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MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
General Description
Benefits and Features
The MAX25231 is a small, synchronous automotive buck
converter with integrated high-side and low-side switches.
The MAX25231 is designed to deliver up to 1.2A, with
3.5V to 36V input voltages, while using only 3.5µA quiescent current at no load. The device provides an accurate
output voltage of ±2% within the normal operation input
range of 6V to 18V. With 65ns minimum on-time capability, the converter is capable of large input-to-output conversion ratios. Voltage quality can be monitored by observing the PGOOD signal. The device can operate in dropout
by running at 99% duty cycle, making it ideal for automotive and industrial applications. The device offers two fixed
output voltages of 5V and 3.3V. Frequency is internally
fixed at 2.1MHz, which allows for small external components and reduced output ripple, and guarantees no AM
interference. The device automatically enters skip mode
at light loads with ultra-low quiescent current of 3.5µA at
no load. The device offers pin-enabled spread-spectrum
frequency modulation designed to minimize EMI-radiated
emissions due to the modulation frequency.
● Synchronous DC-DC Converter with Integrated FETs
• 3.5μA Quiescent Current when in Standby Mode
The MAX25231 is available in a small (3mm x 3mm)
12-pin TDFN package with an exposed pad, and uses
very few external components.
Applications
●
●
●
●
Automotive
Always-On, Low-Quiescent-Current Systems
Industrial
High-Voltage DC-DC Converters
19-100881; Rev 2; 5/21
● Small Solution Size Saves Space
• 2.1MHz Frequency
• Fixed 5V/3.3V Output Voltage Options Available
• Fixed 2.5ms Internal Soft-Start
• Fixed Output Voltage with ±2% Output Accuracy
• Adjustable Voltage Output 3V to 10V Available
(MAX25231ATCD)
• Innovative Current-Mode-Control Architecture
Minimizes Total Board Space and BOM Count
● PGOOD Output and High-Voltage EN Input Simplify
Power Sequencing
● Protection Features and Operating Range Ideal for
Automotive Applications
• 3.5V to 36V Operating VIN Range
• 40V Load-Dump Protection
• 99% Duty-Cycle Operation with Low Dropout
• -40°C to +125°C Automotive Temperature Range
• AEC-Q100 Qualified
Ordering Information appears at end of data sheet.
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Simplified Block Diagram
SPS
SYNC
MAX25231
HVLDO
EN
REF
BANDGAP
OSC
BST
BIAS
SUP
CLK
CURRENT SENSE
+
SOFTSTART
SLOPE COMP
LOGIC
OUT
CONTROL
PWM
BIAS
LX
EAMP
FB
FB
V/RESET
COMP
GND
MAX25231ATCD
PGOOD
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Maxim Integrated | 2
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Absolute Maximum Ratings
SUP ........................................................................ -0.3V to +40V
EN............................................................... -0.3V to VSUP + 0.3V
BST to LX (Note 1) ................................................................. +6V
BST......................................................................... -0.3V to +45V
FB ...............................................................-0.3V to VBIAS + 0.3V
SYNC..........................................................-0.3V to VBIAS + 0.3V
SPS ............................................................-0.3V to VBIAS + 0.3V
OUT ........................................................................ -0.3V to +12V
OUT (MAX25231AFOD)......................................... -0.3V to +12V
PGOOD .................................................................... -0.3V to +6V
PGND to AGND..................................................... -0.3V to +0.3V
BIAS ......................................................................... -0.3V to +6V
OUT Short-Circuit Duration.........................................Continuous
ESD Protection
Human Body Model........................................................... ±2kV
Continuous Power Dissipation (TA = +70°C)
12-pin TDFN/SW TDFN ............................................................
(derate 24.4mW/°C above +70°C) ..............................1951mW
Operating Junction Temperature (Note 4) ..........-40ºC to +150ºC
Storage Temperature Range .............................. -65ºC to +150ºC
Junction Temperature ....................................................... +150ºC
Lead Temperature (Soldering, 10s) .................................. +300ºC
Soldering Temperature (Reflow)....................................... +260ºC
Note 1: LX has internal clamp diodes to PGND/AGND and SUP. Applications that forward bias these diodes should take care not to
exceed the IC’s package power-dissipation limits.
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.
Package Information
12 TDFN-CU
Package Code
TD1233+2C
Outline Number
21-0664
Land Pattern Number
90-0397
THERMAL RESISTANCE, FOUR-LAYER BOARD
Junction-to-Ambient (θJA)
41°C/W
Junction-to-Case Thermal Resistance (θJC)
8.5°C/W
For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages.
Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different
suffix character, but the drawing pertains to the package regardless of RoHS status.
Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a
four-layer board. For detailed information on package thermal considerations, refer to www.maximintegrated.com/
thermal-tutorial.
Electrical Characteristics
(VSUP = VEN, VSUP = 14V, VSYNC = 0V, VOUT = 5V, TJ = -40°C to +150°C, unless otherwise noted.) (Notes 3, 4)
PARAMETER
Supply Voltage Range
SYMBOL
VSUP
VSUP
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CONDITIONS
MIN
3.5
t < 1s
After soft-start
TYP
MAX
40
3
UNITS
36
V
36
Maxim Integrated | 3
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Electrical Characteristics (continued)
(VSUP = VEN, VSUP = 14V, VSYNC = 0V, VOUT = 5V, TJ = -40°C to +150°C, unless otherwise noted.) (Notes 3, 4)
PARAMETER
SYMBOL
CONDITIONS
MIN
VEN = low
Supply Current
LX Leakage
ISUP
ILX,LEAK
Undervoltage Lockout
UVLO
BIAS Voltage
VBIAS
TYP
MAX
1
5
MAX25231ATCB/V+, no load, no
switching
3.5
8
MAX25231ATCB/V+, no load, switching
(Note 2)
4.5
MAX25231ATCA/V+, no load, no
switching
5.6
MAX25231ATCA/V+, no load, switching
(Note 2)
6.6
VSUP = 40V, LX = 0 or 40V, TA = +25°C
OUT rising
-1
2.52
Hysteresis
µA
10
+1
2.73
2.93
0.16
5.5V ≤ VSUP ≤ 36V, FPWM mode
UNITS
5
µA
V
V
BUCK CONVERTER
Voltage Accuracy, 5V
VOUT,5V
MAX25231ATCA/V+ skip mode (Note 2)
4.87
5
5.08
MAX25231ATCA/V+ fixed-frequency
PWM mode
4.93
5
5.07
MAX25231ATCB/V+, skip mode
3.18
3.3
3.37
MAX25231ATCB/V+, fixed-frequency
PWM mode
3.25
3.3
3.35
V
Voltage Accuracy, 3.3V
VOUT,3.3V
Output Voltage Range
VOUT
FB Voltage Accuracy
VFB
MAX25231ATCD/V+ only
FB Current
IFB
VFB = 1V, TA = +25ºC
0.02
μA
VSUP = 6V to 36V
0.02
%/V
FB Line Regulation
MAX25231ATCD/V+
3
0.985
10
1
1.015
V
V
V
High-Side Switch OnResistance
RON,HS
VBIAS = 5V, ILX = 1.2A
300
mΩ
Low-Side Switch OnResistance
RON,LS
VBIAS = 5V, ILX= 1.2A
200
mΩ
High-Side Current-Limit
Threshold
Low-Side Negative
Current- Limit Threshold
MAX25231
1.67
1.9
2.13
A
INEG
-0.6
Soft-Start Ramp Time
ISS
2.5
5
ms
Minimum On-Time
tON
66
85
ns
Maximum Duty Cycle
PWM Switching
Frequency
fSW
Fixed
Spread-Spectrum
Range
SS
VSPS = 5V
VTHR,PGD
VOUT rising
98
99
1.925
2.1
A
%
2.275
±6
MHz
%
PGOOD
PGOOD Threshold,
Rising
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MAX25231ATCA/
V+
90
93.5
97
%
Maxim Integrated | 4
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Electrical Characteristics (continued)
(VSUP = VEN, VSUP = 14V, VSYNC = 0V, VOUT = 5V, TJ = -40°C to +150°C, unless otherwise noted.) (Notes 3, 4)
PARAMETER
SYMBOL
PGOOD Threshold,
Falling
VTHF,PGD
PGOOD Debounce
Time
tDEB
CONDITIONS
VOUT falling
MAX25231ATCA/
V+
MIN
TYP
MAX
UNITS
89.5
93
96.5
%
PWM mode, VOUT falling
65
Skip mode, VOUT rising
100
µs
PGOOD High-Leakage
Current
ILEAK,PGD
TA = +25°C
1
µA
PGOOD Low Level
VOUT,PGD
Sinking 1mA
0.4
V
LOGIC LEVELS
EN Level, High
VIH,EN
EN Level, Low
VIL,EN
EN Input Current
IIN,EN
2.4
VEN = VSUP = 14V, TA = +25°C
External Input Clock
Frequency
1.7
SYNC Threshold, High
VIH,SYNC
SYNC Threshold, Low
VIL,SYNC
SYNC Internal Pulldown
V
VIH,SPS
SPS Threshold, Low
VIL,SPS
V
1
µA
2.6
MHz
1.4
V
0.4
RPD,MODE
SPS Threshold, High
0.6
1000
kΩ
1.4
V
0.4
SPS Internal Pulldown
V
V
1000
kΩ
THERMAL PROTECTION
Thermal Shutdown
TSHDN
(Note 3)
175
°C
Thermal-Shutdown
Hysteresis
TSHDN.HYS
(Note 3)
15
°C
Note 2: Guaranteed by design; not production tested.
Note 3: Limits are 100% tested at TA + +25°C. Limits over the operating range and relevant supply voltage are guaranteed by design
and characterization. Typical values are at TA = +25°C.
Note 4: The device is designed for continuous operation up to TJ = +125°C for 95,000 hours and TJ = +150°C for 5,000 hours.
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Maxim Integrated | 5
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Typical Operating Characteristics
(VSUP = VEN = +14V, (TA = +25°C, unless otherwise noted.)
EFFICIENCY vs. LOAD
100
QUIESCENT SUPPLY CURRENT
vs.INPUT VOLTAGE (SKIP MODE)
toc01
25
VEN = 0V
20
5V
60
5V
3.3V
15
3.3V
50
FPWM MODE
40
IQ (µA)
70
IQ (µA)
EFFICIENCY (%)
80
toc03
10
NO LOAD
SKIP MODE
90
SHUTDOWN SUPPLY CURRENT
vs. INPUTVOLTAGE
toc02
5VOUT
10
1
30
5
20
3.3VOUT
10
0.1
0
0
0.001
0.01
0.1
6
1
9
12
LOAD CURRENT (A)
2
SKIPMODE
VOUT = 5V
300
250
200
150
100
30
33
6
36
0.2
0.4
0.6
12
15
2
0.8
0.5
SKIP
FPWM
-0.5
-1
12.0
18.0
33
36
toc07
FPWM
-0.5
-1
-1.5
24.0
30.0
0
36.0
0.2
0.4
0.6
0.8
1
1.2
IOUT (A)
SHUTDOWN WAVEFORM
(1.2A LOAD)
toc10
toc09
VIN = 14V
VOUT = 5V
VIN = 14V
VOUT = 5V
VEN
70
30
SKIP
0
STARTUP WAVEFORM
(1.2A LOAD)
toc08
VSPS = 5V
VOUT = 5V
27
1
0.5
VIN (V)
ILOAD (mA)
SPECTRAL ENERGY DENSITY
vs. FREQUENCY
24
-2
6.0
1.0
21
VIN = 14V
1.5
1
0
18
LOAD REGULATION
toc05
1A LOAD
-2
0
0.0
9
VIN (V)
-1.5
50
OUTPUT SPECTRUM (dBµV)
27
OUTPUT-VOLTAGE CHANGE (%)
OUTPUT-VOLTAGE CHANGE (%)
ISUP (μA)
350
80
24
1.5
400
90
21
LINE REGULATION
(5VOUT)
toc04
500
100
18
VIN (V)
STA NDB Y CURRENT
v s . L OA D CURRENT
450
15
5V/div
VEN
5V/div
1A/div
IINDUCTOR
1A/div
5V/div
VPGOOD
5V/div
5V/div
VOUT
5V/div
60
IINDUCTOR
50
40
30
20
VPGOOD
10
VOUT
0
1.85
2.1
FREQUENCY (MHz)
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2.35
1ms/div
100µs/div
Maxim Integrated | 6
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Typical Operating Characteristics (continued)
(VSUP = VEN = +14V, (TA = +25°C, unless otherwise noted.)
UNDERVOLTAGE PULSE
(NO LOAD)
SLOW VIN RAMP
STEADY-STATE SWITCHING WAVEFORMS
(5VOUT)
toc14
toc12
toc11
VOUT = 5V
10mA LOAD
VIN = 14V
NO LOAD
14V
3.5V
VIN
VIN
5V/div
VLX
5V/div
5V/div
VOUT
5V/div
5V/div
100mA/div
IINDUCTOR
VOUT
5V/div
VOUT
2V/div
VPGOOD
VPGOOD
5V/div
ILOAD
200ns/div
500mA/div
5s/div
10ms/div
LOAD-TRANSIENT RESPONSE
(5VOUT)
LOAD-DUMP TEST
toc15
NO LOAD
VIN = 14V
FPWM
40V
toc16
1A
ILOAD
500mA/div
VOUT
(AC)
100mV/div
14V
VIN
10V/div
VOUT
5V/div
100ms/div
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20µs/div
Maxim Integrated | 7
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Pin Configuration
AGND
7
FB
8
OUT
9
BIAS
SYNC
11
10
PGOOD
12
MAX25231
3
4
5
6
SUP
LX
PGND
2
EN
BST
1
SPS
MAX25231
TDFN-EP
(3mm x 3mm)
Pin Description
PIN
NAME
FUNCTION
Spread-Spectrum Enable. Connect logic-high to enable spread spectrum of internal oscillator or
logic-low to disable spread spectrum. This pin has a 1MΩ internal pulldown.
1
SPS
2
EN
High-Voltage-Compatible Enable Input. If this pin is low, the part is off.
3
BST
Bootstrap Pin for HS Driver. It is recommended to use 0.1μF from BST to LX.
4
SUP
Supply Input. Connect a 4.7μF ceramic capacitor from SUP to PGND.
5
LX
6
PGND
Power Ground. Ground return path for all high-current/high-frequency noisy signals.
7
AGND
Analog Ground. Ground return path for all ‘quiet’ signals.
8
FB
Feedback Pin. Connect a resistor-divider from OUT to FB to ground for external adjustment of the
output voltage (MAX25231ATCD only). Connect to bias for internal fixed voltage configurations.
Buck Switching Node. High impedance when part is off. Connect a 4.7μH inductor between LX and
OUT.
9
OUT
Buck Regulator Output-Voltage-Sense Input. Bypass OUT to PGND with 22μF ceramic capacitor.
10
BIAS
5V Internal BIAS Supply. Connect a 1μF (min) ceramic capacitor to AGND.
11
SYNC
Sync Input. If connected to ground or open, skip-mode operation is enabled under light loads; if
connected to BIAS, forced-PWM mode is enabled. This pin has a 1MΩ internal pulldown.
12
PGOOD
-
EP
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Open-Drain Reset Output. External pullup required.
Exposed Pad. EP must be connected to ground plane on PCB, but is not a current-carrying path
and is only needed for thermal transfer.
Maxim Integrated | 8
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Detailed Description
The MAX25231 family of small, current-mode-controlled buck converters features synchronous rectification and requires
no external compensation network. The devices are designed for 1.2A and can stay in dropout by running at 99% duty
cycle. They provide an accurate output voltage within the 5.5V to 18V input range. Voltage quality can be monitored
by observing the PGOOD signal. The devices operate at a frequency of 2.1MHz (typ), which allows for small external
components, reduced output ripple, and guarantees no AM band interference.
The devices feature an ultra-low 3.5μA (typ) quiescent supply current in standby mode. The devices center standby mode
automatically at light loads if HSFET does not turn on for eight consecutive clock cycles. The devices operate from a
3.5V to 36V supply voltage and can tolerate transients up to 40V, making them ideal for automotive applications. The
devices are available in factory-trimmed fixed output voltages of 5V and 3.3V and are programmable with an external
resistor-divider (MAX25231ATCD only). For fixed output voltages outside of 3.3V and 5V, contact factory for availability.
Enable Input (EN)
The device is activated by driving EN high. EN is compatible from a 3.3V logic level to automotive battery levels. EN
can be controlled by microcontrollers and automotive KEY or CAN inhibit signals. The EN input has no internal pullup/
pulldown current to minimize the overall quiescent supply current. To realize a programmable undervoltage-lockout level,
use a resistor-divider from SUP to EN to AGND.
BIAS/UVLO
The device features undervoltage lockout. When the device is enabled, an internal bias generator turns on. LX begins
switching after VBIAS has exceeded the internal undervoltage-lockout level, VUVLO = 2.73V (typ).
Soft-Start
The device features an internal soft-start timer. The output voltage soft-start ramp time is 2.5ms (typ). If a short circuit or
undervoltage is encountered after the soft-start timer has expired, the device is disabled for 6ms (typ) and then reattempts
soft-start again. This pattern repeats until the short circuit has been removed.
Oscillator/Synchronization and Efficiency (SYNC)
The device has an on-chip oscillator that provides a 2.1MHz (typ) switching frequency. Depending on the condition of
SYNC, two operation modes exist. If SYNC is unconnected or at AGND, the device operates in highly efficient pulseskipping mode. If SYNC is at BIAS or has a clock applied to it, the device is in forced-PWM mode (FPWM). The device
can be switched during operation between FPWM mode and skip mode by switching SYNC.
Skip-Mode Operation
Skip mode is entered when the SYNC pin is connected to ground or is unconnected and the peak load current is < 150mA
(typ). In this mode, the high-side FET is turned on until the current in the inductor is ramped up to 150mA (typ) peak
value and the internal feedback voltage is above the regulation voltage (1.0V, typ). At this point, both the high-side and
low-side FETs are turned off. Depending on the choice of the output capacitor and the load current, the high-side FET
turns on when OUT (valley) drops below the 1.0V (typ) feedback voltage. For fixed output voltage parts, when the device
is in skip mode, the internal high-voltage LDO is turned off after the startup is complete to reduce the input current. VBIAS
is supplied by the output in this condition.
Achieving High Efficiency at Light Loads
The device operates with very low quiescent current at light loads to enhance efficiency and conserve battery life. When
the device enters skip mode, the output current is monitored to adjust the quiescent current.
When the output current is less than approximately 5mA, the devices operate in the lowest-quiescent-current mode, also
called standby mode. In this mode, the majority of the internal circuitry (excluding that necessary to maintain regulation)
in the device, including the internal high-voltage LDO, is turned off to save current. Under no load and with skip mode
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Maxim Integrated | 9
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
enabled, the IC typically draws 4.5μA for the 3.3V parts, and 6.6μA for the 5.0V parts. For load currents greater than
5mA, the device enters normal skip mode, still maintaining very high efficiency.
Controlled EMI with Forced Fixed Frequency
In FPWM mode, the device attempts to operate at a constant switching frequency for all load currents. For tightest
frequency control, apply the operating frequency to SYNC. The advantage of this mode is a constant switching frequency,
which improves EMI performance; the disadvantage is that considerable current can be wasted. If the load current during
a switching cycle is less than the current flowing through the inductor, the excess current is diverted to AGND.
Extended Input Voltage Range
In some cases, the device is forced to deviate from its operating frequency, independent of the state of SYNC. For input
voltages above 18V, the required duty cycle to regulate its output may be smaller than the minimum on- time (66ns, typ).
In this event, the device is forced to lower its switching frequency by skipping pulses.
If the input voltage is reduced and the device approaches dropout, the device tries to turn on the high-side FET
continuously. To maintain gate charge on the high-side FET, the BST capacitor must be periodically recharged. To
ensure proper charge on the BST capacitor when in dropout, the high-side FET is turned off every 20μs and the low-side
FET is turned on for about 200ns. This gives an effective duty cycle of > 99% and a switching frequency of 50kHz when
in dropout.
Spread-Spectrum Option
The device has an optional spread spectrum enabled by the SPS pin. If SPS is pulled high, then the internal operating
frequency varies by ±6% relative to the internally generated operating frequency of 2.1MHz (typ). Spread spectrum is
offered to improve EMI performance of the device.
The internal spread spectrum does not interfere with the external clock applied on the SYNC pin. It is active only when
the device is running with an internally generated switching frequency.
Power-Good (PGOOD)
The device features an open-drain power-good output. PGOOD is an active-high output that pulls low when the output
voltage is below 93% of its nominal value. PGOOD is high impedance when the output voltage is above 93.5% of its
nominal value. Connect a 20kΩ (typ) pullup resistor to an external supply or the on-chip BIAS output.
Overcurrent Protection
The device limits the peak output current to 1.9A (typ). The accuracy of the current limit is ±12%, which makes selection
of external components very easy. To protect against short-circuit events, the device shuts off when OUT is below 50%
of OUT voltage and an overcurrent event is detected. The device attempts a soft-start restart every 7ms and remains off
if the short circuit has not been removed. When the current limit is no longer present, it reaches the output voltage by
following the normal soft-start sequence. If the device's die reaches the thermal limit of 175°C (typ) during the currentlimit event, it immediately shuts off.
Thermal-Overload Protection
The device features thermal-overload protection, turning off when the junction temperature exceeds +175°C (typ). Once
the device cools by 15°C (typ), it turns back on with a soft-start sequence.
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Maxim Integrated | 10
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Applications Information
Setting the Output Voltage
MAX25231ATCA and MAX25231ATCB are configured with a fixed output voltage.
MAX25231ATCD is configured with an adjustable output voltage between 3V and 10V. Connect a resistive divider from
output (OUT) to FB to AGND as the following figure. Select RFB2 (FB to AGND resistor) less than or equal to 500kΩ.
Calculate RFB1 (OUT to FB resistor) with the following equation:
RFB1 = RFB2 [(VOUT/VFB)-1)]
where VFB = 1V.
VOUT
RFB1
MAX25231ATCD
FB
RFB2
Inductor Selection
The design is optimized with 4.7μH inductor for all input and output voltage conditions. The nominal standard value
selected should be within ±50% of 4.7μH.
Input Capacitor
A low-ESR ceramic input capacitor of 4.7μF is recommended for proper device operation. This value can be adjusted
based on application input-voltage ripple requirements.
The discontinuous input current of the buck converter causes large input ripple current. The switching frequency,
peak inductor current, and the allowable peak-to-peak input-voltage ripple dictate the input-capacitance requirement.
Increasing the switching frequency or the inductor value lowers the peak-to-average current ratio, yielding a lower inputcapacitance requirement.
The input ripple is primarily composed of ΔVQ (caused by the capacitor discharge) and ΔVESR (caused by the ESR of the
input capacitor). The total voltage ripple is the sum of ΔVQ and ΔVESR. Assume that input-voltage ripple from the ESR
and the capacitor discharge is equal to 50% each. The following equations show the ESR and capacitor requirement for
a target voltage ripple at the input:
Equation 1:
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Maxim Integrated | 11
MAX25231
ESR =
CIN =
36V, 1.2A Mini Buck Converter with 3.5μA IQ
∆ VESR
/
IOUT + ( ∆ IP − P 2)
IOUT × D(1 − D)
∆ VQ × fSW
where:
∆ IP − P =
(VIN − VOUT) × VOUT
VIN × fSW × L
and:
VOUT
D= V
IN
where IOUT is the output current, D is the duty cycle, and fSW is the switching frequency. Use additional input capacitance
at lower input voltages to avoid possible undershoot below the UVLO threshold during transient loading.
Output Capacitor
For optimal phase margin (> 70 deg, typ) with internal fixed-voltage options, a 22μF output capacitor is recommended. A
lower output capacitor can be used at the expense of lower phase margin. For all other designs, a minimum 10μF output
capacitor is required. Additional output capacitance may be needed based on application-specific output-voltage ripple
requirements. If the total output capacitance required is > 70μF, contact the factory for an optimized solution.
The allowable output-voltage ripple and the maximum deviation of the output voltage during step-load currents determine
the output capacitance and its ESR. The output ripple comprises ΔVQ (caused by the capacitor discharge) and ΔVESR
(caused by the ESR of the output capacitor). Use low-ESR ceramic or aluminum electrolytic capacitors at the output. For
aluminum electrolytic capacitors, the entire output ripple is contributed by ΔVESR. Use the ESROUT equation to calculate
the ESR requirement and choose the capacitor accordingly. If using ceramic capacitors, assume the contribution to the
output ripple voltage from the ESR and the capacitor discharge to be equal. The following equations show the output
capacitance and ESR requirement for a specified output-voltage ripple.
Equation 2:
∆ VESR
ESR = ∆ I
P−P
∆ IP − P
COUT = 8 × ∆ V × f
Q SW
where:
∆ IP − P =
(VIN − VOUT) × VOUT
VIN × fSW × L
and:
VOUT_RIPPLE = ∆ VESR + ∆ VQ
ΔIP-P is the peak-to-peak inductor current as calculated above, and fSW is the converter’s switching frequency. The
allowable deviation of the output voltage during fast transient loads also determines the output capacitance and its ESR.
The output capacitor supplies the step-load current until the converter responds with a greater duty cycle. The response
time (tRESPONSE) depends on the closed-loop bandwidth of the converter. The high switching frequency of the devices
allows for a higher closed-loop bandwidth, thus reducing tRESPONSE and the output-capacitance requirement. The
resistive drop across the output capacitor’s ESR and the capacitor discharge causes a voltage droop during a step load.
Use a combination of low-ESR tantalum and ceramic capacitors for better transient load and ripple/noise performance.
Keep the maximum output-voltage deviations below the tolerable limits of the electronics being powered. When using
a ceramic capacitor, assume an 80% and 20% contribution from the output-capacitance discharge and the ESR drop,
respectively. Use the following equations to calculate the required ESR and capacitance value:
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Maxim Integrated | 12
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Equation 3:
∆ VESR
ESROUT = I
STEP
COUT =
ISTEP × tRESPONSE
∆ VQ
where ISTEP is the load step and tRESPONSE is the response time of the converter. The converter response time
depends on the control-loop bandwidth.
PCB Layout Guidelines
Careful PCB layout is critical to achieve low switching power losses and clean, stable operation. Use a multilayer board
wherever possible for better noise immunity. Follow the guidelines below for a good PCB layout:
1. The input capacitor (4.7μF, see Circuit1 - Fixed Output Typical Application Circuit) should be placed right next to
the SUP pin. Since the MAX25231 operates at 2.1MHz switching frequency, this placement is critical for effective
decoupling of high-frequency noise from the SUP pins.
2. Solder the exposed pad to a large copper-plane area under the device. To effectively use this copper area as a heat
exchanger between the PCB and ambient, expose the copper area on the top and bottom side. Add a few small vias
or one large via on the copper pad for efficient heat transfer. Connect the exposed pad to PGND, ideally at the return
terminal of the output capacitor.
3. Isolate the power components and high-current paths from sensitive analog circuitry.
4. Keep the high-current paths short, especially at the ground terminals. This practice is essential for stable, jitter-free
operation.
5. Connect PGND and AGND together, preferably at the return terminal of the output capacitor. Do not connect them
anywhere else.
6. Keep the power traces and load connections short. This practice is essential for high efficiency. Use thick copper
PCB to enhance full-load efficiency and power-dissipation capability.
7. Route high-speed switching nodes away from sensitive analog areas. Use internal PCB layers as PGND to act as
EMI shields to keep radiated noise away from the device and analog bypass capacitor.
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Maxim Integrated | 13
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Typical Application Circuits
Circuit1 - Fixed Output
MAX25231ATCA
MAX25231ATCB
SUP
BST
CIN
4.7µF
LX
CBST
0.1µF
L
4.7µH
NH
OUT
SYNC
FB
EN
PGOOD
SPS
COUT
22µF
NL
BIAS
AGND
CBIAS
1µF
PGND
Circuit2 - Adjustable Output
MAX25231ATCD
SUP
BST
CIN
4.7µF
LX
CBST
0.1µF
L
4.7µH
NH
OUT
SYNC
NL
FB
EN
PGOOD
SPS
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RFB1
BIAS
AGND
PGND
COUT
22µF
RFB2
CBIAS
1µF
Maxim Integrated | 14
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
DESCRIPTION
IOUT
MAX25231ATCA/V+
-40°C to +125°C
TD1233+2C
Fixed 5V output
1.2A
MAX25231ATCB/V+
-40°C to +125°C
TD1233+2C
Fixed 3.3V output
1.2A
MAX25231ATCD/V+
-40°C to +125°C
TD1233+2C
Adjustable output voltage between 3V to 10V
1.2A
Note: All parts are OTP versions, no metal mask differences.
/V denotes an automotive qualified part.
+Denotes a lead(Pb)-free/RoHS-compliant package.
Y = Side-wettable package.
* Future product—contact factory for availability
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Maxim Integrated | 15
MAX25231
36V, 1.2A Mini Buck Converter with 3.5μA IQ
Revision History
REVISION
NUMBER
REVISION
DATE
0
9/20
Initial release
—
1
4/21
Removing future products notation for MAX25231ATCD/V+
15
2
5/21
Update Package Information
3
DESCRIPTION
PAGES
CHANGED
For pricing, delivery, and ordering information, please visit Maxim Integrated’s online storefront at https://www.maximintegrated.com/en/storefront/storefront.html.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent
licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max
limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
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