LT1936H Просмотр технического описания (PDF) - Linear Technology

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LT1936H

1.4A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1936

APPLICATIONS INFORMATION

FB Resistor Network

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resis-

tors according to:

R1=

R2

VOUT

1.200

–



1

R2 should be 20k or less to avoid bias current errors.

Reference designators refer to the Block Diagram.

Input Voltage Range

The input voltage range for LT1936 applications depends

on the output voltage and the Absolute Maximum Ratings

of the VIN and BOOST pins.

The minimum input voltage is determined by either the

LT1936’s minimum operating voltage of ~3.45V or by its

maximum duty cycle. The duty cycle is the fraction of

time that the internal switch is on and is determined by

the input and output voltages:

DC

=

VOUT + VD

VIN – VSW + VD

where VD is the forward voltage drop of the catch diode

(~0.5V) and VSW is the voltage drop of the internal switch

(~0.5V at maximum load). This leads to a minimum input

voltage of:

VIN(MIN)

=

VOUT + VD

DCMAX

–

VD

+

VSW

with DCMAX = 0.87.

The maximum input voltage is determined by the absolute

maximum ratings of the VIN and BOOST pins and by the

minimum duty cycle DCMIN = 0.08:

VIN(MAX)

=

VOUT + VD

DCMIN

–

VD

+

VSW

Note that this is a restriction on the operating input voltage;

the circuit will tolerate transient inputs up to the absolute

maximum ratings of the VIN and BOOST pins.

8

Inductor Selection and Maximum Output Current

A good first choice for the inductor value is

L = 2.2 (VOUT + VD)

where VD is the voltage drop of the catch diode (~0.4V)

and L is in μH. With this value the maximum output cur-

rent will be above 1.2A at all duty cycles and greater than

1.4A for duty cycles less than 50% (VIN > 2 VOUT). The

inductor’s RMS current rating must be greater than the

maximum load current and its saturation current should be

about 30% higher. For robust operation in fault conditions

(start-up or short circuit) and high input voltage (>30V),

the saturation current should be above 2.6A. To keep the

efficiency high, the series resistance (DCR) should be less

than 0.1Ω, and the core material should be intended for

high frequency applications. Table 1 lists several vendors

and suitable types.

Table 1. Inductor Vendors

VENDOR URL

Murata www.murata.com

TDK

www.component.tdk.com

Toko

www.toko.com

Sumida www.sumida.com

PART SERIES

LQH55D

SLF7045

SLF10145

D62CB

D63CB

D75C

D75F

CR54

CDRH74

CDRH6D38

CR75

TYPE

Open

Shielded

Shielded

Shielded

Shielded

Shielded

Open

Open

Shielded

Shielded

Open

Of course, such a simple design guide will not always

result in the optimum inductor for your application. A

larger value provides a slightly higher maximum load

current and will reduce the output voltage ripple. If your

load is lower than 1.2A, then you can decrease the value

of the inductor and operate with higher ripple current. This

allows you to use a physically smaller inductor, or one

with a lower DCR resulting in higher efficiency. Be aware

that if the inductance differs from the simple rule above,

then the maximum load current will depend on input volt-

age. There are several graphs in the Typical Performance

Characteristics section of this data sheet that show the

maximum load current as a function of input voltage and

inductor value for several popular output voltages. Low

1936fd

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