MAX1744 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1744

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

Maxim Integrated 

High-Voltage, Step-Down DC-DC

Controllers in µMAX

INPUT

4.5V TO 36V

C2

4.7μF

LOW ESR

ON

OFF

5V

3.3V

0.47μF

IN

SHDN

3/5

C3

4.7μF

D2

VH

EXT

MAX1744 CS

VL

OUT

REF

GND

P M1

FAIRCHILD

NDS9407

L1

22μH

D1

NIHON

EC2IQ506

RSENSE

40mΩ

C1

220μF

OUT

3.3V OR 5V

2A

Figure 1. Typical Application Circuit

100% Duty Cycle and Dropout

The MAX1744/MAX1745 operate with a duty cycle up to

100%. This feature extends the input voltage range by

turning the MOSFET on continuously when the supply

voltage approaches the output voltage. This services

the load when conventional switching regulators with

less than 100% duty cycle would fail. Dropout voltage is

defined as the difference between the input and output

voltages when the input is low enough for the output to

drop out of regulation. Dropout depends on the

MOSFET drain-to-source on-resistance, current-sense

resistor, and inductor series resistance, and is propor-

tional to the load current:

Dropout voltage=

IOUT x ⎡⎣RDS(ON) + RSENSE+ RINDUCTOR ⎤⎦

Regulation Control Scheme

The MAX1744/MAX1755 have a unique operating

scheme that allows PWM operation at medium and high

current, with automatic switching to pulse-skipping

mode at lower currents to improve light-load efficiency.

Figure 2 shows the simplified block diagram.

Under medium- and heavy-load operation, the inductor

current is continuous and the part operates in PWM

mode. In this mode, the switching frequency is set by

either the 1μs minimum on-time or the 2μs minimum off-

time, depending on the duty cycle. The duty cycle is

approximately the output voltage divided by the input

voltage. If the duty cycle is less than 33%, the minimum

on-time controls the frequency; and the frequency is

approximately f ≈ 1MHz ✕ D, where D is the duty cycle.

If the duty cycle is greater than 33%, the off-time sets the

frequency; and the frequency is approximately f ≈ 500kHz

✕ (1 - D).

In both cases, the voltage is regulated by the error

comparator. For low duty cycles (<33%), the MOSFET

is turned on for the minimum on-time, causing fixed-on-

time operation. During the MOSFET on-time, the output

voltage rises. Once the MOSFET is turned off, the volt-

age drops to the regulation threshold (set by the inter-

nal voltage-divider for the MAX1745 and by the external

voltage-divider for the MAX1744), at which time another

cycle is initiated. For high duty cycles (>33%), the

MOSFET remains off for the minimum off-time, causing

fixed-off-time operation. In this case, the MOSFET

remains on until the output voltage rises to the regula-

tion threshold. Then the MOSFET turns off for the mini-

mum off-time, initiating another cycle.

By switching between fixed-on-time and fixed-off-time

operation, the MAX1744/MAX1745 can operate at high

input-output ratios, yet still operate up to 100% duty

cycle for low dropout. Note that when transitioning from

fixed-on-time to fixed-off-time operation, the output volt-

age drops slightly due to the output ripple voltage. In

fixed-on-time operation, the minimum output voltage is

regulated, but in fixed-off-time operation, the maximum

output voltage is regulated. Thus, as the input voltage

drops below approximately three times the output volt-

age, a decrease in line regulation can be expected.

The drop in voltage is approximately VDROP ≈ VRIPPLE / 2.

At light output loads, the inductor current is discontinu-

ous, causing the MAX1744/MAX1745 to operate at

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