5V/3.3V or Adjustable, 100% Duty-Cycle,

High-Efficiency, Step-Down DC-DC Controllers

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_______________Detailed Description

The MAX1626/MAX1627 are step-down DC-DC con-

trollers designed primarily for use in portable comput-

ers and battery-powered devices. Using an external

MOSFET and current-sense resistor allows design flexi-

bility and the improved efficiencies associated with

high-performance P-channel MOSFETs. A unique, cur-

rent-limited, pulse-frequency-modulated (PFM) control

scheme gives these devices excellent efficiency over

load ranges up to three decades, while drawing around

90µA under no load. This wide dynamic range opti-

mizes the MAX1626/MAX1627 for battery-powered

applications, where load currents can vary consider-

ably as individual circuit blocks are turned on and off to

conserve energy. Operation to a 100% duty cycle

allows the lowest possible dropout voltage, extending

battery life. High switching frequencies and a simple

circuit topology minimize PC board area and compo-

nent costs. Figure 1 shows a typical operating circuit

for the MAX1626.

PFM Control Scheme

The MAX1626/MAX1627 use a proprietary, third-genera-

tion, current-limited PFM control scheme. Improvements

include a reduced current-sense threshold and operation

to a 100% duty cycle. These devices pulse only as need-

ed to maintain regulation, resulting in a variable switching

frequency that increases with the load. This eliminates the

current drain associated with constant-frequency pulse-

width-modulation (PWM) controllers, caused by switching

the MOSFET unnecessarily.

When the output voltage is too low, the error compara-

tor sets a flip-flop, which turns on the external P-chan-

nel MOSFET and begins a switching cycle (Figures 1

and 2). As shown in Figure 3, current through the

inductor ramps up linearly, storing energy in a magnet-

ic field while dumping charge into an output capacitor

and servicing the load. When the MOSFET is turned off,

the magnetic field collapses, diode D1 turns on, and

the current through the inductor ramps back down,

transferring the stored energy to the output capacitor

and load. The output capacitor stores energy when the

inductor current is high and releases it when the induc-

tor current is low.

The MAX1626/MAX1627 use a unique feedback and

control system to govern each pulse. When the output

voltage is too low, the error comparator sets a flip-flop,

which turns on the external P-channel MOSFET. The

MOSFET turns off when the current-sense threshold is

exceeded or when the output voltage is in regulation. A

one-shot enforces a 2µs minimum on-time, except in

current limit. The flip-flop resets when the MOSFET

turns off. Otherwise the MOSFET remains on, allowing a

duty cycle of up to 100%. This feature ensures the low-

est possible dropout. Once the MOSFET is turned off,

the minimum off-time comparator keeps it off. The mini-

mum off-time is normally 2µs, except when the output is

significantly out of regulation. If the output is low by

30% or more, the minimum off-time increases, allowing

soft-start. The error comparator has 0.5% hysteresis for

improved noise immunity.

In the MAX1626, the

3/5 pin selects the output voltage

(Figure 2). In the MAX1627, external feedback resistors

at FB adjust the output.

Operating Modes

When delivering low and medium output currents, the

MAX1626/MAX1627 operate in discontinuous-conduc-

tion mode. Current through the inductor starts at zero,

rises as high as the peak current limit set by the cur-

rent- sense resistor, then ramps down to zero during

each cycle (Figure 3). Although efficiency is still excel-

lent, output ripple increases and the switch waveform

exhibits ringing. This ringing occurs at the resonant fre-

quency of the inductor and stray capacitance, due to

residual energy trapped in the core when the commuta-

tion diode (D1 in Figure 1) turns off. It is normal and

poses no operational problems.

When delivering high output currents, the MAX1626/

MAX1627 operate in continuous-conduction mode

(Figure 4). In this mode, current always flows through

the inductor and never ramps to zero. The control cir-

cuit adjusts the switch duty cycle to maintain regulation

without exceeding the peak switching current set by

the current-sense resistor. This provides reduced out-

put ripple and high efficiency.

100% Duty Cycle and Dropout

The MAX1626/MAX1627 operate with a duty cycle up

to 100%. This feature extends usable battery life by

turning the MOSFET on continuously when the supply

voltage approaches the output voltage. This services

the load when conventional switching regulators with

less than 100% duty cycle would fail. Dropout voltage

is defined as the difference between the input and out-

put voltages when the input is low enough for the out-

put to drop out of regulation. Dropout depends on the

MOSFET drain-to-source on-resistance, current-sense

resistor, and inductor series resistance, and is propor-

tional to the load current:

Dropout Voltage =

I

x R

+ R

+ R

OUT

DS(ON)

SENSE

INDUCTOR

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