Low-Cost Voltage-Mode PWM

Step-Down Controllers

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Detailed Description

The MAX1966/MAX1967 are BiCMOS switch-mode

power-supply controllers designed to implement sim-

ple, buck-topology regulators in cost-sensitive applica-

tions. The main power-switching circuit consists of two

N-channel MOSFETs (or a dual MOSFET), an inductor,

and input and output filter capacitors. An all N-channel

synchronous-rectified design provides high efficiency

at reduced cost. Gate drive for the N-channel high-side

switch is provided by a flying capacitor boost circuit

that uses a 0.1µF capacitor connected to BST.

Major circuit blocks of the MAX1966/MAX1967 are

shown in Figures 1 and 2:

•

Control Logic

•

Gate Driver Outputs

•

Current-Limit Comparator

•

Clock Generator

•

Ramp Generator

•

Error Amplifier

•

Error Comparator

•

Soft-Start

•

5V Linear Regulator (MAX1967)

•

800mV Reference

•

Thermal Shutdown

the MAX1967, an internal 5V linear regulator steps

down the input voltage to supply both the IC and the

gate drivers. The synchronous-rectified gate driver is

nal diode-capacitor boost circuit.

Resistorless Current Limit

side N-channel MOSFET to sense the current. This

eliminates the need for an external sense resistor usual-

ly placed in series with the output. The voltage mea-

inductor current limit is given by the equation below:

MOSFET Gate Drivers

The DH and DL drivers are optimized for driving

MOSFETs with low gate charge. An adaptive dead-time

circuit monitors the DL output and prevents the high-

side FET from turning on until the low-side MOSFET is

fully off. There must be a low-resistance, low-inductance

connection from the DL driver to the MOSFET gate for

the adaptive dead-time circuit to work properly.

Otherwise, the sense circuitry in the MAX1966/

MAX1967 detects the MOSFET gate as off while there

is charge left on the gate. Use very short, wide traces

measuring no less than 50mils to 100mils wide if the

MOSFET is 1in away from the MAX1966/MAX1967. The

same type of adaptive dead-time circuit monitors the

DH off edge. The same recommendations apply for the

gate connection of the high-side MOSFET.

The internal pulldown transistor that drives DL low is

robust, with a 1.1

Ω typical on-resistance. This helps

prevent DL from being pulled up due to capacitive cou-

pling from the drain to the gate of the low-side synchro-

nous-rectifier MOSFET during the fast rise time of the

inductor node. The gate drivers are capable of driving

up to 1A. Use MOSFETs with combined total gate

Internal Soft-Start

The MAX1966/MAX1967 feature an internally set soft-

start function that limits inrush current. It accomplishes

this by ramping the internal reference input to the con-

troller transconductance amplifier from 0 to the 0.8V

reference voltage. The ramp time is 1024 oscillator

cycles that begins when initial power is applied. At the

nominal 100kHz switching rate, the soft-start ramp is

approximately 10ms. The soft-start does not function if

the MAX1966/MAX1967 are shut down by pulling

COMP/EN low.

High-Side Gate-Drive Supply (BST)

Gate-drive voltage for the high-side N-channel switch is

generated by a flying-capacitor boost circuit (Figures 3

and 4). The flying capacitor is connected between BST

and LX.

On startup, the synchronous rectifier (low-side MOS-

FET) forces LX to ground and charges the boost

capacitor to 5V. On the second half-cycle, the

MAX1966/MAX1967 turn on the high-side MOSFET by

closing an internal switch between BST and DH. This

provides the necessary gate-to-source voltage to drive

the high-side FET gate above its source at the input

voltage.

Internal 5V Linear Regulator

(MAX1967)

All MAX1967 functions are internally powered from an

on-chip, low-dropout 5V regulator. The MAX1967 has a

VCC pin must be connected to VL through a 10

Ω resis-

tor and VL must be bypassed with a 2.2µF capacitor to

ImV R

PEAK

DS ON

= 305

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