MAX1744 Ver la hoja de datos (PDF) - Maxim Integrated

Número de pieza

componentes Descripción

Fabricante

MAX1744

High-Voltage / Step-Down DC-DC Controller in MAX

Maxim Integrated 

High-Voltage, Step-Down DC-DC

Controllers in µMAX

Pin Description

NAME

PIN

MAX1744 MAX1745

FUNCTION

1

GND

2

VL

3

REF

4

3/5

4

—

5

OUT

6

CS

7

SHDN

8

VH

9

EXT

GND

VL

REF

—

FB

OUT

CS

SHDN

VH

EXT

Ground

5V Linear Regulator Output. VL provides power to the internal circuitry and can supply up

to 1mA to an external load. Bypass VL to GND with 4.7μF or greater capacitor.

1.25V Reference Output. REF can supply up to 100μA to an external load. Bypass REF to

GND with a 0.1μF or greater ceramic capacitor.

3.3V or 5V Selection. Connect 3/5 to GND to set the output voltage to 3.3V. Connect 3/5 to

VL to set the output voltage to 5V.

Feedback Input for Adjustable Output Operation. Connect to an external voltage-divider

between the output and FB to set the output voltage. The regulation voltage threshold is

1.25V.

Sense Input for Fixed 5V or 3.3V Output Operation (MAX1744) and Negative Current-Sense

Input (MAX1744/5). OUT is connected to an internal voltage-divider (MAX1744). OUT does

not supply current.

Current-Sense Input. Connect the current-sense resistor between CS and OUT. External

MOSFET is turned off when the voltage across the resistor is equal to or greater than the

current limit trip level (100mV).

Active-Low Shutdown Input. Connect SHDN to IN for normal operation. Drive SHDN to low

to shut the part off. In shutdown mode, the reference, output, external MOSFET, and

internal regulators are turned off.

High-Side Linear Regulator Output. VH provides a regulated output voltage that is 5V below

IN. The external P-channel MOSFET gate is driven between IN and VH. Bypass VH to IN

with a 4.7μF or greater capacitor (see the Capacitor Selection section).

Gate Drive for External P-Channel MOSFET. EXT swings between IN and VH.

10

IN

IN

Positive Supply Input. Bypass IN to GND with a 0.47μF or greater ceramic capacitor.

Detailed Description

The MAX1744/MAX1745 are high-voltage step-down

DC-DC converter controllers. These devices offer high

efficiency over a wide range of input/output voltages

and currents, making them optimal for use in applica-

tions such as telecom, automotive, and industrial con-

trol. Using an external P-channel MOSFET and

current-sense resistor allows design flexibility and

improved efficiency. The MAX1744/MAX1745 automati-

cally switch from PWM operation at medium and heavy

loads to pulse-skipping operation at light loads to

improve light-load efficiency. The low 90μA quiescent

current further optimizes these parts for applications

where low input current is critical. Operation to 100%

duty cycle allows the lowest possible dropout voltage,

which allows a wider input voltage variation. The small

size, high switching frequency, and low parts count

minimize the required circuit board area and compo-

nent cost. Figure 1 shows the MAX1744 typical applica-

tion circuit.

Operating Modes

When delivering low output currents, the MAX1744/

MAX1745 operate in discontinuous-conduction mode.

Current through the inductor starts at zero, rises as

high as the current limit, then ramps down to zero dur-

ing each cycle (Figure 3). The switch waveform exhibits

ringing, which occurs at the resonant frequency of the

inductor and stray capacitance, due to residual energy

trapped in the core when the commutation diode (D1 in

Figure 1) turns off.

When delivering medium-to-high output currents, the

MAX1744/MAX1745 operate in PWM continuous-con-

duction mode (Figure 4). In this mode, current always

flows through the inductor and never ramps to zero.

The control circuit adjusts the switch duty cycle to

maintain regulation without exceeding the peak switch-

ing current set by the current-sense resistor.

_______________________________________________________________________________________ 7

Share Link: