MAX1837 データシートの表示（PDF） - Maxim Integrated

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MAX1837

24V Internal Switch, 100% Duty Cycle, Step-Down Converters

Maxim Integrated 

MAX1836/MAX1837

24V Internal Switch, 100% Duty Cycle,

Step-Down Converters

Input-Output (Dropout) Voltage

A step-down converter’s minimum input-to-output volt-

age differential (dropout voltage) determines the lowest

useable input supply voltage. In battery-powered sys-

tems, this limits the useful end-of-life battery voltage. To

maximize battery life, the MAX1836/MAX1837 operate

with duty cycles up to 100%, which minimizes the input-

to-output voltage differential. When the supply voltage

approaches the output voltage, the P-channel MOSFET

remains on continuously to supply the load.

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. For a step-down con-

verter with 100% duty cycle, the dropout voltage depends

on the MOSFET drain-to-source on-resistance (RDS(ON))

and inductor series resistance; therefore, it is proportional

to the load current:

( ) VDROPOUT = IOUT × RDS(ON) + RINDUCTOR

Shutdown (SHDN)

A logic-level low voltage on SHDN shuts down the

MAX1836/MAX1837. When shut down, the supply cur-

rent drops to 3μA to maximize battery life, and the internal

P-channel MOSFET turns off to isolate the output from the

input. The output capacitance and load current determine

the rate at which the output voltage decays. A logic-level

high voltage on SHDN activates the MAX1836/MAX1837.

Do not leave SHDN floating. If unused, connect SHDN to

IN. When setting output voltages above 5.5V, the shut-

down feature cannot be used, so SHDN must be perma-

nently connected to IN. The SHDN input voltage slew rate

must be greater than 10V/ms.

Thermal-Overload Protection

Thermal-overload protection limits total power dissipa-

tion in the MAX1836/MAX1837. When the junction tem-

perature exceeds TJ = +160°C, a thermal sensor turns off

the pass transistor, allowing the IC to cool. The thermal

sensor turns the pass transistor on again after the IC’s

junction temperature cools by 10°C, resulting in a pulsed

output during continuous thermal-overload conditions.

Design Information

Output Voltage Selection

The feedback input features dual-mode operation.

Connect the output to OUT and FB to GND for the preset

output voltage. The MAX1836/MAX1837 are supplied

with factory-set output voltages of 3.3V or 5V. The two-

digit part number suffix identifies the output voltage. See

the Selector Guide. For example, the MAX1836EUT33

has a preset 3.3V output voltage.

The MAX1836/MAX1837 output voltage may be adjusted

by connecting a voltage divider from the output to FB

(Figure 5). When externally adjusting the output voltage,

connect OUT to GND. Select R2 in the 10kΩ to 100kΩ

range. Calculate R1 with the following equation:

= R1

R2







VOUT

VFB







−



1



where VFB = 1.25V, and VOUT may range from 1.25V to

VIN. When setting output voltages above 5.5V, the shut-

down feature cannot be used, so SHDN must be perma-

nently connected to IN.

Inductor Selection

When selecting the inductor, consider these four param-

eters: inductance value, saturation current rating, series

resistance, and size. The MAX1836/MAX1837 operate

with a wide range of inductance values. For most applica-

tions, values between 10μH and 100μH work best with

the controller’s switching frequency. Calculate the mini-

mum inductance value as follows:

( ) L(MIN) =

VIN(MAX) − VOUT

ILIM

t ON(MIN)

where tON(MIN) = 1.0μs. Inductor values up to six times

L(MIN) are acceptable. Low-value inductors may be small-

er in physical size and less expensive, but they result in

higher peak-current overshoot due to current-sense com-

parator propagation delay (300ns). Peak-current over-

shoot reduces efficiency and could exceed the current

ratings of the internal switching MOSFET and external

components.

INPUT

4.5V OR 24V

CIN

IN

LX

SHDN

FB

MAX1836

MAX1837

GND

OUT

OUTPUT

L1

1.25V TO VIN

D1

R1

COUT

R2

NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES.

Figure 5. Adjustable Output Voltage

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