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 ..
MAX1845EEI+T ,Dual, High-Efficiency, Step-Down Controller with Accurate Current LimitMAX184519-1955; Rev 2; 1/03Dual, High-Efficiency, Step-DownController with Accurate Current Limit
MAX1845ETX ,Dual / High-Efficiency / Step-Down Controller with Accurate Current LimitFeaturesThe MAX1845 is a dual PWM controller configured for Ultra-High Efficiencystep-down (buck) ..
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MAX474CSA ,Single/Dual/Quad, 10MHz Single-Supply Op AmpsGeneral Description ________
MAX474CUA ,Single/Dual/Quad, 10MHz Single-Supply Op Ampsapplications. Withtheir fast slew rate and settling time, they can replace ' Rail-to-Rail Output Sw ..
MAX474EPA ,Single/Dual/Quad, 10MHz Single-Supply Op AmpsApplicationsMAX473CSA 0°C to +70°C 8 SOPortable EquipmentMAX473CUA 0°C to +70°C 8 µMAXBattery-Power ..
MAX1845EEI-MAX1845ETX
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
Quick-PWM and Dual Mode are trademarks of Maxim Integrated
Products.
Minimal Operating Circuit
Ordering Information
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 otherwise
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.)
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.)
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
ELECTRICAL CHARACTERISTICS
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current LimitMAX1845 toc01
LOAD CURRENT (A)
FREQUENCY (kHz)
FREQUENCY vs. LOAD CURRENTMAX1845 toc02
INPUT VOLTAGE (V)
FREQUENCY (kHz)
FREQUENCY vs. INPUT VOLTAGE
(TON = FLOAT, SKIP = VCC)
__________________________________________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.
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)MAX1845 toc03B
SUPPLY VOLTAGE V+ (V)
SUPPLY CURRENT (1015202530
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE (SKIP = GND)MAX1845 toc04a
LOAD CURRENT (A)
EFFICIENCY (%)
EFFICIENCY vs. LOAD CURRENT
(8A COMPONENTS, SKIP = VCC)MAX1845 toc04b
LOAD CURRENT (A)
EFFICIENCY (%)
EFFICIENCY vs. LOAD CURRENT
(8A COMPONENTS, SKIP = GND)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)MAX1845 toc04d
LOAD CURRENT (A)
EFFICIENCY (%)
EFFICIENCY vs. LOAD CURRENT
(4A COMPONENTS, SKIP = GND)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 Description
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
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-
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.
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 VDROP1 is 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:
Dual, High-Efficiency, Step-Down
Controller with Accurate Current Limit
Table 1. Component Selection for
Standard Applications
Table 2. Component Suppliers
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
MAX1845
Dual, High-Efficiency, Step-Down
Controller with Accurate Current LimitFigure 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
