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19-1142 Rev 0; 9/96
NUAL KIT MA UATION BLE EVAL AVAILA
100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter
____________________________Features
o 95% Efficiency o 600mA Output Current o Cycle-by-Cycle Current Limiting o Low-Dropout, 100% Duty-Cycle Operation, 300mV at 500mA o Internal 0.6 (typ) MOSFET o Internal Synchronous Rectifier o High-Frequency Current-Mode PWM o External SYNC or Internal 300kHz Oscillator o Guaranteed 260kHz to 340kHz Internal Oscillator Frequency Limits o 2.5A Shutdown Mode
_______________General Description
The MAX887 high-efficiency, step-down DC-DC converter provides an adjustable output from 1.25V to 10.5V. It accepts inputs from 3.5V to 11V and delivers 600mA. Operation to 100% duty cycle minimizes dropout voltage (300mV typ at 500mA). Synchronous rectification reduces output rectifier losses, resulting in efficiency as high as 95%. Fixed-frequency pulse-width modulation (PWM) reduces noise in sensitive communications applications. Using a high-frequency internal oscillator allows tiny surface-mount components to reduce PC board area, and eliminates audio-frequency interference. A SYNC input allows synchronization to an external clock to avoid interference with sensitive RF and dataacquisition circuits. The MAX887 features current-mode operation for superior load/line-transient response. Cycle-by-cycle current limiting protects the internal MOSFET and rectifier. A low-current (2.5A typ) shutdown mode conserves battery life.
MAX887
________________________Applications
Portable Instruments Cellular Phones and Radios Personal Communicators Distributed Power Systems Computer Peripherals
______________Ordering Information
PART MAX887HC/D MAX887HESA TEMP. RANGE 0C to +70C -40C to +85C PIN-PACKAGE Dice* 8 SO
*Contact factory for availability. Dice are tested at TA = +25C.
__________Typical Operating Circuit
VIN = 3.5V to 11V V+ 47F ON 0.33F LX R1 165k 47F 33H VOUT = 3.3V
__________________Pin Configuration
TOP VIEW
SHDN 1 FB 2 REF 3 8 V+ LX SYNC GND
MAX887
SHDN SYNC VL REF
C1 100pF
OFF
MAX887
7 6 5
FB R2 100k
VL 4
GND
2.2F
0.047F VOUT = 1.25V (R1/R2 + 1)
SO
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter MAX887
ABSOLUTE MAXIMUM RATINGS
REF, FB, SYNC, VL to GND..................................... -0.3V to +6V V+ to GND ............................................................. -0.3V to +12V SHDN, LX to GND ....................................... -0.3V to (V+ + 0.3V) PGND to GND ...................................................... -0.3V to +0.3V Continuous Power Dissipation (TA = +70C) SO (derate 9.09mW/C above +70C) .........................471mW Operating Temperature Ranges MAX887HC/D.......................................................0C to +70C MAX887HESA ...................................................-40C to +85C Storage Temperature Range ........................... -65C to +165C Lead Temperature (soldering, 10sec) ............................ +300C
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.
ELECTRICAL CHARACTERISTICS
(V+ = +7V, PGND = GND = 0V, SHDN = V+, (TA = 0C to TMAX), unless otherwise noted.) PARAMETER Supply Range Quiescent Supply Current (PWM Mode) Quiescent Supply Current (PFM Mode) Shutdown Supply Current Output Voltage Range Load Regulation Line Regulation PWM FB Feedback Threshold FB Input Current SYNC Frequency SYNC Pulse Width High or Low PWM Maximum Duty Cycle PWM Switching Frequency High-Side Current Limit LX On-Resistance LX Leakage Current LX Reverse Leakage Current, Regulator Off Undervoltage Lockout Startup Voltage SYNC Input High Voltage SYNC Input Low Voltage SYNC Input Current SHDN Input High Voltage SHDN Input Low Voltage SHDN Input Current, Sinking SHDN Input Capacitance VL Output Voltage REF Output Voltage VFB IFB fSYNC SYNC, PW PWM, DUTY fOSC ILIM+ RON, LX ILXLKG ILXLKGR V+, UVLO V+, START VIH, SYNC VIL, SYNC IIN, SYNC VIH, SHDN VIL, SHDN IIN-, SHDN CIN, SHDN VL VREF SHDN = GND or V+ (Note 1) IVL = 0mA to 1mA 0A to 30A 3.3 1.25 SYNC = GND or 3V 2.4 0.8 1 10 ILX = 100mA V+ = 12V, LX = GND to 12V V+ = floating, LX = 5V, SHDN = GND V+ falling V+ rising 2.5 0.5 1 -10 SYNC = 3.0V, FB = 1.18V SYNC = 3.0V SYMBOL V+ IV+, PWM IV+, PFM IV+, SHDN VOUT, RANGE IOUT = 0mA, SYNC = 3.0V IOUT = 0mA, SYNC = GND SHDN = GND Circuit of Figure 2 IOUT = 0mA to 500mA VIN = 4V to 11V, PWM mode SYNC = 3.0V, PWM duty cycle = 50% FB = 1.30V 25 500 100 260 0.75 300 1.0 0.6 1.0 1.0 3.0 3.1 10 20 3.3 3.5 340 1.40 1.225 1.25 0.005 0.2 1.250 1.275 0.10 440 CONDITIONS MIN 3.5 2.7 0.2 2.5 TYP MAX 11.0 4.0 0.5 5 10.50 UNITS V mA mA A V %/mA %/V V A kHz ns % kHz A A A V V V V A V V A pF V V
Note 1: Guaranteed by design and not production tested. 2 _______________________________________________________________________________________
100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter
ELECTRICAL CHARACTERISTICS
(V+ = +7V, PGND = GND = 0V, SHDN = V+, (TA = -40C to +85C), unless otherwise noted.) (Note 2) PARAMETER Supply Range Quiescent Supply Current (PWM Mode) Quiescent Supply Current (PFM Mode) Shutdown Supply Current Output Voltage Range PWM FB Feedback Threshold FB Input Current PWM Switching Frequency High-Side Current Limit Undervoltage Lockout Startup Voltage SYMBOL V+ IV+, PWM IV+, PFM IV+, SHDN VOUT, RANGE VFB IFB fOSC ILIM+ V+, UVLO V+, START V+ falling V+ rising IOUT = 0mA, SYNC = 3.0V IOUT = 0mA, SYNC = GND SHDN = GND Circuit of Figure 2 SYNC = 3.0V, PWM duty cycle = 50% FB = 1.30V SYNC = 3.0V 250 0.75 300 1.00 3.0 3.1 1.25 1.222 1.250 CONDITIONS MIN 3.5 2.7 0.2 2.5 TYP MAX 11.0 4.0 0.6 5 10.50 1.278 0.10 350 1.50 3.3 3.5 UNITS V mA mA A V V A kHz A V V
MAX887
Note 2: Specifications from 0C to -40C are guaranteed by design and not production tested.
__________________________________________Typical Operating Characteristics
(Circuit of Figure 2, TA = +25C, unless otherwise noted.)
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX887-01
EFFICIENCY vs. OUTPUT CURRENT
MAX887-02
EFFICIENCY vs. OUTPUT CURRENT
90 80 PWM MODE (SYNC = VL) VOUT = 5V VIN = 7V VIN = 9V
MAX887-03
0.7 0.6 DROPOUT VOLTAGE (V) 0.5 0.4 0.3 0.2 0.1 0 0 200 400 600 800 LOAD CURRENT (mA) 5V SETTING VOUT = 4.75V 3.3V SETTING VOUT = 3.135V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 PWM MODE (SYNC = VL) VOUT = 3.3V VIN = 4V VIN = 5V VIN = 7V VIN = 11V VIN = 9V
100
EFFICIENCY (%)
70 60 50 40 30 20 10 VIN = 5.5V
VIN = 11V
1000
0.0001
0.001 0.01 0.1 OUTPUT CURRENT (A)
0.6
0 0.0001
0.001
0.01
0.1
0.6
OUTPUT CURRENT (A)
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100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter MAX887
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
MAX887-04
EFFICIENCY vs. OUTPUT CURRENT
MAX887-05
MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE
MAXIMUM OUTPUT CURRENT (mA) 3.3V SETTING, VOUT = 3.135V 1000 800 600 400 200 0 GUARANTEED OUTPUT CURRENT OF FIGURE 2 IS 600mA 3 5 7 9 11 13 15 17 5V SETTING, VOUT = 4.75V
MAX887-06
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.0001 0.001 0.01 0.1 VIN = 9V IDLE MODE (SYNC = GND) VOUT = 3.3V VIN = 4V VIN = 5V VIN = 11V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10
VIN = 5.5V VIN = 11V
1200
VIN = 9V
VIN = 7V IDLE MODE (SYNC = GND) VOUT = 5V 0.001 0.01 0.1 0.6
VIN = 7V
0.6
0 0.0001
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
SUPPLY VOLTAGE (V)
MAXIMUM OUTPUT CURRENT vs. SYNC FREQUENCY
MAX887-07
QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX887-08
QUIESCENT CURRENT vs.TEMPERATURE
VIN = 5.3V VOUT = 3.3V PWM MODE 2.0 1.5 1.0 0.5 PFM MODE 0 -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
MAX887-09
1200 MAXIMUM OUTPUT CURRENT (mA) 1000 800 600 400 200 0 0 100 200 300 400 500 SYNC FREQUENCY (kHz) C2, C3 = 47F L1 = 33H VIN = 5V VOUT = 3.3V VOUT = -5% at IOUT(MAX)
3.5 A 3.0 QUIESCENT CURRENT (mA) 2.5 2.0 1.5 1.0 0.5 0 C B 0 2 4 A: VOUT = 3.3V, PWM MODE B: VOUT = 3.3V, PFM MODE A
3.0 2.5
600
6 8 10 12 14 16 18 SUPPLY VOLTAGE (V)
SWITCHING FREQUENCY vs. SUPPLY VOLTAGE
MAX887-10
SWITCHING FREQUENCY vs. TEMPERATURE
MAX887-11
QUIESCENT CURRENT (mA)
OUTPUT RIPPLE AND HARMONICS
VIN = 5V VOUT = 3.3V IOUT = 500mA PWM MODE
MAX887 TOC-20
350 340 SWITCHING FREQUENCY (kHz) 330 VOUT = 3.3V
350 340 330 FREQUENCY (kHz) 320 310 300 290 280 270 260 250 VOUT = 3.3V
4
3 OUTPUT NOISE (mV) -60 -40 -20 0 20 40 60 80 100 120 140 TEMPERATURE (C)
320 310 300 290 280 270 260 250 0 2 4 6 8 10 12 14 16 SUPPLY VOLTAGE (V) 18
2
1
0
-1 10k 100k 1M 10M FREQUENCY (Hz)
4
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100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25C, unless otherwise noted.)
MAX887
HEAVY-LOAD, PWM-MODE SWITCHING WAVEFORMS
MAX887-12
LIGHT-LOAD, PWM-MODE SWITCHING WAVEFORMS
MAX887-13
A
A
B C 0mA 1s/div VIN = 5V, VOUT = 3.3V, LOAD = 500mA A: LX, 5V/div B: VOUT, 20mV/div, AC COUPLED C: INDUCTOR CURRENT, 500mA/div
B
C 1s/div VIN = 5V, VOUT = 3.3V, LOAD = 0mA A: LX, 5V/div B: VOUT, 20mV/div, AC COUPLED C: INDUCTOR CURRENT, 500mA/div
LIGHT-LOAD, PFM-MODE SWITCHING WAVEFORMS
MAX887-14
MEDIUM-LOAD, PFM-MODE SWITCHING WAVEFORMS
MAX887-15
A
A
B
B
C 1s/div VIN = 5V, VOUT = 3.3V, LOAD = 0mA A: LX, 5V/div B: VOUT, 20mV/div, AC COUPLED C: INDUCTOR CURRENT, 200mA/div
C 10s/div VIN = 5V, VOUT = 3.3V, LOAD = 70mA A: LX, 5V/div B: VOUT, 20mV/div, AC COUPLED C: INDUCTOR CURRENT, 200mA/div
_______________________________________________________________________________________
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100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter MAX887
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25C, unless otherwise noted.)
LOAD-TRANSIENT RESPONSE
MAX887-16
LINE-TRANSIENT RESPONSE
MAX887-17
A
A
B B C 40s/div VIN = 5V, VOUT = 3.3V, LOAD = 0mA TO 500mA, PWM MODE A: LX, 5V/div B: VOUT, 50mV/div, AC COUPLED C: LOAD CURRENT, 500mA/div 200s/div VIN = 5V TO 11V, VOUT = 3.3V, LOAD = 500mA, PWM MODE A: VIN, 5V/div B: VOUT, 20mV/div, AC COUPLED
RECOVERY FROM 100% DUTY CYCLE (DROP OUT)
MAX887-18
SHUTDOWN AND STARTUP RESPONSE
A
MAX887-19
A B C B
C
D 500s/div VIN = 5V, VOUT = 3.3V, LOAD = 100mA, PWM MODE A: SHDN, 5V/div B: VOUT, 2V/div, AC COUPLED C: LX, 5V/div D: INDUCTOR CURRENT, 500mA/div
200s/div VIN = 3.3V TO 11V, VOUT = 3.3V, LOAD = 500mA, PWM MODE A: VIN, 5V/div B: VOUT, 50mV/div, AC COUPLED C: LX, 10V/div
6
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100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter
______________________________________________________________Pin Description
PIN 1 2 3 4 5 NAME SHDN FB REF VL GND FUNCTION Shutdown, Active-Low, Logic-Level Input. Connect SHDN to V+ for normal operation. Feedback Input. Connect FB to a resistor voltage divider between the output and GND. Reference Bypass Output. Connect a 0.047F capacitor to GND very close to the MAX887, within 0.2 in. (5mm). 3.3V Internal Logic Regulator Output. Bypass VL to GND with a 2.2F capacitor very close to the MAX887, within 0.2 in. (5mm). Ground Oscillator Synchronization and PWM Control Input. SYNC is a logic-level input. Tie SYNC to VL for internal 300kHz PWM operation at all loads. The oscillator synchronizes to the negative edge of an external clock between 10kHz and 400kHz. The MAX887 operates in PWM mode when SYNC is clocked. Tying SYNC to GND allows a reduced supply-current mode at light loads. Inductor Connection to the drain of an internal P-channel MOSFET Supply-Voltage Input. 3.5V min to 11V max. Bypass V+ to GND with a 0.33F and large-value electrolytic capacitor in parallel. These capacitors must be as close to the V+ and GND pins as possible. Place the 0.33F capacitor within 0.2 in. (5mm) of the MAX887.
MAX887
6
SYNC
7 8
LX V+
V+ SHDN VL VL REF REF GND GND 25mV 100mV
PFM CURRENT COMPARATOR V+
1
LEVEL SHIFTER ILIM COMPARATOR 0.1X SENSE FET PWM COMPARATOR CONTROL & DRIVER LOGIC FB REF LX
RAMP GEN SYNC CELL PWM SLOPE COMPENSATION FROM CONTROL LOGIC
SENSE FET 0.1X
SYNC
PWM ON SIGNAL FB FB
50mV 1 NEGLIM COMPARATOR OVERVOLTAGE COMPARATOR 0mV in PFM ADJ. IN PWM GND
REF
PFM COMPARATOR
REF
Figure 1. Simplified Functional Block Diagram
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100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter MAX887
_______________Detailed Description
The MAX887 is a step-down, pulse-width modulation (PWM) DC-DC converter that provides an adjustable output from 1.25V to 10.5V. It accepts inputs from 3.5V to 11V and delivers up to 600mA. An internal MOSFET and synchronous rectifier reduce PC board area while maintaining high efficiency. Cycle-by-cycle current limiting protects the internal MOSFETs and reduces system stress during overload conditions. Operation with up to 100% duty cycle for an output of 3V and higher minimizes dropout voltage. Fixed-frequency PWM operation reduces interference in sensitive communications and data-acquisition applications. A SYNC input allows synchronization to an external clock. When enabled, Idle ModeTM extends battery life under light loads by placing the regulator in low quiescent current (200A typ) pulse-frequency modulation (PFM) operation. Shutdown quiescent current is 2.5A typ. or the PWM comparator signals that the output is in regulation. When the switch turns off, during the second half of each cycle, the inductor's magnetic field collapses, releasing the stored energy and forcing current through the output diode to the output filter capacitor and load. The output filter capacitor stores charge when the inductor current is high and releases it when the inductor current is low, smoothing the voltage across the load. During normal operation, the MAX887 regulates output voltage by switching at a constant frequency and then modulating the power transferred to the load per pulse using the PWM comparator. A multi-input comparator sums three weighted differential signals (the output voltage with respect to the reference, the main switch current sense, and the slope-compensation ramp) and changes states when a threshold is reached. It modulates output power by adjusting the inductor peak current during the first half of each cycle, based on the output error voltage. The MAX887's loop gain is relatively low to enable the use of a small, low-valued output filter capacitor. The resulting load regulation is 2.5% typ at 500mA. Slope compensation is added to account for the inductor current waveform's down slope during the second half of each cycle, and to eliminate the inductor current staircasing characteristic of current-mode controllers at high duty cycles.
PWM Control Scheme
The MAX887 uses an oscillator-triggered minimum/ maximum on-time current-mode control scheme. The minimum on-time is approximately 280ns unless in dropout. The maximum on-time is approximately 4/fOSC, allowing operation to 100% duty cycle. Currentmode feedback provides cycle-by-cycle current limiting for superior load and line response and protection of the internal MOSFET and rectifier. At each falling edge of the internal oscillator, the SYNC cell sends a PWM ON signal to the control and drive logic, turning on the internal P-channel MOSFET (main switch) (Figures 1 and 2). This allows current to ramp up through the inductor (Figure 2) to the load, and stores energy in a magnetic field. The switch remains on until either the current-limit (ILIM) comparator is tripped, the maximum on-time is reached (not shown),
100% Duty-Cycle Operation
For the internal oscillator frequency, the fOSC/4 maximum on-time exceeds one cycle and permits operation to 100% duty cycle. As the input voltage drops, the duty cycle increases until the P-channel MOSFET is held on continuously and 100% duty cycle is reached. Dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal switch and inductor around 300mV (IOUT = 500mA). In PWM mode, subharmonic oscillation can occur near dropout, but subharmonic voltage ripple is small, since the ripple current is low. When using synchronization to an external oscillator, 100% duty cycle is available for SYNC frequencies higher than fOSC/4.
VIN = 3.5V to 11V V+ 47F ON 0.33F LX
33H
VOUT = 3.3V
MAX887
SHDN SYNC VL REF
R1 165k
C1 100pF
47F
Synchronous Rectification
Although an external Schottky diode is used as the primary output rectifier, an N-channel synchronous rectifier turns on to reduce power loss across the diode and improve efficiency. During the second half of each cycle, when the inductor current ramps below the threshold set by the NEGLIM comparator or when the end of the oscillator period is reached, the synchronous rectifier turns off. This keeps excess current from flowing
OFF
FB R2 100k
GND
2.2F
0.047F VOUT = 1.25V (R1/R2 + 1)
Figure 2. Typical Operating Circuit
8 _______________________________________________________________________________________
100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter
backward through the inductor, from the output filter capacitor to GND, or through the switch and synchronous rectifier to GND. During PWM operation, the NEGLIM threshold adjusts to permit small amounts of reverse current to flow from the output during light loads. This allows regulation with a constant switching frequency and eliminates minimum load requirements. The NEGLIM comparator threshold is 0mA if VFB < 1.25V, and decreases as VFB exceeds 1.25V to prevent the output from rising. The NEGLIM threshold in PFM mode is 0mA. (See Forced PWM and Idle Mode operation.) sition from PFM to PWM modes with loads around 100mA, and has no adverse impact on regulation. Output ripple is higher during PFM operation, and the output filter capacitor should be selected on this basis when PFM mode is used. Output ripple and noise are higher during PFM operation.
MAX887
SYNC Input and Frequency Control
The MAX887H comes with an internal oscillator set for a fixed switching frequency of 300kHz. Connect SYNC to VL for normal forced-PWM operation. Do not leave SYNC floating. Connecting SYNC to GND enables Idle Mode operation to reduce supply current at light loads. SYNC is a logic-level input useful for operating-mode selection and frequency control. It is a negative edge triggered input that allows synchronization to an external frequency between 25kHz and 440kHz. When SYNC is clocked by an external signal, the converter operates in PWM mode. If SYNC is low or high for more than 100s, the oscillator defaults to 300kHz. Operating at a lower switching frequency reduces quiescent current, but reduces maximum load current as well (Table 1). For example, at 330kHz, maximum output current is 600mA, while at 30kHz, maximum output current is only 30mA. Note that 100% duty cycle will only occur for fSYNC > fOSC/4.
Forced PWM and Idle Mode Operation
Connect SYNC to VL for normal forced PWM operation. Forced PWM operation is desirable in sensitive RF and data-acquisition applications, to ensure that switchingnoise harmonics do not interfere with sensitive IF and data-sampling frequencies. A minimum load is not required during forced PWM operation, since the synchronous rectifier passes reverse inductor current as needed to allow constant-frequency operation with no load. Connecting SYNC to GND enables Idle Mode operation. This proprietary control scheme places the MAX887 in PFM mode at light loads to improve efficiency and reduce quiescent current to 200A typ. With Idle Mode enabled, the MAX887 initiates PFM operation when the output current drops below 100mA. During PFM operation, the MAX887 switches only as needed to service the load, reducing the switching frequency and associated losses in the internal switch and synchronous rectifier, Schottky diode, and external inductor. During PFM mode, a switching cycle is initiated when the PFM comparator senses that the output voltage has dropped too low. The P-channel MOSFET switch turns on and conducts current to the output filter capacitor and load until the inductor current reaches the PFM peak current limit (100mA). Then the switch turns off and the magnetic field in the inductor collapses, forcing current through the output diode to the output filter capacitor and load. The output filter capacitor stores charge when the inductor current is high and releases charge when it is low, smoothing the voltage across the load. Then the MAX887 waits until the PFM comparator senses a low output voltage again. During PFM mode, the synchronous rectifier is disabled and the external Schottky diode is used as an output rectifier. The PFM current comparator controls both entry into PWM mode and the peak switching current during PFM mode. Consequently, some jitter is normal during tran-
VL Regulator
The MAX887 uses an internal 3.3V linear regulator for logic power in the IC. This logic supply is brought out using the VL pin for bypassing and compensation with an external 2.2F capacitor to GND. Connect this capacitor close to the MAX887, within 0.2in (5mm).
Shutdown
Connecting SHDN to GND places the MAX887 in a lowcurrent shutdown mode (IQ = 2.5A typ at V+ = 7V). In shutdown, the reference, VL regulator, control circuitry, internal switching MOSFET, and the synchronous rectifier turn off and the output falls to 0V. Connect SHDN to V+ for normal operation.
Current-Sense Comparators
Several internal current-sense comparators are used inside the MAX887. In PWM operation, the PWM comparator is used for current-mode control. Current-mode control imparts cycle-by-cycle current limiting and provides improved load and line response, allowing tighter specification of the inductor saturation current limit to reduce inductor cost. A second 100mA current-sense comparator is used across the P-channel switch to control entry into PFM mode. A third current-sense comparator monitors current through the internal N-channel MOSFET to set the NEGLIM threshold and determine
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100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter MAX887
when to turn off this synchronous rectifier. A fourth comparator (ILIM) is used at the P-channel MOSFET switch for overcurrent detection. This protects the system, external components, and internal MOSFETs under overload conditions. ple and transient response. Lower oscillator frequencies require a larger-value output capacitor. When Idle Mode is used, verify capacitor selection with light loads during PFM operation, since output ripple is higher under these conditions. Low-ESR capacitors are recommended. Capacitor ESR is a major contributor to output ripple (usually more than 60%). Ordinary aluminum-electrolytic capacitors have high ESR and should be avoided. Low-ESR aluminum-electrolytic capacitors are acceptable and relatively inexpensive. Low-ESR tantalum capacitors are better and provide a compact solution for spaceconstrained surface-mount designs. Do not exceed the ripple current ratings of tantalum capacitors. Ceramic capacitors have the lowest ESR overall, and OS-CON capacitors have the lowest ESR of the highvalue electrolytic types. It is generally not necessary to use ceramic and OS-CON capacitors for the MAX887; they need only be considered in very compact, highreliability, or wide-temperature applications, where the expense is justified. When using very-low-ESR capacitors, such as ceramic or OS-CON, check for stability while examining load-transient response, and increase the compensation capacitor C1 if needed. Table 2 lists suppliers for the various components used with the MAX887.
________________Design Information
Output Voltage Selection
To select an output voltage between 1.25V and 10.5V, connect FB to a resistor voltage divider between the output and GND (Figure 2). Select feedback resistor R2 in the 5k to 100k range, since FB input leakage is 100nA max. R1 is then given by:
V R1 = R2 OUT - 1 VFB
where VFB = 1.25V. A small ceramic capacitor (C1) around 100pF to 470pF should be added in parallel with R1 to compensate for stray capacitance at the FB pin, and output capacitor equivalent series resistance (ESR).
Inductor Selection
A 1.3A inductor with the value recommended in Table 1 is sufficient for most applications. However, the exact inductor value is not critical, and values within 50% of those in Table 1 are acceptable. For best efficiency, the inductor's DC resistance should be less than 0.25. The inductor saturation current rating must exceed the 1A ILIM current limit. Table 2 lists component suppliers.
Table 2. Component Suppliers
COMPANY AVX Coilcraft Coiltronics Dale International Rectifier Motorola Nichicon Nihon Sanyo USA USA USA USA USA USA USA Japan USA Japan USA Japan USA USA USA Japan USA PHONE (803) 946-0690 (800) 282-4975 (847) 639-6400 (561) 241-7876 (605) 668-4131 (310) 322-3331 (602) 303-5454 (847) 843-7500 81-7-5231-8461 (805) 867-2555 81-3-3494-7411 (619) 661-6835 81-7-2070-6306 (408) 988-8000 (800) 554-5565 (603) 224-1961 (847) 956-0666 81-3-3607-5111 (714) 255-9500 FAX (803) 626-3123 (847) 639-1469 (561) 241-9339 (605) 665-1627 (310) 322-3332 (602) 994-6430 (847) 843-2798 81-7-5256-4158 (805) 867-2698 81-3-3494-7414 (619) 661-1055 81-7-2070-1174 (408) 970-3950 (603) 224-1430 (847) 956-0702 81-3-3607-5144 (714) 255-9400
Capacitor Selection
Input and output filter capacitors should be chosen to service inductor currents with acceptable voltage ripple. The input filter capacitor also reduces peak currents and noise at the voltage source. See Table 1 for suggested values. The MAX887's loop gain is relatively low, to enable the use of small, low-valued output filter capacitors. Higher values provide improved output rip-
Table 1. Inductor and Output Filter vs. Sync Frequency
SYNC RANGE (kHz) 300-400 200-300 150-200 100-150 75-100 10 L1 (H) 33 47 68 100 150 COUT (F) 33 47 68 100 150
Siliconix Sprague Sumida United Chemi-Con
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100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter
Bypass V+ to GND using a 0.33F capacitor. Also bypass VL to GND with a 2.2F capacitor, and VREF to GND using a 0.047F capacitor. These capacitors should be placed within 0.2in (5mm) of their respective pins. A small ceramic capacitor (C1) of around 100pF to 470pF should be added in parallel with R1 to compensate for stray capacitance at the FB pin and output capacitor ESR.
___________________Chip Information
TRANSISTOR COUNT: 2006 SUBSTRATE CONNECTED TO GND
MAX887
Output Diode Selection
A 1A external diode (D1) is required as an output rectifier to pass inductor current during the second half of each cycle. This diode operates in PFM mode and during transition periods while the synchronous rectifier is off. Use a Schottky diode to prevent the slow internal diode of the N-channel MOSFET from turning on.
PC Board Layout and Routing
High switching frequencies and large peak currents make PC board layout a very important part of design. Poor design can result in excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors. Power components, such as the MAX887, inductor, input filter capacitor, and output filter capacitor should be placed as close together as possible, and their traces kept short, direct, and wide. Connect their ground pins at a common node in a star-ground configuration. Keep the extra copper on the board and integrate into ground as a pseudo-ground plane. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces, such as from the LX pin, away from the voltagefeedback network, and separate using grounded copper. Place the small bypass capacitors (C1, C3, C5, and C6) within 0.2in (5mm) of their respective pins. The MAX887 evaluation kit manual illustrates an example PC board layout, routing, and pseudo-ground plane.
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100% Duty Cycle, Low-Noise, Step-Down, PWM DC-DC Converter MAX887
________________________________________________________Package Information
DIM INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27
D A e B
0.101mm 0.004in.
0-8
A1
C
L
A A1 B C E e H L
E
H
Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.)
DIM PINS D D D 8 14 16
INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00
21-0041A
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