MAX1745EUB+ Datasheet MAX1744EUB+, MAX1745EUB+, MAX1744EUB+T | P7-P9

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

_______________________________________________________________________________________ 7

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.

Pin Description

NAME

MAX1744 MAX1745

FUNCTION

1 GND GND Ground

2VL VL

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.

3 REF REF

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.

4 3/5 —

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.

4— FB

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.

5 OUT OUT

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.

6CS CS

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).

7 SHDN SHDN

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.

8VH VH

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).

9 EXT EXT 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.

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

8 _______________________________________________________________________________________

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 ≈ 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 V

DROP

≈ V

RIPPLE

/ 2.

At light output loads, the inductor current is discontinu-

ous, causing the MAX1744/MAX1745 to operate at

Figure 1. Typical Application Circuit

GND

REF

MAX1744

3/5

3.3V

OFF

SHDN

EXT

INPUT

4.5V TO 36V

NIHON

EC2IQ506

FAIRCHILD

NDS9407

OUT

3.3V OR 5V

220μF

22μH

SENSE

40mΩ

4.7μF

LOW ESR

0.47μF

Dropout voltage=

IxR +R

OUT DS(ON) SENNSE INDUCTOR

⎡

⎣

⎤

⎦

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

_______________________________________________________________________________________ 9

lower frequencies, reducing the MOSFET gate drive

and switching losses. In discontinuous mode, under

most circumstances, the on-time will be the fixed mini-

mum on-time of 1μs. If the inductor value is small, or

the current-sense resistor large, the current limit will be

tripped before the minimum on-time, terminating the

on-time and thus setting the fixed on-time.

If the inductance is too large, or the output capacitance

high and equivalent series resistance (ESR) low, then

the MOSFET remains on longer than the minimum on-

time, until the output capacitor charges beyond the

error comparator’s (V

OUT

/ 1.25V)

8mV hysteresis,

causing the part to operate in hysteretic mode.

Operating in hysteretic mode results in lower frequency

operation. The transition to hysteretic mode occurs at

the critical output capacitor ESR:

ESR

CRITICAL

= (V

OUT

/ 1.25V)

8mV / I

RIPPLE

where I

RIPPLE

is the inductor ripple current, and can be

determined by:

RIPPLE

= (V

- V

OUT

)

ON(MIN)

/ L

where t

ON(MIN)

is the minimum on-time (1μs) for mini-

mum on-time-control, or:

RIPPLE

= (V

OUT

)

OFF(MIN)

/ L

where t

OFF(MIN)

is the minimum off-time (2μs) for mini-

mum off-time-control.

REF

MINIMUM

OFF-TIME

ONE SHOT

Q TRIG

LINEAR

REGULATOR

LINEAR

REGULATOR

1.25

REFERENCE

(FB)

ERROR

COMPARATOR

OUT

3/5

SHDN

100mV

SHDN

EXT

MINIMUM

ON-TIME

ONE SHOT

Q TRIG

( ) MAX1745 ONLY

- - - MAX1744 ONLY

Figure 2. Simplified Functional Diagram

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15

MAX1745EUB+

Mfr. #:

Buy MAX1745EUB+

Manufacturer:

Maxim Integrated

Description:

Switching Controllers Step-Down DC/DC Controller

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MAX1745EUB+

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