND <1 5 µAShutdown Supply Current (VDD) SHDN = GND <1 5 µAShutdown Supply Current (V+) SHDN = GND, V+ = 28V, VCC = VDD = 0 or 5V <1 5 µAReference Voltage VCC = 4.5V to 5.5V, no external REF load 1.98 2.00 2.02 VReference Load Regulation IREF = 0 to 50µA 0.01 V
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
ELECTRICAL CHARACTERISTICS (continued)(Circuit of Figure 1, V+ = 15V, VCC= VDD= 5V, SKIP= GND, TA
= 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITSREF Sink Current REF in regulation 10 µAREF Fault Lockout Voltage Falling edge, hysteresis = 40mV 1.6 VOvervoltage Trip Threshold (Fixed-Threshold Mode) With respect to error comparator threshold, no load OVP = GND, rising edge, hysteresis = 1% 12 14.5 17 %External feedback, measured at FB with respect to
VOVP, 1V < VOVP < 1.8V, rising edge, hysteresis =1% -30 +30 mVOvervoltage Comparator Offset (Adjustable-Threshold Mode) Internal feedback, measured at OUT with respect to the
nominal OUT regulation voltage, 1V < VOVP < 1.8V,
rising edge, hysteresis = 1% -3.5 +3.5 %OVP Input Leakage Current 1V < VOVP < 1.8V -100 0 +100 nAOvervoltage Fault
Propagation Delay FB forced 2% above trip threshold 1.5 µsOutput Undervoltage Protection
Trip Threshold (Fixed-Threshold
Mode) With respect to error comparator threshold, UVP = VCC 65 70 75 %External feedback, measured at FB with respect to
VUVP, 0.4V < VUVP < 1V -40 +40 mV Output Undervoltage Protection
Trip Threshold (Adjustable-
Threshold Mode) Internal feedback, measured at OUT with respect to the
nominal OUT regulation voltage, 0.4V < VUVP < 1V -5 +5 %UVP Input Leakage Current 0.4V < VUVP < 1V -100 <1 +100 nAOutput Undervoltage Protection
Blanking Time From rising edge of SHDN 10 30 msPGOOD Trip Threshold (Lower) With respect to error comparator threshold, no load -12.5 -10 -8 %PGOOD Trip Threshold (Upper) With respect to error comparator threshold, no load 8 10 12.5 %PGOOD Propagation Delay FB forced 2% beyond PGOOD trip threshold, falling 10 µsPGOOD Output Low Voltage ISINK = 1mA 0.4 VPGOOD Leakage Current High state, forced to 5.5V 1 µAILIM Adjustment Range 0.25 3 VCurrent-Limit Threshold (Fixed) GND - VCS, ILIM = VCC 90 100 110 mVVILIM = 0.5V 40 50 60 Current-Limit Threshold
(Adjustable) GND - VCS VILIM = 2V 170 200 230 mVCurrent-Limit Threshold
(Negative Direction) GND - VCS, SKIP = VCC, ILIM = VCC ,TA = +25°C -140 -117 -95 mVCurrent-Limit Threshold
(Zero Crossing) GND - VCS, SKIP = GND 3 mVThermal Shutdown Threshold Hysteresis = 10°C 150 °CVCC Undervoltage
Lockout Threshold Rising edge, hysteresis = 20mV, PWM disabled below this level 4.1 4.4 V
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
ELECTRICAL CHARACTERISTICS (continued)(Circuit of Figure 1, V+ = 15V, VCC= VDD= 5V, SKIP= GND, TA
= 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITSMAX1844EEP 1.5 5 DH Gate-Driver On-Resistance
(Note 4) BST - LX forced to 5V MAX1844EGP, MAX1844ETP 1.5 6 ΩMAX1844EEP 1.5 5 DL Gate-Driver On-Resistance (Note 4) DL, high state MAX1844EGP, MAX1844ETP 1.5 6 ΩMAX1844EEP 0.5 1.7 DL Gate-Driver On-Resistance (Note 4) DL, low state MAX1844EGP, MAX1844ETP 0.5 2.7 ΩDH Gate-Driver Source/Sink
Current DH forced to 2.5V, BST-LX forced to 5V 1 ADL Gate-Driver Source Current DL forced to 2.5V 1 ADL Gate-Driver Sink Current DL forced to 5V 3 ADL rising 35 Dead Time DH rising 26 nsLogic Input High Voltage LATCH, SHDN, SKIP 2.4 VLogic Input Low Voltage LATCH, SHDN, SKIP 0.8 VLogic Input Current LATCH, SHDN, SKIP -1 +1 µADual Mode Threshold, Low OVP, UVP, FB 0.15 0.20 0.25 VOVP, UVP VCC - 1.5 VCC - 0.4 Dual Mode Threshold, High FB 1.9 2.0 2.1 VTON VCC Level VCC - 0.4 VTON Float Voltage 3.15 3.85 VTON Reference Level 1.65 2.35 VTON GND Level 0.5 VTON Input Current Forced to GND or VCC -3 +3 µAILIM Input Leakage Current -100 0 +100 nA
PARAMETER CONDITIONS MIN TYP MAX UNITSBattery voltage, V+ 2 28 Input Voltage Range VCC , VDD 4.5 5.5 VFB = OUT 0.985 1.015FB = GND 2.462 2.538 Error Comparator Threshold (DC Output Voltage Accuracy) (Note 1)V+ = 4.5V to 28V, SKIP = VCC FB = VCC 1.773 1.827VTON = GND(600kHz) 140 180TON = REF(450kHz) 175 225TON = Unconnected(300kHz) 260 320 On-Time V+ = 24V, VOUT = 2V (Note 2) TON = VCC(200kHz) 380 470ns
ELECTRICAL CHARACTERISTICS(Circuit of Figure 1, V+ = 15V, VCC= VDD= 5V, SKIP= GND, TA
= -40°C to +85°C, unless otherwise noted.) (Note 3)
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook ComputersPARAMETER CONDITIONS MIN TYP MAX UNITSMinimum Off-Time (Note 2) 500 nsQuiescent Supply Current (VCC) FB forced above the regulation point 800 µAQuiescent Supply Current (VDD) FB forced above the regulation point 5 µAQuiescent Supply Current (V+) Measured at V+ 40 µAShutdown Supply Current (VCC) SHDN = GND 5 µAShutdown Supply Current (VDD) SHDN = GND 5 µAShutdown Supply Current (V+) SHDN = GND, V+ = 28V, VCC = VDD = 0 or 5V 5 µAReference Voltage VCC = 4.5V to 5.5V, no external REF load 1.98 2.02 VOvervoltage Trip Threshold (Fixed-Threshold Mode) With respect to error comparator threshold, no load OVP = GND, rising edge, hysteresis = 1% 12 17 %External feedback, measured at FB with respect to
VOVP, 1V < VOVP 1.8V, rising edge, hysteresis = 1% -30 +30 mV Overvoltage Comparator Offset (Adjustable-Threshold Mode) Internal feedback, measured at OUT with respect to the
nominal OUT regulation voltage, 1V < VOVP < 1.8V -3.5 +3.5 %PGOOD Trip Threshold (Lower) With respect to error comparator threshold, no load OUT falling edge, hysteresis = 1% -12.5 -7.5 %PGOOD Trip Threshold (Upper) With respect to error comparator threshold, no load OUT rising edge, hysteresis = 1% 7.5 12.5 %PGOOD Output Low Voltage ISINK = 1mA 0.4 VPGOOD Leakage Current High state, forced to 5.5V 1 µACurrent-Limit Threshold (Fixed) GND - VCS, ILIM = VCC 85 115 mVGND - VCS, VILIM = 0.5V 35 65 Current-Limit Threshold
(Adjustable) GND - VCS, VILIM = 2V 160 240 mVVCC Undervoltage
Lockout Threshold Rising edge, hysteresis = 20mV, PWM disabled below this level 4.1 4.4 VLogic Input High Voltage LATCH, SHDN, SKIP 2.4 VLogic Input Low Voltage LATCH, SHDN, SKIP 0.8 VLogic Input Current LATCH, SHDN, SKIP -1 1 µA
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, V+ = 15V, VCC= VDD= 5V, SKIP= GND, TA= -40°C to +85°C, unless otherwise noted.) (Note 3)
Note 1:When the inductor is in continuous conduction, the output voltage will have a DC regulation level higher than the error com-
parator threshold by 50% of the ripple. In discontinuous conduction (SKIP= GND, light load), the output voltage will have a
DC regulation level higher than the trip level by approximately 1.5% due to slope compensation.
Note 2:On-time and off-time specifications are measured from 50% point to 50% point at theDH pin with LX = GND, VBST= 5V,
and a 250pF capacitor connected from DH to LX. Actual in-circuit times may differ due to MOSFET switching speeds.
Note 3:Specifications to -40°C are guaranteed by design, not production tested.
Note 4:Production testing limitations due to package handling require relaxed maximum on-resistance specifications for the QFN
package. The MAX1844EEP, MAX1844EGP, and MAX1844ETP contain the same die and the QFN package imposes no
additional resistance in-circuit.
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
EFFICIENCY vs. LOAD CURRENTMAX1844 toc01
LOAD CURRENT (A)
EFFICIENCY (%)
VIN = 7V
VIN = 12V
VIN = 20V
FREQUENCY vs. LOAD CURRENT
MAX1844 toc02
LOAD CURRENT (A)
FREQUENCY (kHz)
VIN = 15V, SKIP = VCC
VIN = 15V, SKIP = GND
VIN = 7V, SKIP = GND
VIN = 7V, SKIP = VCC
FREQUENCY vs. INPUT VOLTAGE
MAX1844 toc03
INPUT VOLTAGE (V)
FREQUENCY (kHz)
ILOAD = 1A
FREQUENCY vs. TEMPERATURE
MAX1844 toc04
TEMPERATURE (°C)
FREQUENCY (kHz)ILOAD = 1A
ILOAD = 4A
CURRENT LIMIT vs. TEMPERATURE
MAX1844 toc07
TEMPERATURE (°C)
CURRENT LIMIT (A)
CONTINUOUS-TO-DISCONTINUOUS INDUCTOR
CURRENT vs. INPUT VOLTAGE
MAX1844 toc05
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
CONTINUOUS INDUCTOR CURRENT
DISCONTINUOUS INDUCTOR CURRENT
CURRENT LIMIT vs. INPUT VOLTAGE
MAX1844 toc06
INPUT VOLTAGE (V)
CURRENT LIMIT (A)
NORMALIZED OVERVOLTAGE
TRIP THRESHOLD vs. VOVP
MAX1844 toc08
VOVP (V)
NORMALIZED THRESHOLD (V/V)
OVERVOLTAGE TRIP THRESHOLD
OUTPUT VOLTAGE SET POINT
OVERVOLTAGE TRIP THRESHOLD
vs. TEMPERATURE
MAX1844 toc09
TEMPERATURE (°C)
OVERVOLTAGE TRIP THRESHOLD (%)
__________________________________________Typical Operating Characteristics(Circuit of Figure 1, VIN= 15V, SKIP= GND, TON= unconnected, TA= +25°C, unless otherwise noted.)
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook ComputersNO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE (PWM MODE)
MAX1844 toc10
INPUT VOLTAGE (V)
SUPPLY CURRENT (mA)
IIN
IDD
ICC
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE (SKIP MODE)
MAX1844 toc11
INPUT VOLTAGE (V)
SUPPLY CURRENT (mA)
ICC
IINIDD
20μs/div
LOAD-TRANSIENT RESPONSE
(PWM MODE)INDUCTOR
CURRENT
2A/div
VOUT
AC COUPLED
100mV/div
5V/div
MAX1844 toc12A
20μs/div
LOAD-TRANSIENT RESPONSE
(SKIP MODE)INDUCTOR
CURRENT
2A/div
VOUT
AC COUPLED
100mV/div
5V/div
MAX1844 toc12B
500μs/div
STARTUP WAVEFORMINDUCTOR
CURRENT
5A/div
VOUT
1V/div
MAX1844 toc14
SHDN
5V/div
5V/div
40μs/div
OUTPUT OVERLOAD WAVEFORM
(UVP = GND)VOUT
1V/div
5V/div
INDUCTOR
CURRENT
5A/div
MAX1844 toc13A
RLOAD = 112mΩ
40μs/div
OUTPUT OVERLOAD WAVEFORM
(UVP = VCC, OVP = GND)VOUT
1V/div
5V/div
INDUCTOR
CURRENT
5A/div
MAX1844 toc13B
RLOAD = 112mΩ
100μs/div
SHUTDOWN WAVEFORM
(OVP = GND)VOUT
1V/div
5V/div
INDUCTOR
CURRENT
5A/div
MAX1844 toc15A
RLOAD = 1Ω
100μs/div
SHUTDOWN WAVEFORM
(OVP = VCC)VOUT
1V/div
5V/div
INDUCTOR
CURRENT
5A/div
MAX1844 toc15B
RLOAD = 1Ω
Typical Operating Characteristics (continued)(Circuit of Figure 1, VIN= 15V, SKIP= GND, TON= unconnected, TA= +25°C, unless otherwise noted.)
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook ComputersPINQSOP QFN NAME FUNCTION1 18 CSCurrent-Sense Input. Connect a low-value current-sense resistor between CS and GND for accurate
current sensing. For lower power dissipation (less accurate) current sensing, connect CS to LX to
use the synchronous rectifier as the sense resistor. The PWM controller will not begin a cycle unless
the current sensed at CS is less than the current-limit threshold programmed at ILIM.2 19 LATCHOvervoltage Protection Latch Control Input. The synchronous rectifier MOSFET is always forced to
the on state when an overvoltage fault is detected. If LATCH is low, the synchronous rectifier remains
on until either OVP is brought high, SHDN is toggled, or VCC is cycled below 1V. If LATCH is high,
normal operation resumes when the overvoltage condition ends.3 20 SHDN Shutdown Control Input. Drive SHDN to GND to force the MAX1844 into shutdown. Drive or connect
to VCC for normal operation. A rising edge on SHDN clears the overvoltage and undervoltage
protection fault latches.4 1 OVPOvervoltage Protection Control Input. An overvoltage fault occurs if the internal or external feedback
voltage exceeds the voltage at OVP. Apply a voltage between 1V and 1.8V to set the overvoltage
limit between 100% and 180% of nominal output voltage. Connect to GND to assert the default
overvoltage limit at 114% of the nominal output voltage. Connect to VCC to disable overvoltage fault
detection and clear the overvoltage protection fault latch.5 2 FB Feedback Input. Connect to VCC for a 1.8V fixed output or to GND for a 2.5V fixed output. For an
adjustable output (1V to 5.5V), connect FB to a resistive-divider from the output voltage. The FB
regulation level is 1V.6 3 OUT Output Voltage Sense Connection. Connect directly to the junction of the external output filter
capacitors. OUT senses the output voltage to determine the on-time for the high-side switching
MOSFET. OUT also serves as the feedback input in fixed-output modes.7 4 ILIMCurrent-Limit Threshold Adjustment. The current-limit threshold at CS is 0.1 times the voltage at ILIM.
Connect ILIM to a resistive-divider (typically from REF) to set the current-limit threshold between
25mV and 300mV (with 0.25V to 3V at ILIM). Connect to VCC to assert the 100mV default current-limit
threshold.8 5 REF 2V Reference Voltage Output. Bypass to GND with a 0.22µF (min) bypass capacitor. Can supply
50µA for external loads. Reference turns off in shutdown.9 6 UVPUndervoltage Protection Control Input. An undervoltage fault occurs if the internal or external
feedback voltage is less than the voltage at UVP. Apply a voltage between 0.4V and 1V to set the
undervoltage limit between 40% and 100% of the nominal output voltage. Connect to VCC to assert
the default undervoltage limit of 70% of the nominal output voltage. Connect to GND to disable
undervoltage fault detection and clear the undervoltage protection latch.10 7 PGOOD Power-Good Open-Drain Output. PGOOD is low when the output voltage is more than 10% above or
below the normal regulation point or during soft-start. PGOOD is high impedance when the output is
in regulation and the soft-start circuit has terminated. PGOOD is low in shutdown.11 8 GND Analog and Power Ground12 9 DL Synchronous Rectifier Gate-Driver Output. Swings from GND to VDD.13 10 VDD Supply Input for the DL Gate Drive. Connect to the system supply voltage, 4.5V to 5.5V. Bypass to
GND with a 1µF (min) ceramic capacitor.
Pin Description
Standard Application CircuitThe standard application circuit (Figure 1) generates a
2.5V rail for general-purpose use in a notebook computer.
See Table 1 for component selections. Table 2 lists com-
ponent manufacturers.
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook ComputersPINQSOP QFN NAME FUNCTION14 11 VCC Analog Supply Input. Connect to the system supply voltage, 4.5V to 5.5V, with a series 20Ω resistor.
Bypass to GND with a 1µF (min) ceramic capacitor.15 12 TON On-Time Selection-Control Input. This four-level logic input sets the nominal DH on-time. Connect to
GND, REF, VCC, or leave TON unconnected to select the following nominal switching frequencies:
GND = 600kHz, REF = 450kHz, floating = 300kHz, and VCC = 200kHz.16 13 V+ Battery Voltage Sense Connection. Connect to input power source. V+ is used only to set the PWM
one-shot timing.17 14 SKIP Pulse-Skipping Control Input. Connect to VCC for low-noise, forced-PWM mode. Connect to GND to
enable pulse-skipping operation.18 15 BST Boost Flying-Capacitor Connection. Connect to an external capacitor and diode according to the
Standard Application Circuit (Figure 1). See the MOSFET Gate Drivers (DH, DL) section.19 16 LX External Inductor Connection. Connect LX to the switched side of the inductor. LX serves as the
lower supply rail for the DH high-side gate driver.20 17 DH High-Side Gate-Driver Output. Swings from LX to BST.
Pin Description (continued)
Table 1. Component Selection for
Standard Applications
COMPONENT2.5V AT 4AC1 Input Capacitor
10μF, 25V
Taiyo Yuden TMK432BJ106KM or
TDK C4532X5R1E106M
C2 Output Capacitor
330μF, 6V
Kemet T510X477108M006AS or
Sanyo 6TPB330M
D1 SchottkyNihon EP10QY03
L1 Inductor
4.7μH
Coilcraft DO33116P-682 or
Sumida CDRH124-4R7MC
Q1 High-Side MOSFETFairchild Semiconductor
1/2 FDS6982A
Q2 Low-Side MOSFETFairchild Semiconductor
1/2 FDS6982A
RSENSE
0.015Ω ±1%, 0.5W resistor
IRC LR2010-01-R015F or
Dale WSL-2010-R015F
Table 2. Component Suppliers*Distributor
SUPPLIERUSA PHONEFACTORY FAXCoilcraft847-639-64001-847-639-1469
Dale-Vishay203-452-56641-203-452-5670
Fairchild408-822-21811-408-721-1635
IRC800-752-87081-828-264-7204
Kemet408-986-04241-408-986-1442
NIEC (Nihon)805-867-2555*81-3-3494-7414
Sanyo619-661-683581-7-2070-1174
Sumida847-956-066681-3-3607-5144
Taiyo Yuden408-573-41501-408-573-4159
TDK847-390-44611-847-390-4405
Detailed DescriptionThe MAX1844 buck controller is targeted for low-voltage
power supplies for notebook computers. Maxim’s propri-
etary Quick-PWM pulse-width modulator in the MAX1844
is specifically designed for handling fast load steps while
maintaining a relatively constant 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 also avoiding the problems
caused by widely varying switching frequencies in con-
ventional constant-on-time and constant-off-time PWM
schemes.
5V Bias Supply (VCCand VDD)The MAX1844 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 reg-
ulator 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 exter-
nal linear regulator such as the MAX1615.
The battery and 5V bias inputs can be connected
together if the input source is a fixed 4.5V to 5.5V supply.
If the 5V bias supply is powered up prior to the
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook ComputersVCC
UVP
VIN
7V TO 20V
BIAS SUPPLY
330μF
SEE TABLE 1 FOR OTHER COMPONENT SELECTIONS.
POWER-GOOD
INDICATOR
4.7μH
VOUT
2.5V
SHDN
CMPSH-3
3.3μF
0.1μF
0.22μF270kΩ
130kΩ
100kΩ
RSENSE
15mΩ
4.7μFR1
20Ω
SKIP
ILIM
ON/OFF
CONTROL
LOW-NOISE
CONTROL
BST
OUT
LATCH
OVP
PGOOD
VDD
MAX1844
TON
REF
GND
10μF
Figure 1. Standard Application Circuit
battery supply, the enable signal (SHDN) must be
delayed until the battery voltage is present in order to
ensure startup. The 5V bias supply provides VCCand
gate-drive power, so the maximum current drawn is:
IBIAS= ICC+ f (QG1+ QG2) = 5mA to 30mA (typ)
where ICCis 550µA (typ), 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-fre-
quency, constant-on-time on-demand PWM with voltage
feed-forward (Figure 2). This architecture relies on the out-
put filter capacitor’s ESR to act as a current-sense resistor,
so the output ripple voltage provides the PWM ramp sig-
nal. The control algorithm is simple: the high-side switch
on-time is determined solely by a one-shot whose pulse
MAX1844
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook ComputersFigure 2. MAX1844 Functional Diagram
REF
-10%
FROM
OUT
REF
ERROR
AMP
TOFF
TON
REF
+10%
FEEDBACK
MUX
(SEE FIGURE 6)
CHIP
SUPPLY
1.0V
0.1V
POR
OVP
ILIM
VCC - 1V
VCC - 1V
VCC - 1V
SHDN
PGOOD
ON-TIME
COMPUTE
TON
1-SHOT
1-SHOT
TRIG
2V TO 28V
TRIG
REF
REF
OUTPUT
GND
VCC
VDD
ZERO CROSSING
CURRENT
LIMIT
BST
+5V
SKIP
TON
LATCH
MAX1844
0.7V
1.14V
UVP
20ms
TIMER
OVP/UVP
LATCH
0.1V
OUT
MAX1844width 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 trig-
gered if the error comparator is low, the low-side switch
current is below the current-limit threshold, and the mini-
mum off-time one-shot has timed out.
On-Time One-Shot (TON)The heart of the PWM core is the one-shot that sets the
high-side switch on-time. 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 proportional to the battery
voltage as measured by the V+ input, and proportional
to the output voltage. This algorithm 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 volt-
age ripple. The on-time is given by:
On-Time = K (VOUT+ 0.075V) / VIN
where K (switching period) 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 600kHz and 450kHz, and
±10% at the two slower settings. This translates to
reduced switching-frequency accuracy at higher frequen-
cies (Table 5). Switching frequency increases as a func-
tion of load current due to the increasing drop across the
low-side MOSFET, which causes a faster inductor-current
discharge ramp. The on-times guaranteed in the
Electrical Characteristicsare influenced by switching
delays in the external high-side power MOSFET.
Two 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 contribu-
tors to the change of frequency with changing load cur-
rent. The dead-time effect increases the effective
on-time, reducing the switching frequency as one or
both dead times are added to the effective on-time. It
occurs only in PWM mode (SKIP= high) when the induc-
tor current reverses at light or negative load currents.
With reversed inductor current, the inductor’s EMF caus-
es LX to go high earlier than normal, 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:VV
t(VV)
OUTDROP1INDROP2=+
High-Speed Step-Down Controller with
Accurate Current Limit for Notebook Computers
Table 3. Operating Mode Truth TableNormal operation with automatic PWM/PFM switchover for pulse skipping at light loads.
Best light-load efficiency.
Run
(PFM/PWM)SwitchingGND1
Low-noise operation with no automatic switchover. Fixed-frequency PWM action is
forced regardless of load. Inductor current reverses at light load levels. Low noise,
high IQ.
Run (PWM),
Low NoiseSwitchingVCC1
Fault latch has been set by overvoltage protection, output UVLO, or thermal shutdown.
Device will remain in FAULT mode until VCCpower is cycled or SHDNis toggled.FaultHighX1
Low-power shutdown state. DL is forced to VDDif OVPis enabled and to GND if OVP is
disabled. ICC< 1µA typ.ShutdownHigh or
LowX0
COMMENTSMODEDLSKIPSHDN
Good operating point for
compound buck designs
or desktop circuits.
+5V input600
TON = GND
TON = REF3-cell Li+ notebook
Useful in 3-cell systems
for lighter loads than the
CPU core or where size is
key.
Considered mainstream
by current standards.4-cell Li+ notebook 300
TON = Float
200
TON = VCC4-cell Li+ notebook Use for absolute best
efficiency.
COMMENTSTYPICAL
APPLICATION
FREQUENCY
(kHz)
Table 4. Frequency Selection Guidelines
