3A, 1MHz, Low-Voltage, Step-Down Regulators with

Synchronous Rectification and Internal Switches

_______________________________________________________________________________________

9

Synchronous Rectification

In a step-down regulator without synchronous rectifica-

tion, an external Schottky diode provides a path for cur-

rent to flow when the inductor is discharging. Replacing

the Schottky diode with a low-resistance NMOS syn-

chronous switch reduces conduction losses and

improves efficiency.

The NMOS synchronous-rectifier switch turns on follow-

ing a short delay after the PMOS power switch turns off,

thus preventing cross conduction or “shoot through.” In

constant-off-time mode, the synchronous-rectifier

switch turns off just prior to the PMOS power switch

turning on. While both switches are off, inductor current

flows through the internal-body diode of the NMOS

switch. The internal-body diode’s forward voltage is rel-

atively high. An external Schottky diode from PGND to

LX can improve efficiency.

Thermal Resistance

Junction-to-ambient thermal resistance,

dependent on the amount of copper area immediately

surrounding the IC leads. The MAX1830/MAX1831

resistance of +71°C/W with no forced airflow. Airflow

over the board significantly reduces the junction-to-

ambient thermal resistance. For heatsinking purposes,

evenly distribute the copper area connected at the IC

among the high-current pins.

Power Dissipation

Power dissipation in the MAX1830/MAX1831 is domi-

nated by conduction losses in the two internal power

switches. Power dissipation due to supply current in the

control section and average current used to charge

and discharge the gate capacitance of the internal

switches (i.e., switching losses) is approximately:

cy in PWM mode.

This number is reduced when the switching frequency

decreases as the part enters Idle Mode. Combined con-

duction losses in the two power switches are approxi-

mated by:

The junction-to-ambient thermal resistance required to

dissipate this amount of power is calculated by:

where:

Design Procedure

For typical applications, use the recommended compo-

nent values in Table 1. For other applications, take the

following steps:

1) Select the desired PWM-mode switching frequency;

1MHz is a good starting point. See Figure 3 for maxi-

mum operating frequency.

2) Select the constant off-time as a function of input

voltage, output voltage, and switching frequency.

4) Select the inductor as a function of output voltage,

off-time, and peak-to-peak inductor current.

Setting the Output Voltage

The output of the MAX1830/MAX1831 is selectable

between one of three preset output voltages. For a pre-

set output voltage, connect FB to the output voltage

and connect FBSEL as indicated in Table 2. For an

adjustable output voltage, connect FBSEL to GND and

connect FB to a resistive divider between the output

Figure 4. Adjustable Output Voltage

Table 2. Output Voltage Programming

LX

R2

R1

R1 = 30k

Ω

FB

MAX1830

MAX1831

PIN

OUTPUT VOLTAGE (V)

FBSEL

FB

MAX1830

MAX1831

Output voltage

2.5

2.5

Unconnected

Output voltage

1.5

1.5

REF

Output voltage

1.8

3.3

GND

Resistive divider

Adjustable