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19-5478; Rev 0; 12/10
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
General Description
The MAX15058 high-efficiency, current-mode, synchronous step-down switching regulator with integrated power switches delivers up to 3A of output current. The device operates from 2.7V to 5.5V and provides an output voltage from 0.6V up to 94% of the input voltage, making the device ideal for distributed power systems, portable devices, and preregulation applications. The MAX15058 utilizes a current-mode control architecture with a high-gain transconductance error amplifier. The current-mode control architecture facilitates easy compensation design and ensures cycle-by-cycle current limit with fast response to line and load transients. The MAX15058 offers selectable skip-mode functionality to reduce current consumption and achieve a higher efficiency at light output load. The low RDS(ON) integrated switches ensure high efficiency at heavy loads while minimizing critical inductances, making the layout design a much simpler task with respect to discrete solutions. Utilizing a simple layout and footprint assures first-pass success in new designs. The MAX15058 features a 1MHz, factory-trimmed, fixedfrequency PWM mode operation. The high switching frequency, along with the PWM current-mode architecture, allows for a compact, all-ceramic capacitor design. The MAX15058 offers a capacitor-programmable softstart reducing inrush current, startup into PREBIAS operations, and a PGOOD open-drain output that can be used as an interrupt and for power sequencing. The MAX15058 is available in a 9-bump (3 x 3 array), 1.65mm x 1.65mm WLP package and is specified over the -40NC to +85NC temperature range. (typ) Low-Side MOSFETs at 5V
TION KIT EVALUA BLE ILA AVA
Features
S Internal 30mI (typ) RDS(ON) High-Side and 18mI S Continuous 3A Output Current Over Temperature S 95% Efficiency with 3.3V Output at 3A S 1% Output Voltage Accuracy Over Load, Line, and
MAX15058
Temperature
S Operates from 2.7V to 5.5V Supply S Cycle-by-Cycle Overcurrent Protection S Adjustable Output from 0.6V to Up to 0.94 x VIN S Programmable Soft-Start S Factory-Trimmed, 1MHz Switching Frequency S Stable with Low-ESR Ceramic Output Capacitors S Safe-Startup Into Prebiased Output S External Reference Input S Skip-Mode Functionality S Enable Input/Power-Good Output S Fully Protected Against Overcurrent and
Overtemperature
S Input Undervoltage Lockout
Ordering Information
PART MAX15058EWL+ TEMP RANGE -40C to +85C PIN-PACKAGE 9 WLP
+Denotes a lead(Pb)-free/RoHS-compliant package.
Applications
Distributed Power Systems Preregulators for Linear Regulators Portable Devices Notebook Power Server Power IP Phones
_______________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
ABSOLUTE MAXIMUM RATINGS
IN, PGOOD to GND ................................................-0.3V to +6V LX to GND ..................................................-0.3V to (VIN + 0.3V) LX to GND .......................................-1V to (VIN + 0.3V) for 50ns EN, COMP, FB, SS/REFIN, SKIP to GND ...-0.3V to (VIN + 0.3V) LX Current (Note 1) ................................................... -6A to +6A Output Short-Circuit Duration ....................................Continuous Continuous Power Dissipation (TA = +70NC) 9-Bump WLP Multilayer Board (derate 14.1mW/NC above TA = +70NC) ....................1127mW Junction-to-Case Thermal Resistance (BJC) (Note 2) .....26NC/W Junction-to-Ambient Thermal Resistance (BJA) (Note 2) ..........................................71NC/W Operating Temperature Range .......................... -40NC to +85NC Storage Temperature Range............................ -65NC to +150NC Soldering Temperature (reflow) ......................................+260NC
Note 1: LX has internal clamp diodes to GND and IN. Applications that forward bias these diodes should not exceed the IC's package power dissipation limits. Note 2: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial.
ELECTRICAL CHARACTERISTICS
(VIN = 5V, TA = -40NC to +85NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 3) PARAMETER IN Voltage Range IN Shutdown Supply Current IN Supply Current VIN Undervoltage Lockout Threshold VIN Undervoltage Lockout Hysteresis ERROR AMPLIFIER Transconductance Voltage Gain FB Set-Point Accuracy FB Input Bias Current COMP to Current-Sense Transconductance COMP Clamp Low POWER SWITCHES LX On-Resistance, High-Side pMOS LX On-Resistance, Low-Side nMOS High-Side Switch Current-Limit Threshold Low-Side Switch Sink CurrentLimit Threshold Low-Side Switch Source CurrentLimit Threshold IHSCL 30 18 5 4 5 mI mI A A A gMV AVEA VFB IFB gMC VFB = 0.65V, VSS = 0.6V Over line, load, and temperature VFB = 0.6V 594 -500 18 0.94 1.5 90 600 606 +500 mS dB mV nA A/V V IIN SYMBOL VIN VEN = 0V VEN = 5V, VFB = 0.65V, no switching LX starts switching, VIN rising LX stops switching, VIN falling CONDITIONS MIN 2.7 0.2 1.56 2.6 200 TYP MAX 5.5 2 2.3 2.7 UNITS V FA mA V mV
2
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
ELECTRICAL CHARACTERISTICS (continued)
(VIN = 5V, TA = -40NC to +85NC, unless otherwise noted. Typical values are at TA = +25NC.) (Note 3) PARAMETER LX Leakage Current RMS LX Output current OSCILLATOR Switching Frequency Maximum Duty Cycle Minimum Controllable On-Time Slope Compensation Ramp Valley Slope Compensation Ramp Amplitude ENABLE EN Input High Threshold Voltage EN Input Low Threshold Voltage EN Input Leakage Current SKIP Input Leakage Current SOFT-START, PREBIAS, REFIN Soft-Start Current SS/REFIN Discharge Resistance SS/REFIN Prebias Mode Stop Voltage External Reference Input Range HICCUP Number of Consecutive CurrentLimit Events to Hiccup Timeout POWER-GOOD OUTPUT PGOOD Threshold PGOOD Threshold Hysteresis PGOOD VOL PGOOD Leakage THERMAL SHUTDOWN Thermal Shutdown Threshold Thermal Shutdown Hysteresis Temperature falling 150 20 NC NC VFB rising VFB falling IPGOOD = 5mA, VFB = 0.5V VPGOOD = 5V, VFB = 0.65V 0.535 8 1024 0.555 28 20 0.013 60 0.575 Events Clock Cycles V mV mV FA ISS RSS VSS/REFIN = 0.45V, sourcing ISS/REFIN = 10mA, sinking VSS/REFIN rising 0 10 8.3 0.58 IN - 1.8 FA I V V VEN rising VEN falling VEN = 5V VSKIP = VEN = 5V 1.45 0.4 0.025 25 V V FA FA VSLOPE Extrapolated to 100% duty cycle fSW DMAX 850 1000 94 70 1.15 320 1150 kHz % ns V mV SYMBOL VEN = 0V 3 CONDITIONS MIN TYP MAX 10 UNITS FA A
MAX15058
3
Note 3: Specifications are 100% production tested at TA = +25NC. Limits over the operating temperature range are guaranteed by design and characterization.
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
Typical Operating Characteristics
(VIN = 5V, VOUT = 1.8V, ILOAD = 3A, Circuit of Figure 5, TA = +25NC, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT (PWM MODE)
MAX15058 toc01
EFFICIENCY vs. OUTPUT CURRENT (FORCED PWM)
MAX15058 toc02
EFFICIENCY vs. LOAD CURRENT (SKIP MODE)
MAX15058 toc03
100 95 EFFICIENCY (%) 90 85 80
VOUT = 3.3V
VOUT = 2.5V
100
100 95 EFFICIENCY (%) 90
VOUT = 2.5V
95 EFFICIENCY (%) 90 85 80
VOUT = 1.8V VOUT = 1.5V VOUT = 1.2V
VOUT = 1.8V
VOUT = 1.5V
85 80 75
VOUT = 1.5V VOUT = 1.2V
VOUT = 1.8V VOUT = 2.5V VOUT = 3.3V
VOUT = 1.2V
VIN = 5V
75 0 0.5 1.0 1.5 2.0 2.5 3.0 OUTPUT CURRENT (A) 75 0 0.5 1.0 1.5 2.0
VIN = 3.3V
2.5 3.0 70 0 0.5 1.0 1.5 2.0
VIN = 5V
2.5 3.0
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
EFFICIENCY vs. OUTPUT CURRENT (SKIP MODE)
VOUT = 2.5V
95 EFFICIENCY (%) 90 85 80
MAX15058 toc04
SWITCHING FREQUENCY vs. INPUT VOLTAGE
1080 SWITCHING FREQUENCY (kHz) 1060 1040 1020 1000 980 960 940 920 900 3.0 2.7 3.2 3.7 4.2 4.7 5.2 INPUT VOLTAGE (V)
MAX15058 toc05
100
1100
VOUT = 1.8V
VOUT = 1.5V VOUT = 1.2V
VIN = 3.3V
75 0 0.5 1.0 1.5 2.0 2.5 OUTPUT CURRENT (A)
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
MAX15058 toc06
OUTPUT VOLTAGE vs. OUTPUT CURRENT
1.89 1.87 OUTPUT VOLTAGE (V) 1.85 1.83 1.81 1.79 1.77
MAX15058 toc07
1.89 1.87 OUTPUT VOLTAGE (V) 1.85 1.83 1.81 1.79 1.77 1.75 2.7
VOUT = 3.3V
VOUT = 5V
IOUT = 0.5A
3.2 3.7 4.2 4.7 5.2
1.75
0
0.5
1.0
1.5
2.0
2.5
3.0
SUPPLY VOLTAGE (V)
OUTPUT CURRENT (A)
4
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
Typical Operating Characteristics (continued)
(VIN = 5V, VOUT = 1.8V, ILOAD = 3A, Circuit of Figure 5, TA = +25NC, unless otherwise noted.)
MAX15058
LOAD-TRANSIENT RESPONSE
MAX15058 toc08
SWITCHING WAVEFORMS (IOUT = 3A)
MAX15058 toc09a
50mV/div AC-COUPLED
VOUT 20mV/div AC-COUPLED ILX 1AV/div
1A/div 0A VLX 5V/div
100s/div
400ns/div
SWITCHING WAVEFORMS (IOUT = 3A)
MAX15058 toc09b
SWITCHING WAVEFORM IN SKIP MODE (IOUT = 10mA)
MAX15058 toc10
VOUT 20mV/div AC-COUPLED ILX 1AV/div
VOUT 50mV/div AC-COUPLED
ILX 1A/div 0A VIN = 3.3V 400ns/div 10s/div VLX 5V/div VLX 5V/div
INPUT AND OUTPUT WAVEFORMS (IOUT = 3A)
MAX15058 toc11
SHUTDOWN WAVEFORM
MAX15058 toc12
VENABLE 5V/div
INPUT 20mV/div AC-COUPLED OUTPUT 100mV/div AC-COUPLED
VOUT 1V/div ILX 1A/div VPGOOD 5V/div
400ns/div
10s/div
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5
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
Typical Operating Characteristics (continued)
(VIN = 5V, VOUT = 1.8V, ILOAD = 3A, Circuit of Figure 5, TA = +25NC, unless otherwise noted.)
SOFT-START WAVEFORMS (PWM MODE) (IOUT = 3A) SOFT-START WAVEFORMS (SKIP MODE) (IOUT = 3A)
VENABLE 5V/div VOUT 1V/div
MAX15058 toc13a
MAX15058 toc13b
VENABLE 5V/div VOUT 1V/div
ILX 1A/div VPGOOD 5V/div
ILX 1A/div VPGOOD 5V/div
200s/div
200s/div
SHUTDOWN CURRENT vs. INPUT VOLTAGE
90 SHUTDOWN CURRENT (nA) 80 70 60 50 40 30 20 10 0 2.7 3.2 3.7 4.2 4.7 5.2 INPUT VOLTAGE (V)
SHORT-CIRCUIT HICCUP MODE
MAX15058 toc15 MAX15058 toc14
100
VEN = 0V
IIN 500mA/div VOUT 200mV/div
IOUT 5A/div 200s/div
RMS INPUT CURRENT vs. INPUT VOLTAGE
MAX15058 toc16
FB VOLTAGE vs. TEMPERATURE
MAX15058 toc17
200 160 120 80 40 0
606 604 FEEDBACK VOLTAGE (V) 602 600 598 596
RMS INPUT CURRENT (mA)
SHORT CIRCUIT ON OUTPUT
2.8 3.1 3.4 3.7 4.0 4.3 4.6 4.9 5.2 5.5 INPUT VOLTAGE (V)
NO LOAD
594 -40 -20 0 20 40 60 80 AMBIENT TEMPERATURE (C)
6
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
Typical Operating Characteristics (continued)
(VIN = 5V, VOUT = 1.8V, ILOAD = 3A, Circuit of Figure 5, TA = +25NC, unless otherwise noted.)
SOFT-START WAVEFORMS (EXTERNAL REFIN) (PWM MODE)
MAX15058 toc18a
MAX15058
SOFT-START WAVEFORMS (EXTERNAL REFIN) (SKIP MODE)
MAX15058 toc18b
NO LOAD
VSS/REFIN 500mV/div VOUT 1V/div
NO LOAD
VSS/REFIN 500mV/div VOUT 1V/div
ILX 1A/div VPGOOD 5V/div
ILX 1A/div VPGOOD 5V/div
200s/div
200s/div
STARTING INTO A PREBIASED OUTPUT (IOUT = 2A)
STARTING INTO A PREBIASED OUTPUT (NO LOAD)
MAX15058 toc20a
MAX15058 toc19
VENABLE 5V/div VOUT 1V/div
VENABLE 5V/div VOUT 1V/div
ILX 1A/div VPGOOD 5V/div 200s/div
ILX 1A/div VPGOOD 5V/div 200s/div
PWM MODE
PWM MODE
STARTING INTO A PREBIASED OUTPUT (NO LOAD)
MAX15058 toc20b
STARTING INTO A PREBIASED OUTPUT HIGHER THAN SET OUTPUT
MAX15058 toc21
1.8V
VENABLE 5V/div VOUT 1V/div
VOUT 500mV/div
ILX 1A/div VPGOOD 5V/div 200s/div
IL 1A/div VSS/REFIN 500mV/div
SKIP MODE
10I LOAD AT OUT
400s/div
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7
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
Typical Operating Characteristics (continued)
(VIN = 5V, VOUT = 1.8V, ILOAD = 3A, Circuit of Figure 5, TA = +25NC, unless otherwise noted.)
CASE TEMPERATURE vs. AMBIENT TEMPERATURE
MAX15058 toc22
INPUT CURRENT IN SKIP MODE vs. OUTPUT VOLTAGE
4.5 4.0 INPUT CURRENT (mA) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0
80 CASE TEMPERATURE (C) 60 40 20 0 -20 -40 -40 -20 0 20 40 60 80 AMBIENT TEMPERATURE (C)
NO LOAD
VCC = 5.0V
VCC = 3.3V
1.2
1.7
2.2
2.7
3.2
OUTPUT VOLTAGE (V)
8
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MAX15058 toc23
100
5.0
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
Pin Configuration
TOP VIEW (BUMPS ON BOTTOM) MAX15058
GND A1 COMP B1 FB C1 LX A2 SKIP B2 SS/REFIN C2 IN A3 EN B3 PGOOD C3
MAX15058
WLP
Pin Description
BUMP A1 A2 A3 B1 B2 B3 C1 NAME GND LX IN COMP SKIP EN FB FUNCTION Analog Ground/Low-Side Switch Source Terminal. Connect to the PCB copper plane at one point near the input bypass capacitor return terminal. Inductor Connection. Connect LX to the switched side of the inductor. LX is high impedance when the IC is in shutdown mode. Input Power Supply. Input supply range is from 2.7V to 5.5V. Bypass with a minimum 10FF ceramic capacitor to GND. See Figures 5 and 6. Voltage Error-Amplifier Output. Connect the necessary compensation network from COMP to GND. See the Closing the Loop: Designing the Compensation Circuitry section. Skip-Mode Input. Connect to EN to select skip mode or leave unconnected for normal operation. Enable Input. EN is a digital input that turns the regulator on and off. Drive EN high to turn on the regulator. Connect to IN for always-on operation. Feedback Input. Connect FB to the center tap of an external resistor-divider from the output to GND to set the output voltage from 0.6V up to 94% of VIN. Soft-Start/External Voltage Reference Input. Connect a capacitor from SS/REFIN to GND to set the startup time. See the Setting the Soft-Start Time section for details on setting the soft-start time. Apply a voltage reference from 0V to VIN - 1.5V to drive soft-start externally. Open-Drain Power-Good Output. PGOOD goes high when FB is above 555mV and pulls low if FB is below 527mV.
C2
SS/REFIN
C3
PGOOD
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9
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
Block Diagram
SKIP EN IN BIAS GENERATOR EN LOGIC, IN UVLO THERMAL SHDN SHDN SKIP-MODE LOGIC SKPM HIGH-SIDE CURRENT LIMIT
VOLTAGE REFERENCE
LX CURRENT-SENSE AMPLIFIER LX IN IN
STRONG PREBIASED FORCED START 0.58V SS/REFIN SS/REFIN BUFFER 0.6V
SKPM CONTROL LOGIC CK IN LX
GND
10A PWM COMPARATOR
ERROR AMPLIFIER
LOW-SIDE SOURCE-SINK CURRENT LIMIT AND ZEROCROSSING COMPARATOR SINK SOURCE
FB COMP RAMP OSCILLATOR RAMP GEN CK
C
ZX
SKPM
MAX15058
PGOOD
POWER-GOOD COMPARATOR
0.555V RISING, 0.527V FALLING
10
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
Detailed Description
The MAX15058 high-efficiency, current-mode switching regulator can deliver up to 3A of output current. The MAX15058 provides output voltages from 0.6V to 0.94 x VIN from 2.7V to 5.5V input supplies, making the device ideal for on-board point-of-load applications. The MAX15058 delivers current-mode control architecture using a high-gain transconductance error amplifier. The current-mode control architecture facilitates easy compensation design and ensures cycle-by-cycle current limit with fast response to line and load transients. The MAX15058 features a 1MHz fixed switching frequency, allowing for all-ceramic capacitor designs and fast transient responses. The high operating frequency minimizes the size of external components. The MAX15058 is available in a 1.65mm x 1.65mm (3 x 3 array) x 0.5mm pitch WLP package. The MAX15058 offers a selectable skip-mode functionality to reduce current consumption and achieve a higher efficiency at light output loads. The low RDS(ON) integrated switches (30mI high-side and 18mI low-side, typ) ensure high efficiency at heavy loads while minimizing critical inductances, making the layout design a much simpler task with respect to discrete solutions. Utilizing a simple layout and footprint assures first-pass success in new designs. The MAX15058 features 1MHz Q15%, factory-trimmed, fixed-frequency PWM mode operation. The MAX15058 also offers capacitor-programmable, soft-start reducing inrush current, startup into PREBIAS operation, and a PGOOD open-drain output for sequencing with other devices. The controller logic block is the central processor that determines the duty cycle of the high-side MOSFET under different line, load, and temperature conditions. Under normal operation, where the current-limit and temperature protection are not triggered, the controller logic block takes the output from the PWM comparator and generates the driver signals for both high-side and low-side MOSFETs. The control logic block controls the break-before-make logic and all the necessary timing. limit is reached. The low-side MOSFET turns on for the remainder of the oscillation cycle. The MAX15058 can soft-start into a prebiased output without discharging the output capacitor. In safe prebiased startup, both low-side and high-side MOSFETs remain off to avoid discharging the prebiased output. PWM operation starts when the voltage on SS/REFIN crosses the voltage on FB. The MAX15058 can start into a prebiased voltage higher than the nominal set point without abruptly discharging the output. Forced PWM operation starts when the SS/ REFIN voltage reaches 0.58V (typ), forcing the converter to start. In case of prebiased output, below or above the output nominal set point, if low-side sink current-limit threshold (set to the reduced value of -0.4A (typ) for the first 32 clock cycles and then set to -5A (typ)) is reached, the low-side switch turns off before the end of the clock period, and the high-side switch turns on until one of the following conditions is satisfied: * High-sidesourcecurrenthitsthereducedhigh-side current limit (0.4A, typ); in this case, the high-side switch is turned off for the remaining time of the clock period. The clock period ends. Reduced high-side current limit is activated to recirculate the current into the high-side power switch rather than into the internal high-side body diode, which could be damaged. Low-side sink current limit is provided to protect the low-side switch from excessive reverse current during prebiased operation.
MAX15058
Starting into a Prebiased Output
*
Controller Function--PWM Logic
In skip mode operation, the prebias output needs to be lower than the set point. The MAX15058 features independent device enable control and power-good signal that allow for flexible power sequencing. Drive the enable input (EN) high to enable the regulator, or connect EN to IN for always-on operation. Power-good (PGOOD) is an open-drain output that asserts when VFB is above 555mV (typ), and deasserts low if VFB is below 527mV (typ). The MAX15058 utilizes a soft-start feature to slowly ramp up the regulated output voltage to reduce input inrush current during startup. Connect a capacitor from SS/ REFIN to GND to set the startup time (see the Setting the Soft-Start Time section for capacitor selection details).
Enable Input
The high-side MOSFET turns on at the beginning of the oscillator cycle and turns off when the COMP voltage crosses the internal current-mode ramp waveform, which is the sum of the slope compensation ramp and the current-mode ramp derived from inductor current (current-sense block). The high-side MOSFET also turns off if the maximum duty cycle is 94%, or when the current
Programmable Soft-Start (SS/REFIN)
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11
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
A high-gain error amplifier provides accuracy for the voltage-feedback loop regulation. Connect the necessary compensation network between COMP and GND (see the Compensation Design Guidelines section). The error-amplifier transconductance is 1.5mS (typ). COMP clamp low is set to 0.94V (typ), just below the slope ramp compensation valley, helping COMP to rapidly return to the correct set point during load and line transients. The PWM comparator compares COMP voltage to the current-derived ramp waveform (LX current to COMP voltage transconductance value is 18A/V typ). To avoid instability due to subharmonic oscillations when the duty cycle is around 50% or higher, a slope compensation ramp is added to the current-derived ramp waveform. Confirm the compensation ramp slope (0.3V x 1MHz = 0.3V/Fs) is equivalent to half the inductor current downslope in the worst case (load 3A, current ripple 30% and maximum duty-cycle operation of 94%). The slope compensation ramp valley is set to 1.15V (typ). When the converter output is shorted or the device is overloaded, each high-side MOSFET current-limit event (5A typ) turns off the high-side MOSFET and turns on the low-side MOSFET. On each current-limit event a 3-bit counter is incremented. The counter is reset after three consecutive high-side MOSFETs turn on without reaching current limit. If the current-limit condition persists, the counter fills up reaching eight events. The control logic then discharges SS/REFIN, stops both high-side and low-side MOSFETs, and waits for a hiccup period (1024 clock cycles typ) before attempting a new softstart sequence. The hiccup mode is also enabled during soft-start time. The MAX15058 contains an internal thermal sensor that limits the total power dissipation to protect the device in the event of an extended thermal fault condition. When the die temperature exceeds +150NC (typ), the thermal sensor shuts down the device, turning off the DC-DC converter to allow the die to cool. After the die temperature falls by 20NC (typ), the device restarts, following the soft-start sequence. The MAX15058 operates in skip mode when SKIP is connected to EN. When in skip mode, LX output becomes high impedance when the inductor current falls below
Error Amplifier
PWM Comparator
200mA (typ). The inductor current does not become negative. If during a clock cycle the inductor current falls below the 200mA threshold (during off-time), the low side turns off. At the next clock cycle, if the output voltage is above set point, the PWM logic keeps both high-side and low-side MOSFETs off. If instead the output voltage is below the set point, the PWM logic drives the highside on for a minimum fixed on-time (300ns typ). In this way the system can skip cycles, reducing frequency of operations, and switches only as needed to service load at the cost of an increase in output voltage ripple (see the Skip Mode Frequency and Output Ripple section). In skip mode, power dissipation is reduced and efficiency is improved at light loads because power MOSFETs do not switch at every clock cycle.
Applications Information
The MAX15058 output voltage is adjustable from 0.6V up to 94% of VIN by connecting FB to the center tap of a resistor-divider between the output and GND (Figure 1). Choose R1 and R2 so that the DC errors due to the FB input bias current (Q500nA) do not affect the output voltage accuracy. With lower value resistors, the DC error is reduced, but the amount of power consumed in the resistor-divider increases. A typical value for R2 is 10kI, but values between 5kI and 50kI are acceptable. Once R2 is chosen, calculate R1 using: V R1 = R2 x OUT - 1 VFB where the feedback threshold voltage, VFB = 0.6V (typ). When regulating for an output of 0.6V in skip mode, short FB to OUT and keep R2 connected from FB to GND. A high-valued inductor results in reduced inductor ripple current, leading to a reduced output ripple voltage. However, a high-valued inductor results in either a larger physical size or a high series resistance (DCR) and a lower saturation current rating. Typically, choose an inductor value to produce a current ripple equal to 30% of load current. Choose the inductor with the following formula: V VOUT L= x 1 - OUT fSW x LIR x ILOAD VIN where fSW is the internally fixed 1MHz switching frequency, and LIR is the desired inductor current ratio (typically
Setting the Output Voltage
Overcurrent Protection and Hiccup
Thermal-Shutdown Protection
Inductor Selection
Skip Mode Operation
12
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
FEEDBACK DIVIDER ERROR AMPLIFIER POWER MODULATOR SLOPE COMPENSATION RAMP VOUT OUTPUT FILTER AND LOAD VIN
C
FB R1 *CFF VFB R2 COMP VCOMP RC CC COMPARATOR
gMC
QHS PWM CONTROL LOGIC QLS
IL L DCR IOUT
VOUT
ESR COUT
RLOAD
gMV
ROUT
VCOMP
GMOD
IOUT
REF
ROUT = 10AVEA(dB)/20/gMV
*NOTE: CFF IS OPTIONAL AND DESIGNED TO EXTEND THE REGULATOR'S GAIN BANDWIDTH AND INCREASED PHASE MARGIN FOR SOME LOW-DUTY CYCLE APPLICATIONS.
NOTE: THE GMOD STAGE SHOWN ABOVE MODELS THE AVERAGE CURRENT OF THE INDUCTOR, IL, INJECTED INTO THE OUTPUT LOAD, IOUT, e.g., IL = IOUT. THIS CAN BE USED TO SIMPLIFY/MODEL THE MODULATION/CONTROL/POWER STATE CIRCUITRY SHOWN WITHIN THE BOXED AREA.
Figure 1. Peak Current-Mode Regulator Transfer Model
set to 0.3). In addition, the peak inductor current, IL_PK, must always be below the minimum high-side currentlimit value, IHSCL, and the inductor saturation current rating, IL_SAT. Ensure that the following relationship is satisfied: 1 IL_PK = ILOAD + IL < min IHSCL_, IL_SAT 2
(
)
fSW is the switching frequency (1MHz), and ILOAD is the output load. The impedance of the input capacitor at the switching frequency should be less than that of the input source so high-frequency switching currents do not pass through the input source, but are instead shunted through the input capacitor. The input capacitor must meet the ripple current requirement imposed by the switching currents. The RMS input ripple current is given by:
V OUT x (VIN - VOUT ) IRIPPLE = ILOAD VIN
The input capacitor reduces the peak current drawn from the input power supply and reduces switching noise in the device. The total input capacitance must be equal to or greater than the value given by the following equation to keep the input ripple voltage within the specification and minimize the high-frequency ripple current being fed back to the input source: CIN = ILOAD V x OUT fSW x VIN_RIPPLE VIN
Input Capacitor Selection
where IRIPPLE is the input RMS ripple current. The key selection parameters for the output capacitor are capacitance, ESR, ESL, and voltage rating. The parameters affect the overall stability, output ripple voltage, and transient response of the DC-DC converter. The output ripple occurs due to variations in the charge stored in the output capacitor, the voltage drop due to the capacitor's ESR, and the voltage drop due to the
Output Capacitor Selection
where DVIN_RIPPLE is the maximum-allowed input ripple voltage across the input capacitors and is recommended to be less than 2% of the minimum input voltage,
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13
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
capacitor's ESL. Estimate the output-voltage ripple due to the output capacitance, ESR, and ESL as follows:
VOUT = VOUT VOUT 1 x 1 - x R ESR_COUT + fSW x L VIN 8 x fSW x C OUT
the inductor and output capacitor values. After a short time, the controller responds by regulating the output voltage back to the predetermined value. Use higher COUT values for applications that require light load operation or transition between heavy load and light load, triggering skip mode, causing output undershooting or overshooting. When applying the load, limit the output undershoot by sizing COUT according to the following formula: C OUT ILOAD 3fCO x VOUT
For ceramic capacitors, ESR contribution is negligible: R ESR_OUT << 1 8 x fSW x C OUT
For tantalum or electrolytic capacitors, ESR contribution is dominant: R ESR_OUT >> 1 8 x fSW x C OUT
Use these equations for initial output-capacitor selection. Determine final values by testing a prototype or an evaluation circuit. A smaller ripple current results in less output-voltage ripple. Since the inductor ripple current is a factor of the inductor value, the output-voltage ripple decreases with larger inductance. Use ceramic capacitors for low ESR and low ESL at the switching frequency of the converter. The ripple voltage due to ESL is negligible when using ceramic capacitors. Load-transient response also depends on the selected output capacitance. During a load transient, the output instantly changes by ESR x DILOAD. Before the controller can respond, the output deviates further, depending on
where DILOAD is the total load change, fCO is the regulator unity-gain bandwidth (or zero crossover frequency), and DVOUT is the desired output undershooting. When removing the load and entering skip mode, the device cannot control output overshooting, since it has no sink current capability; see the Skip Mode Frequency and Output Ripple section to properly size COUT. In skip mode, the switching frequency (fSKIP) and output ripple voltage (VOUT-RIPPLE) shown in Figure 2 are calculated as follows: tON is a fixed time (300ns, typ); the peak inductor current reached is: V - VOUT I SKIP -LIMIT = IN x t ON L
Skip Mode Frequency and Output Ripple
IL ISKIP-LIMIT
ILOAD tON tOFF1 tOFF2 = n x tCK
VOUT
VOUT-RIPPLE
Figure 2. Skip Mode Waveform
14
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
tOFF1 is the time needed for inductor current to reach the zero-current crossing limit (~0A): t OFF1 = L x I SKIP -LIMIT VOUT the inductor's pole frequency is shifted beyond the gain bandwidth of the regulator. System stability is provided with the addition of a simple series capacitor-resistor from COMP to GND. This pole-zero combination serves to tailor the desired response of the closed-loop system. The basic regulator loop consists of a power modulator (comprising the regulator's pulse-width modulator, current sense and slope compensation ramps, control circuitry, MOSFETs, and inductor), the capacitive output filter and load, an output feedback divider, and a voltage-loop error amplifier with its associated compensation circuitry. See Figure 1. The average current through the inductor is expressed as: IL = G MOD x VCOMP where IL is the average inductor current and GMOD is the power modulator's transconductance. For a buck converter: VOUT = R LOAD x IL where RLOAD is the equivalent load resistor value. Combining the above two relationships, the power modulator's transfer function in terms of VOUT with respect to VCOMP is: R xI = LOAD L = RLOAD x G MOD VCOMP IL GMOD VOUT The peak current-mode controller's modulator gain is attenuated by the equivalent divider ratio of the load resistance and the current-loop gain's impedance. GMOD becomes: 1 G MOD (DC) = g MC x RLOAD x K x (1 - D) - 0.5 1 + fSW x L S where RLOAD = VOUT/IOUT(MAX), fSW is the switching frequency, L is the output inductance, D is the duty cycle (VOUT/VIN), and KS is a slope compensation factor calculated from the following equation: S V xf x L x g MC K S = 1 + SLOPE = 1 + SLOPE SW SN (VIN - VOUT ) where: V S SLOPE = SLOPE = VSLOPE x fSW t SW SN =
MAX15058
During tON and tOFF1, the output capacitor stores a charge equal to (see Figure 2):
1 1 2 + L x (I SKIP -LIMIT - ILOAD ) x VIN - VOUT VOUT Q OUT = 2
During tOFF2 (= n x tCK, number of clock cycles skipped), output capacitor loses this charge:
Q OUT t OFF2 = ILOAD 1 1 2 L x (I SKIP - LIMIT - ILOAD ) x + VIN - VOUT VOUT t OFF2 = 2 xILOAD
Finally, frequency in skip mode is: fSKIP = 1 t ON + t OFF1 + t OFF2
Output ripple in skip mode is: VOUT -RIPPLE = VCOUT -RIPPLE + VESR-RIPPLE -I (I ) x t ON = SKIP -LIMIT LOAD C OUT
+ R ESR,COUT x (I SKIP -LIMIT - ILOAD ) L x I SKIP -LIMIT VOUT -RIPPLE = + RESR,COUT C OUT x (VIN - VOUT ) x (I SKIP -LIMIT - ILOAD )
To limit output ripple in skip mode, size COUT based on the above formula. All the above calculations are applicable only in skip mode. The MAX15058 uses a fixed-frequency, peak-current-mode control scheme to provide easy compensation and fast transient response. The inductor peak current is monitored on a cycle-by-cycle basis and compared to the COMP voltage (output of the voltage error amplifier). The regulator's duty cycle is modulated based on the inductor's peak current value. This cycle-by-cycle control of the inductor current emulates a controlled current source. As a result,
Compensation Design Guidelines
(VIN - VOUT )
L x g MC
15
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
1ST ASYMPTOTE R2 x (R1 + R2)-1 x 10AVEA(dB)/20 x gMC x RLOAD x {1 + RLOAD x [KS x (1 - D) - 0.5] x (L x fSW)-1}-1 GAIN 2ND ASYMPTOTE R2 x (R1 + R2)-1 x gMV x (2GCC)-1 x gMC x RLOAD x {1 + RLOAD x [KS x (1 - D) - 0.5] x (L x fSW)-1}-1 3RD ASYMPTOTE R2 x (R1 + R2)-1 x gMV x (2GCC)-1 x gMC x RLOAD x {1 + RLOAD x [KS x (1 - D) - 0.5] x (L x fSW)-1}-1 x (2GCOUT x {RLOAD-1 + [KS x (1 - D) - 0.5] x (L x fSW)-1}-1)-1 4TH ASYMPTOTE R2 x (R1 + R2)-1 x gMV x RC x gMC x RLOAD x {1 + RLOAD x [KS x (1 - D) - 0.5] x (L x fSW)-1}-1 x (2GCOUT x {RLOAD-1 + [KS x (1 - D) - 0.5] x (L x fSW)-1}-1)-1 3RD POLE (DBL) 2ND ZERO 0.5 x fSW (2GCOUTESR)-1 fCO FREQUENCY
UNITY 1ST POLE [2GCC x (10AVEA(dB)/20 - gMV-1)]-1 2ND POLE fPMOD* 1ST ZERO (2GCCRC)-1
5TH ASYMPTOTE R2 x (R1 + R2)-1 x gMV x RC x gMC x RLOAD x {1 + RLOAD x [KS x (1 - D) - 0.5] x (L x fSW)-1}-1 x (2GCOUT x {RLOAD-1 + [KS x (1 - D) - 0.5] x (L x fSW)-1}-1)-1 x (0.5 x fSW)2 x (2Gf)-2
NOTE: ROUT = 10AVEA(dB)/20 x gMV-1 fPMOD = [2GCOUT x (ESR + {RLOAD-1 + [KS x (1 - D) - 0.5] x (L x fSW)-1}-1)]-1 WHICH FOR ESR << {RLOAD-1 + [KS x (1 - D) - 0.5] x (L x fSW)-1}-1 BECOMES fPMOD = [2GCOUT x {RLOAD-1 + [KS x (1 - D) - 0.5] x (L x fSW)-1}-1]-1 fPMOD = (2GCOUT x RLOAD)-1 + [KS x (1 - D) - 0.5] x (2GCOUT x L x fSW)-1 6TH ASYMPTOTE R2 x (R1 + R2)-1 x gMV x RC x gMC x RLOAD x {1 + RLOAD x [KS x (1 - D) - 0.5] x (L x fSW)-1}-1 x ESR x {RLOAD-1 + [KS x (1 - D) - 0.5] x (L x fSW)-1}-1 x (0.5 x fSW)2 x (2Gf)-2
Figure 3. Asymptotic Loop Response of Current-Mode Regulator
As previously mentioned, the power modulator's dominant pole is a function of the parallel effects of the load resistance and the current-loop gain's equivalent impedance:
fPMOD = 1 1 K S x (1 - D) - 0.5 + 2 x C OUT x ESR + RLOAD fSW x L
-1
which can be expressed as: fPMOD K S x (1 - D) - 0.5 + 2 x C OUT x R LOAD 2 x fSW x L x C OUT 1

And knowing that the ESR is typically much smaller than the parallel combination of the load and the current loop: 1 K S x (1 - D) - 0.5 ESR << + R LOAD fSW x L fPMOD 1 1 K S x (1 - D) - 0.5 + 2 x C OUT x RLOAD fSW x L
-1 -1
Note: Depending on the application's specifics, the amplitude of the slope compensation ramp could have a significant impact on the modulator's dominate pole. For low duty-cycle applications, it provides additional damping (phase lag) at/near the crossover frequency (see the Closing the Loop: Designing the Compensation Circuitry section). There is no equivalent effect on the power modulator zero, fZMOD. fZMOD = fZESR = 1 2 x C OUT x ESR
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
The effect of the inner current loop at higher frequencies is modeled as a double-pole (complex conjugate) frequency term, GSAMPLING(s), as shown: G SAMPLING (s) = 1 s2 s x fSW x Q C The dominant poles and zeros of the transfer loop gain are shown below: fP1 = fP2 = gMV AVEA(dB)/20 x C 2 x 10 1 K S x (1-D) -0.5 -1 1 + 2 x C OUT fSW x L R LOAD 1 (fSW ) 2 1 fZ1 = 2 x C CR C fP3 = fZ2 = 1 2 x C OUTESR
MAX15058
C
( x fSW ) 2
QC =
+
+1
where the sampling effect quality factor, QC, is: 1 x K S x (1 - D) - 0.5
And the resonant frequency is: SAMPLING(s) = x fSW or: f fSAMPLING = SW 2 Having defined the power modulator's transfer function, the total system transfer can be written as follows (see Figure 3): Gain(s) = GFF(s) x GEA(s) x GMOD(DC) x GFILTER(s) x GSAMPLING(s) where: G FF (s) =
The order of pole-zero occurrence is: fP1 < fP2 fZ1 < fCO fP3 < fZ2 Under heavy load, fP2, approaches fZ1. Figure 3 shows a graphical representation of the asymptotic system closed-loop response, including dominant pole and zero locations. The loop response's fourth asymptote (in bold, Figure 3) is the one of interest in establishing the desired crossover frequency (and determining the compensation component values). A lower crossover frequency provides for stable closed-loop operation at the expense of a slower load- and line-transient response. Increasing the crossover frequency improves the transient response at the (potential) cost of system instability. A standard rule of thumb sets the crossover frequency between 1/10 and 1/5 of the switching frequency. First, select the passive power and decoupling components that meet the application's requirements. Then, choose the small-signal compensation components to achieve the desired closed-loop frequency response and phase margin as outlined in the Closing the Loop: Designing the Compensation Circuitry section.
(sC FFR1 + 1) R2 x R1 + R2 sC FF (R1|| R2) + 1
Leaving CFF empty, GFF(s) becomes: R2 G FF (s) = R1 + R2 Also:
GEA (s) = 10 AVEA(dB)/20 x
(sC CR C + 1)
10 AVEA(dB)/20 sC C R C + + 1 g MV
which simplifies to: GEA (s) = 10 AVEA(dB)/20 x
(sC CR C + 1)
10 AVEA(dB)/20 sC C + 1 g MV
when R C <<
10 AVEA(dB)/20 g MV
1) Select the desired crossover frequency. Choose fCO approximately 1/10 to 1/5 of the switching frequency (fSW).
Closing the Loop: Designing the Compensation Circuitry
G FILTER (s) = R LOAD x
(sC OUTESR + 1)
-1 K S x (1 - D) - 0.5 1 sC + 1 + OUT R fSW x L LOAD
2) Determine RC by setting the system transfer's fourth asymptote gain equal to unity (assuming fCO > fZ1, fP2, and fP1) where:
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17
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
R LOADK S (1 - D) - 0.5 1 + L x fSW R1 + R2 x 2f C RC = x CO OUT x R2 g MV x g MC x R LOAD 1 ESR + 1 K S (1 - D) - 0.5 + R LOAD L x fSW
Using CFF the zero-pole order is adjusted as follows:
fP1 < fP2 fZ1 < 1 1 < 2C FFR1 2C FF (R1|| R2)
fCO < fP3 < fZ2
Confirm the desired operation of CFF empirically. The phase lead of CFF diminishes as the output voltage is a smaller multiple of the reference voltage, e.g., below about 1V. Do not use CFF when VOUT = VFB. The soft-start feature ramps up the output voltage slowly, reducing input inrush current during startup. Size the CSS capacitor to achieve the desired soft-start time, tSS, using: I xt C SS = SS SS VFB ISS, the soft-start current, is 10FA (typ) and VFB, the output feedback voltage threshold, is 0.6V (typ). When using large COUT capacitance values, the high-side current limit can trigger during the soft-start period. To ensure the correct soft-start time, tSS, choose CSS large enough to satisfy: C SS >> C OUT x VOUT x I SS (IHSCL_ - IOUT ) x VFB
and where the ESR is much smaller than the parallel combination of the equivalent load resistance and the current loop impedance, e.g.,: ESR << 1 1 K S (1 - D) - 0.5 + R LOAD L x fSW R1 + R2 2fCO x C OUT x R2 g MV x g MC
Setting the Soft-Start Time
RC becomes: RC =
3) Determine CC by selecting the desired first system zero, fZ1, based on the desired phase margin. Typically, setting fZ1 below 1/5 of fCO provides sufficient phase margin. fZ1 = therefore: CC 5 2 x fCO x R C f 1 CO 2 x C CR C 5
IHSCL_ is the typical high-side MOSFET current-limit value. An external tracking reference with steady-state value between 0V and VIN - 1.8V can be applied to SS/REFIN. In this case, connect an RC network from external tracking reference and SS/REFIN, as shown in Figure 4. The recommended value for RSS is approximately 1kI. RSS is needed to ensure that, during hiccup period, SS/ REFIN can be internally pulled down. When an external reference is connected to SS/REFIN, the soft-start must be provided externally.
4) For low duty-cycle applications, the addition of a phase-leading capacitor (CFF in Figure 1) helps mitigate the phase lag of the damped half-frequency double pole. Adding a second zero near to but below the desired crossover frequency increases both the closed-loop phase margin and the regulator's unitygain bandwidth (crossover frequency). Select the capacitor as follows: 1 C FF = 2 x fCO x (R1|| R2) This guarantees the additional phase-leading zero occurs at a frequency lower than fCO from: 1 fPHASE_LEAD = 2 x C FF x R1
18
VREF_EXT
RSS SS/REFIN CSS
MAX15058
Figure 4. RC Network for External Reference at SS/REFIN
_____________________________________________________________________________________
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
INPUT 2.8V TO 5.5V CIN 22F RPULL 20kI IN LX 1.2I LOUT (ICE IN06142) 1H
OUTPUT 1.8V AT 3A
MAX15058
PGOOD ON GND FB COMP
COUT 22F x 2
1nF
CFF 100pF
R1 8.06kI
ENABLE OFF
EN SKIP SS/REFIN
R2 4.02kI RC 5.36kI CC 1nF
CSS 22nF
Figure 5. Application Circuit for PWM Mode Operation
The MAX15058 is available in a 9-bump WLP package and can dissipate up to 1127mW at TA = +70NC. When the die temperature exceeds +150NC, the thermal-shutdown protection is activated (see the Thermal-Shutdown Protection section). Careful PCB layout is critical to achieve clean and stable operation. It is highly recommended to duplicate the MAX15058 Evaluation Kit layout for optimum performance. If deviation is necessary, follow these guidelines for good PCB layout: 1) Connect the signal and ground planes at a single point immediately adjacent to the GND bump of the IC.
Power Dissipation
2) Place capacitors on IN and SS/REFIN as close as possible to the IC and the corresponding pad using direct traces. 3) Keep the high-current paths as short and wide as possible. Keep the path of switching current short and minimize the loop area formed by LX, the output capacitors, and the input capacitors. 4) Connect IN, LX, and GND separately to a large copper area to help cool the IC to further improve efficiency. 5) Ensure all feedback connections are short and direct. Place the feedback resistors and compensation components as close as possible to the IC. 6) Route high-speed switching nodes (such as LX) away from sensitive analog areas (such as FB and COMP).
Layout Procedure
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19
High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator MAX15058
INPUT 2.8V TO 5.5V CIN 22F RPULL 20kI IN LX 1.2I COUT 22F x 2 LOUT (ICE IN06142) 1H
OUTPUT 1.8V AT 3A
MAX15058
PGOOD ON GND FB COMP
1nF
CFF 100pF
R1 8.06kI
ENABLE OFF
EN SKIP SS/REFIN
R2 4.02kI RC 5.36kI CC 1nF
CSS 22nF
Figure 6. Application Circuit for Skip Mode Operation
Chip Information
PROCESS: BiCMOS
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.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 TYPE 9 WLP PACKAGE CODE W91B1+1 OUTLINE NO. 21-0067 LAND PATTERN NO. Refer to Application Note 1891
20
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High-Efficiency, 3A, Current-Mode Synchronous, Step-Down Switching Regulator
Revision History
REVISION NUMBER 0 REVISION DATE 12/10 Initial release DESCRIPTION PAGES CHANGED --
MAX15058
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
(c)
21
2010 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.

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