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

Synchronous Rectification and Internal Switches

6

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_______________Detailed Description

The MAX1830/MAX1831 synchronous, current-mode,

constant-off-time, PWM DC-DC converters step down

input voltages of +3V to +5.5V to preset output voltages,

MAX1830 has preset outputs +2.5V, +1.8V, and +1.5V.

The MAX1831 has preset outputs of +3.3V, +2.5V or

+1.5V. Both devices deliver up to 3A of continuous output

current. Internal switches composed of a 45m

Ω PMOS

power switch and a 55m

Ω NMOS synchronous rectifier

switch improve efficiency, reduce component count, and

eliminate the need for an external Schottky diode.

The MAX1830/MAX1831 optimize efficiency by operat-

ing in constant-off-time mode under heavy loads and in

Maxim’s proprietary Idle Mode under light loads. A sin-

gle resistor-programmable constant-off-time control

sets switching frequencies up to 1MHz, allowing the

user to optimize performance trade-offs in efficiency,

switching noise, component size, and cost. Under low-

dropout conditions, the device operates in a 100%

duty-cycle mode, where the PMOS switch remains con-

tinuously on. Idle Mode enhances light-load efficiency

by skipping cycles, thus reducing transition and gate-

charge losses.

When power is drawn from a regulated supply, constant-

off-time PWM architecture essentially provides constant-

frequency operation. This architecture has the inherent

advantage of quick response to line and load transients.

The MAX1830/MAX1831 current-mode, constant-off-

time PWM architecture regulates the output voltage by

changing the PMOS switch on-time relative to the con-

stant off-time. Increasing the on-time increases the

peak inductor current and the amount of energy trans-

ferred to the load per pulse.

Modes of Operation

The current through the PMOS switch determines the

mode of operation: constant-off-time mode (for load

currents greater than half the Idle Mode threshold), or

Idle Mode (for load currents less than half the Idle

Mode threshold). Current sense is achieved through a

proprietary architecture that eliminates current-sensing

Constant-Off-Time Mode

Constant-off-time operation occurs when the current

through the PMOS switch is greater than the Idle Mode

threshold current (which corresponds to a load current

of half the Idle Mode threshold). In this mode, the regu-

Pin Description

PIN

NAME

FUNCTION

1, 3, 14, 16

LX

Connection for the drains of the PMOS power switch and NMOS synchronous-rectifier switch.

Connect the inductor from this node to the output filter capacitor and load.

2, 4

IN

Supply Voltage Input for the internal PMOS power switch

5

SHDN

Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal

6

COMP

Integrator Amplifier section.

7

TOFF

Off-Time Select Input. Sets the PMOS power switch off-time during constant-off-time operation.

Connect a resistor from TOFF to GND to adjust the PMOS switch off-time.

8FB

Feedback Input for both preset-output and adjustable-output operating modes. Connect directly to

output for fixed-voltage operation or to a resistive divider for adjustable operating modes.

9

GND

Analog Ground

10

REF

Reference Output. Bypass REF to GND with a 1µF capacitor.

11

FBSEL

Feedback Select Input. Selects output voltage. See Table 3 for programming instructions.

12

Ω and 2.2µF

lowpass filter (Figure 1).

13, 15

PGND

Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.