High-Voltage, Step-Down DC-DC

Controllers in µMAX

8

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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:

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

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

frequency; and the frequency is approximately f

≈ 500kHz

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.

At light output loads, the inductor current is discontinu-

ous, causing the MAX1744/MAX1745 to operate at

Figure 1. Typical Application Circuit

GND

REF

VL

D2

IN

MAX1744

VH

3/5

5V

3.3V

ON

OFF

SHDN

EXT

INPUT

4.5V TO 36V

P

D1

NIHON

EC2IQ506

M1

FAIRCHILD

NDS9407

OUT

CS

OUT

3.3V OR 5V

2A

C1

220

μF

L1

22

μH

40m

Ω

C3

4.7

μF

C2

4.7

μF

LOW ESR

0.47

μF

Dropout voltage=

Ix R

+ R

OUT

DS(ON)

SEN

NSE

INDUCTOR

+R

⎡⎣

⎤⎦