_______________General Description
The MAX887 high-efficiency, step-down DC-DC con-
verter 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 applica-
tions. 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 data-
acquisition circuits.
The MAX887 features current-mode operation for supe-
rior load/line-transient response. Cycle-by-cycle current
limiting protects the internal MOSFET and rectifier. A
low-current (2.5µA typ) shutdown mode conserves bat-
tery life.
________________________Applications
Portable Instruments
Cellular Phones and Radios
Personal Communicators
Distributed Power Systems
Computer Peripherals
____________________________Features
♦95% Efficiency
♦600mA Output Current
♦Cycle-by-Cycle Current Limiting
♦Low-Dropout, 100% Duty-Cycle Operation,
300mV at 500mA
♦Internal 0.6Ω(typ) MOSFET
♦Internal Synchronous Rectifier
♦High-Frequency Current-Mode PWM
♦External SYNC or Internal 300kHz Oscillator
♦Guaranteed 260kHz to 340kHz Internal Oscillator
Frequency Limits
♦2.5µA Shutdown Mode
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
________________________________________________________________
Maxim Integrated Products
1
1
2
8
7
V+
LX
SYNC
GND
SHDN
FB
REF
VL
MAX887
SO
TOP VIEW
3
4
6
5
__________________Pin Configuration
MAX887
REF
R1
165kΩ
R2
100kΩ
0.047µF
2.2µF
47µF47µF
C1
100pF
0.33µF
33µH
V+ LX
FB
GND
SYNC
VL
SHDN
VOUT = 3.3V
VOUT = 1.25V (R1/R2 + 1)
VIN = 3.5V to 11V
ON
OFF
__________Typical Operating Circuit
19-1142 Rev 0; 9/96
PART
MAX887HC/D
MAX887HESA -40°C to +85°C
0°C to +70°C
TEMP. RANGE PIN-PACKAGE
Dice*
8 SO
EVALUATION KIT MANUAL
AVAILABLE
______________Ordering Information
*
Contact factory for availability. Dice are tested at TA= +25°C.
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V+ = +7V, PGND = GND = 0V, SHDN = V+, (TA= 0°C to TMAX), unless otherwise noted.)
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.
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= +70°C)
SO (derate 9.09mW/°C above +70°C) .........................471mW
Operating Temperature Ranges
MAX887HC/D.......................................................0°C to +70°C
MAX887HESA...................................................-40°C to +85°C
Storage Temperature Range ........................... -65°C to +165°C
Lead Temperature (soldering, 10sec)............................ +300°C
SYNC = GND or 3V
IOUT = 0mA, SYNC = GND
V+ rising
IOUT = 0mA, SYNC = 3.0V
V+ falling
V+ = floating, LX = 5V, SHDN = GND
V+ = 12V, LX = GND to 12V
ILX = ±100mA
SYNC = 3.0V
FB = 1.30V
SYNC = 3.0V, PWM duty cycle = 50%
SHDN = GND
Circuit of Figure 2
IOUT = 0mA to 500mA
VIN = 4V to 11V, PWM mode
SYNC = 3.0V, FB = 1.18V
CONDITIONS
V2.4VIH, SHDN
SHDN Input High Voltage µA±1IIN, SYNC
SYNC Input Current V0.5VIL, SYNC
SYNC Input Low Voltage V2.5VIH, SYNC
SYNC Input High Voltage V3.1 3.5V+, START
Startup Voltage V3.0 3.3V+, UVLO
Undervoltage Lockout
µA1.0 20ILXLKGR
LX Reverse Leakage Current,
Regulator Off
µA-10 1.0 10ILXLKG
LX Leakage Current Ω0.6RON, LX
LX On-Resistance A0.75 1.0 1.40ILIM+
High-Side Current Limit kHz260 300 340fOSC
PWM Switching Frequency
SHDN Input Low Voltage VIL, SHDN
%100PWM, DUTYPWM Maximum Duty Cycle
mA0.2 0.5IV+, PFM
Quiescent Supply Current
(PFM Mode)
mA2.7 4.0IV+, PWM
V3.5 11.0V+Supply Range
Quiescent Supply Current
(PWM Mode)
ns500SYNC, PWSYNC Pulse Width High or Low kHz25 440fSYNC
SYNC Frequency µA±0.10IFB
FB Input Current V1.225 1.250 1.275VFB
PWM FB Feedback Threshold
µA2.5 5IV+, SHDN
Shutdown Supply Current V1.25 10.50VOUT, RANGE
Output Voltage Range %/mA0.005Load Regulation %/V0.2Line Regulation
UNITSMIN TYP MAXSYMBOLPARAMETER
0.8 V
SHDN Input Current, Sinking IIN-, SHDN SHDN = GND or V+ ±1 µA
SHDN Input Capacitance CIN, SHDN (Note 1) 10 pF
VL Output Voltage VLIVL = 0mA to 1mA 3.3 V
REF Output Voltage VREF 0µA to 30µA 1.25 V
Note 1: Guaranteed by design and not production tested.
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
_______________________________________________________________________________________ 3
Note 2: Specifications from 0°C to -40°C are guaranteed by design and not production tested.
IOUT = 0mA, SYNC = GND
V+ rising
IOUT = 0mA, SYNC = 3.0V
V+ falling
SYNC = 3.0V
FB = 1.30V
SYNC = 3.0V, PWM duty cycle = 50%
SHDN = GND
Circuit of Figure 2
CONDITIONS
V3.1 3.5V+, START
Startup Voltage V3.0 3.3V+, UVLO
Undervoltage Lockout A0.75 1.00 1.50ILIM+
High-Side Current Limit kHz250 300 350fOSC
PWM Switching Frequency
mA0.2 0.6IV+, PFM
Quiescent Supply Current
(PFM Mode)
mA2.7 4.0IV+, PWM
V3.5 11.0V+Supply Range
Quiescent Supply Current
(PWM Mode)
µA±0.10IFB
FB Input Current V1.222 1.250 1.278VFB
PWM FB Feedback Threshold
µA2.5 5IV+, SHDN
Shutdown Supply Current V1.25 10.50VOUT, RANGE
Output Voltage Range
UNITSMIN TYP MAXSYMBOLPARAMETER
ELECTRICAL CHARACTERISTICS
(V+ = +7V, PGND = GND = 0V, SHDN = V+, (TA= -40°C to +85°C), unless otherwise noted.) (Note 2)
__________________________________________Typical Operating Characteristics
(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 200 400 600 800 1000
LOAD CURRENT (mA)
DROPOUT VOLTAGE
vs. LOAD CURRENT
MAX887-01
DROPOUT VOLTAGE (V)
3.3V SETTING
VOUT = 3.135V
5V SETTING
VOUT = 4.75V
0
10
20
30
40
50
60
70
80
90
100
0.0001 0.001 0.01 0.1 0.6
EFFICIENCY (%)
OUTPUT CURRENT (A)
EFFICIENCY vs. OUTPUT CURRENT
MAX887-02
PWM MODE
(SYNC = VL)
VOUT = 3.3V
VIN = 4V
VIN = 5V
VIN = 7V
VIN = 11V
VIN = 9V
0
10
20
30
40
50
60
70
80
90
100
0.0001 0.001 0.01 0.1 0.6
EFFICIENCY (%)
OUTPUT CURRENT (A)
EFFICIENCY vs. OUTPUT CURRENT
MAX887-03
PWM MODE
(SYNC = VL)
VOUT = 5V
VIN = 9V
VIN = 5.5V VIN = 11V
VIN = 7V
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
4 _______________________________________________________________________________________
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
0
10
20
30
40
50
60
70
80
90
100
0.0001 0.001 0.01 0.1 0.6
EFFICIENCY (%)
OUTPUT CURRENT (A)
EFFICIENCY vs. OUTPUT CURRENT
MAX887-04
IDLE MODE
(SYNC = GND)
VOUT = 3.3V
VIN = 5V
VIN = 7V
VIN = 9V
VIN = 11V
VIN = 4V
0
10
20
30
40
50
60
70
80
90
100
0.0001 0.001 0.01 0.1 0.6
EFFICIENCY (%)
OUTPUT CURRENT (A)
EFFICIENCY vs. OUTPUT CURRENT
MAX887-05
IDLE MODE
(SYNC = GND)
VOUT = 5V
VIN = 5.5V
VIN = 9V
VIN = 7V
VIN = 11V
0
200
400
600
800
1000
1200
357911131517
MAXIMUM OUTPUT CURRENT (mA)
SUPPLY VOLTAGE (V)
MAXIMUM OUTPUT CURRENT
vs. SUPPLY VOLTAGE
MAX887-06
GUARANTEED OUTPUT CURRENT
OF FIGURE 2 IS 600mA
3.3V SETTING, VOUT = 3.135V
5V SETTING, VOUT = 4.75V
0
200
400
600
800
1000
1200
0 100 200 300 400 500 600
MAXIMUM OUTPUT CURRENT (mA)
SYNC FREQUENCY (kHz)
MAXIMUM OUTPUT CURRENT
vs. SYNC FREQUENCY
MAX887-07
C2, C3 = 47µF
L1 = 33µH
VIN = 5V
VOUT = 3.3V
VOUT = -5% at IOUT(MAX)
250
260
270
280
290
300
310
320
330
340
350
024681012141618
SWITCHING FREQUENCY (kHz)
SUPPLY VOLTAGE (V)
SWITCHING FREQUENCY
vs. SUPPLY VOLTAGE
MAX887-10
VOUT = 3.3V
0
0.5
1.5
2.5
3.5
3.0
2.0
1.0
QUIESCENT CURRENT (mA)
SUPPLY VOLTAGE (V)
QUIESCENT SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX887-08
A
A
A: VOUT = 3.3V, PWM MODE
B: VOUT = 3.3V, PFM MODE
024681012141618
C
B0
0.5
1.0
1.5
2.0
2.5
3.0
-60 -40 -20 0 20 40 60 80 100 120 140
QUIESCENT CURRENT (mA)
TEMPERATURE (°C)
QUIESCENT CURRENT
vs.TEMPERATURE
MAX887-09
PWM MODE
VIN = 5.3V
VOUT = 3.3V
PFM MODE
250
260
270
280
290
300
310
320
330
340
350
FREQUENCY (kHz)
TEMPERATURE (°C)
SWITCHING FREQUENCY
vs. TEMPERATURE
MAX887-11
VOUT = 3.3V
-60 -40 -20 0 20 40 60 80 100 120 140
4
-1 10k 10M
OUTPUT RIPPLE AND HARMONICS
0
2
3
1
MAX887 TOC-20
FREQUENCY (Hz)
OUTPUT NOISE (mV)
100k 1M
VIN = 5V
VOUT = 3.3V
IOUT = 500mA
PWM MODE
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
_______________________________________________________________________________________
5
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
VIN = 5V, VOUT = 3.3V, LOAD = 500mA
A: LX, 5V/div
B: VOUT, 20mV/div, AC COUPLED
C: INDUCTOR CURRENT, 500mA/div
1µs/div
HEAVY-LOAD, PWM-MODE
SWITCHING WAVEFORMS
A
B
C
0mA
MAX887-12
VIN = 5V, VOUT = 3.3V, LOAD = 0mA
A: LX, 5V/div
B: VOUT, 20mV/div, AC COUPLED
C: INDUCTOR CURRENT, 500mA/div
1µs/div
LIGHT-LOAD, PWM-MODE
SWITCHING WAVEFORMS
A
B
C
MAX887-13
VIN = 5V, VOUT = 3.3V, LOAD = 0mA
A: LX, 5V/div
B: VOUT, 20mV/div, AC COUPLED
C: INDUCTOR CURRENT, 200mA/div
1µs/div
LIGHT-LOAD, PFM-MODE
SWITCHING WAVEFORMS
A
B
C
MAX887-14
VIN = 5V, VOUT = 3.3V, LOAD = 70mA
A: LX, 5V/div
B: VOUT, 20mV/div, AC COUPLED
C: INDUCTOR CURRENT, 200mA/div
10µs/div
MEDIUM-LOAD, PFM-MODE
SWITCHING WAVEFORMS
A
B
C
MAX887-15
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
6 _______________________________________________________________________________________
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 2, TA = +25°C, unless otherwise noted.)
40µs/div
LOAD-TRANSIENT RESPONSE
A
B
C
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
MAX887-16
200µs/div
LINE-TRANSIENT RESPONSE
A
B
VIN = 5V TO 11V, VOUT = 3.3V,
LOAD = 500mA, PWM MODE
A: VIN, 5V/div
B: VOUT, 20mV/div, AC COUPLED
MAX887-17
200µs/div
RECOVERY FROM 100% DUTY CYCLE
(DROP OUT)
A
B
C
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
MAX887-18
500µs/div
SHUTDOWN AND STARTUP RESPONSE
A
B
C
D
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
MAX887-19
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
_______________________________________________________________________________________ 7
______________________________________________________________Pin Description
NAME FUNCTION
1SHDN Shutdown, Active-Low, Logic-Level Input. Connect SHDN to V+ for normal operation.
2 FB Feedback Input. Connect FB to a resistor voltage divider between the output and GND.
PIN
3 REF Reference Bypass Output. Connect a 0.047µF capacitor to GND very close to the MAX887, within 0.2 in. (5mm).
4VL 3.3V Internal Logic Regulator Output. Bypass VL to GND with a 2.2µF capacitor very close to the MAX887,
within 0.2 in. (5mm).
8 V+ Supply-Voltage Input. 3.5V min to 11V max. Bypass V+ to GND with a 0.33µF and large-value electrolytic
capacitor in parallel. These capacitors must be as close to the V+ and GND pins as possible. Place the
0.33µF capacitor within 0.2 in. (5mm) of the MAX887.
7 LX Inductor Connection to the drain of an internal P-channel MOSFET
6 SYNC
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.
5 GND Ground
PFM CURRENT COMPARATOR
LEVEL
SHIFTER
CONTROL &
DRIVER LOGIC
SLOPE COMPENSATION
FROM CONTROL LOGIC
PWM
COMPARATOR
NEGLIM
COMPARATOR 0mV in PFM
ADJ. IN PWM
GND
0.1X
SENSE FET
SENSE FET
1Ω
LX
V+
0.1X
1Ω
PWM
REF
FB
SYNC
PFM
COMPARATOR OVERVOLTAGE
COMPARATOR
PWM ON
SIGNAL 50mV
FB
REF
REF
FB
25mV
100mV
RAMP
GEN
SYNC
CELL
ILIM COMPARATOR
REF
VL
V+
GND
REF
GND
SHDN VL
Figure 1. Simplified Functional Block Diagram
_______________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 lim-
iting protects the internal MOSFETs and reduces sys-
tem stress during overload conditions. Operation with
up to 100% duty cycle for an output of 3V and higher
minimizes dropout voltage. Fixed-frequency PWM oper-
ation reduces interference in sensitive communications
and data-acquisition applications. A SYNC input allows
synchronization to an external clock. When enabled,
Idle Mode™ extends battery life under light loads by
placing the regulator in low quiescent current (200µA
typ) pulse-frequency modulation (PFM) operation.
Shutdown quiescent current is 2.5µA typ.
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. Current-
mode feedback provides cycle-by-cycle current limit-
ing 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),
or the PWM comparator signals that the output is in
regulation. When the switch turns off, during the sec-
ond half of each cycle, the inductor’s magnetic field
collapses, releasing the stored energy and forcing cur-
rent through the output diode to the output filter capaci-
tor 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 con-
trollers at high duty cycles.
100% Duty-Cycle Operation
For the internal oscillator frequency, the fOSC/4 maxi-
mum 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 cur-
rent 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.
Synchronous Rectification
Although an external Schottky diode is used as the pri-
mary output rectifier, an N-channel synchronous rectifi-
er 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
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
8 _______________________________________________________________________________________
MAX887
REF R2
100kΩ
R1
165kΩ
0.047µF
2.2µF
47µF47µF
C1
100pF
0.33µF
33µH
V+ LX
FB
GND
SYNC
VL
SHDN
VOUT = 3.3V
VOUT = 1.25V (R1/R2 + 1)
VIN = 3.5V to 11V
ON
OFF
Figure 2. Typical Operating Circuit
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
_______________________________________________________________________________________ 9
backward through the inductor, from the output filter
capacitor to GND, or through the switch and synchro-
nous 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 mini-
mum 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.
)
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 switching-
noise harmonics do not interfere with sensitive IF and
data-sampling frequencies. A minimum load is not
required during forced PWM operation, since the syn-
chronous rectifier passes reverse inductor current as
needed to allow constant-frequency operation with no
load.
Connecting SYNC to GND enables Idle Mode opera-
tion. This proprietary control scheme places the
MAX887 in PFM mode at light loads to improve efficien-
cy and reduce quiescent current to 200µA 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-
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.
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 exter-
nal 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 100µs, the oscillator defaults to 300kHz. Operating
at a lower switching frequency reduces quiescent cur-
rent, but reduces maximum load current as well
(Table 1). For example, at 330kHz, maximum output
current is 600mA, while at 30kHz, maximum output cur-
rent is only 30mA. Note that 100% duty cycle will only
occur for fSYNC > fOSC/4.
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 2.2µF capacitor to GND. Connect this
capacitor close to the MAX887, within 0.2in (5mm).
Shutdown
Connecting SHDN to GND places the MAX887 in a low-
current shutdown mode (IQ= 2.5µA typ at V+ = 7V). In
shutdown, the reference, VL regulator, control circuitry,
internal switching MOSFET, and the synchronous recti-
fier 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 com-
parator is used for current-mode control. Current-mode
control imparts cycle-by-cycle current limiting and pro-
vides 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 con-
trol entry into PFM mode. A third current-sense com-
parator monitors current through the internal N-channel
MOSFET to set the NEGLIM threshold and determine
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
10 ______________________________________________________________________________________
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 sys-
tem, external components, and internal MOSFETs
under overload conditions.
________________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 5kΩto 100kΩrange, since FB input leakage is
±100nA max. R1 is then given by:
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 0.25Ω.
The inductor saturation current rating must exceed the
1A ILIM current limit. Table 2 lists component suppliers.
Capacitor Selection
Input and output filter capacitors should be chosen to
service inductor currents with acceptable voltage rip-
ple. The input filter capacitor also reduces peak cur-
rents 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-
ple and transient response. Lower oscillator frequen-
cies 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 alu-
minum-electrolytic capacitors are acceptable and rel-
atively inexpensive. Low-ESR tantalum capacitors
are better and provide a compact solution for space-
constrained 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 high-
value 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, high-
reliability, or wide-temperature applications, where the
expense is justified. When using very-low-ESR capaci-
tors, 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.
R1 R2 V
V1
OUT
FB
=−
Table 1. Inductor and Output Filter
vs. Sync Frequency
Table 2. Component Suppliers
L1
(µH) COUT
(µF)
300–400 33 33
200–300 47 47
150–200 68 68
100–150 100 100
75–100 150 150
SYNC
RANGE (kHz)
COMPANY PHONE FAX
AVX USA (803) 946-0690 (803) 626-3123
(800) 282-4975
Coilcraft USA (847) 639-6400 (847) 639-1469
Coiltronics USA (561) 241-7876 (561) 241-9339
Dale USA (605) 668-4131 (605) 665-1627
International USA (310) 322-3331 (310) 322-3332
Rectifier
Motorola USA (602) 303-5454 (602) 994-6430
Nichicon USA (847) 843-7500 (847) 843-2798
Japan 81-7-5231-8461 81-7-5256-4158
Nihon USA (805) 867-2555 (805) 867-2698
Japan 81-3-3494-7411 81-3-3494-7414
Sanyo USA (619) 661-6835 (619) 661-1055
Japan 81-7-2070-6306 81-7-2070-1174
Siliconix USA (408) 988-8000 (408) 970-3950
(800) 554-5565
Sprague USA (603) 224-1961 (603) 224-1430
Sumida USA (847) 956-0666 (847) 956-0702
Japan 81-3-3607-5111 81-3-3607-5144
United USA (714) 255-9500 (714) 255-9400
Chemi-Con
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
______________________________________________________________________________________ 11
Bypass V+ to GND using a 0.33µF capacitor. Also
bypass VL to GND with a 2.2µF capacitor, and VREF to
GND using a 0.047µF 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 com-
pensate for stray capacitance at the FB pin and output
capacitor ESR.
Output Diode Selection
A 1A external diode (D1) is required as an output recti-
fier to pass inductor current during the second half of
each cycle. This diode operates in PFM mode and dur-
ing 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 feed-
back 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 capaci-
tor 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 voltage-
feedback network, and separate using grounded cop-
per. 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.
___________________Chip Information
TRANSISTOR COUNT: 2006
SUBSTRATE CONNECTED TO GND
MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
12 ______________________________________________________________________________________
DIM
A
A1
B
C
E
e
H
L
MIN
0.053
0.004
0.014
0.007
0.150
0.228
0.016
MAX
0.069
0.010
0.019
0.010
0.157
0.244
0.050
MIN
1.35
0.10
0.35
0.19
3.80
5.80
0.40
MAX
1.75
0.25
0.49
0.25
4.00
6.20
1.27
INCHES MILLIMETERS
21-0041A
Narrow SO
SMALL-OUTLINE
PACKAGE
(0.150 in.)
DIM
D
D
D
MIN
0.189
0.337
0.386
MAX
0.197
0.344
0.394
MIN
4.80
8.55
9.80
MAX
5.00
8.75
10.00
INCHES MILLIMETERS
PINS
8
14
16
1.270.050
L
0°-8°
HE
D
e
A
A1 C
0.101mm
0.004in.
B
________________________________________________________Package Information
