24V, 600mA Internal Switch, 100% Duty Cycle,

Step-Down Converter

8

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Detailed Description

The MAX1776 step-down converter is designed primar-

ily for battery-powered devices and notebook comput-

ers. The unique current-limited control scheme

provides high efficiency over a wide load range.

Operation up to 100% duty cycle allows the lowest pos-

sible dropout voltage, increasing the usable supply

voltage range. Under no load, the MAX1776 draws only

15µA, and in shutdown mode, it draws only 3µA to fur-

ther reduce power consumption and extend battery life.

Additionally, an internal 24V switching MOSFET, inter-

nal current sensing, and a high switching frequency

minimize PC board space and component costs.

Current-Limited Control Architecture

The MAX1776 uses a proprietary current-limited control

scheme with operation to 100% duty cycle. This DC-DC

converter pulses as needed to maintain regulation,

resulting in a variable switching frequency that increas-

es with the load. This eliminates the high supply cur-

rents associated with conventional constant-frequency

pulse-width-modulation (PWM) controllers that switch

the MOSFET unnecessarily.

When the output voltage is too low, the error comparator

sets a flip-flop, which turns on the internal P-channel

MOSFET and begins a switching cycle (Figure 2). As

shown in Figure 3, the inductor current ramps up linear-

ly, storing energy in a magnetic field while charging the

output capacitor and servicing the load. The MOSFET

turns off when the peak current limit is reached, or when

the maximum on-time of 10µs is exceeded and the out-

put voltage is in regulation. If the output is out of regula-

tion and the peak current is never obtained, the

MOSFET remains on, allowing a duty cycle up to 100%.

This feature ensures the lowest possible dropout volt-

age. Once the MOSFET turns off, the flip-flop resets, the

inductor current is pulled through D1, and the current

through the inductor ramps back down, transferring the

stored energy to the output capacitor and load. The

MOSFET remains off until the 0.42µs minimum off-time

expires, and the output voltage drops out of regulation.

Pin Description

PIN

NAME

FUNCTION

1FB

Dual-Mode Feedback Input. Connect to GND for the preset 5V output. Connect to a resistive divider

2

GND

Ground

3

ILIM

Peak Current Control Input. Connect to IN or GND to set peak current limit. ILIM and ILIM2 together set

the peak current limit. See Setting Current Limit.

4

LX

Inductor Connection. Connect LX to external inductor and diode as shown in Figure 1.

5

IN

Input Supply Voltage. Input voltage range is 4.5V to 24V.

6

ILIM2

Peak Current Control Input 2. Connect to IN or GND. ILIM and ILIM2 together set the peak current limit.

See Setting Current Limit.

7

SHDN

Shutdown Input. A logic low shuts down the MAX1776 and reduces the supply current to 3

µA. LX is high

impedance in shutdown. Connect to IN for normal operation.

8

OUT

Regulated Output Voltage High-Impedance Sense Input. Internally connected to a resistive divider.

Do not connect for output voltages higher than 5.5V. Connect to GND when not used.

Figure 1. Typical Application Circuit

MAX1776

IN

GND

ILIM2

ILIM

LX

OUT

FB

µF, 25V CERAMIC

SEE TABLE 3 FOR OTHER COMPONENT VALUES

NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.

OUTPUT

5V

INPUT

4.5V TO 24V

D1

L1

SHDN

J1

J2

J3

J4