Datasheet

ManualsBrandsMAXIM INTEGRATED ManualsPMICsPMIC - DC/DC voltage regulator Holder SOIC 8

_______________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/f

OSC

, 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 f

OSC

/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 (I

OUT

= 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 f

OSC

/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

47µF

C1

100pF

0.33µF

33µH

V+ LX

GND

SYNC

SHDN

OUT

= 3.3V

OUT

= 1.25V (R1/R2 + 1)

= 3.5V to 11V

OFF

Figure 2. Typical Operating Circuit