1766 Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Fabricante

1766

5.5V to 60V 1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1766/LT1766-5

APPLICATIONS INFORMATION

BOOST PIN

For most applications, the boost components are a 0.33μF

capacitor and a 1N4148W diode. The anode is typically

connected to the regulated output voltage to generate a

voltage approximately VOUT above VIN to drive the output

stage. However, the output stage discharges the boost ca-

pacitor during the on time of the switch. The output driver

requires at least 3V of headroom throughout this period

to keep the switch fully saturated. If the output voltage is

less than 3.3V, it is recommended that an alternate boost

supply is used. The boost diode can be connected to the

input, although, care must be taken to prevent the 2× VIN

boost voltage from exceeding the BOOST pin absolute

maximum rating. The additional voltage across the switch

driver also increases power loss, reducing efficiency. If

available, and independent supply can be used with a local

bypass capacitor.

A 0.33μF boost capacitor is recommended for most ap-

plications. Almost any type of film or ceramic capacitor

is suitable, but the ESR should be <1Ω to ensure it can

be fully recharged during the off time of the switch. The

capacitor value is derived from worst-case conditions of

4700ns on time, 42mA boost current and 0.7V discharge

ripple. The boost capacitor value could be reduced under

less demanding conditions, but this will not improve cir-

cuit operation or efficiency. Under low input voltage and

low load conditions, a higher value capacitor will reduce

discharge ripple and improve start-up operation.

SHUTDOWN FUNCTION AND UNDERVOLTAGE

LOCKOUT

Figure 4 shows how to add undervoltage lockout (UVLO)

to the LT1766. Typically, UVLO is used in situations where

the input supply is current limited, or has a relatively high

source resistance. A switching regulator draws constant

power from the source, so source current increases as

source voltage drops. This looks like a negative resistance

load to the source and can cause the source to current limit

or latch low under low source voltage conditions. UVLO

prevents the regulator from operating at source voltages

where these problems might occur.

Threshold voltage for lockout is about 2.38V. A 5.5μA

bias current flows out of the pin at this threshold. The

internally generated current is used to force a default high

state on the shutdown pin if the pin is left open. When

low shutdown current is not an issue, the error due to this

current can be minimized by making RLO 10k or less. If

shutdown current is an issue, RLO can be raised to 100k,

but the error due to initial bias current and changes with

temperature should be considered.

RLO = 10k to 100k (25k suggested)

RHI

=

RLO(

2.38V

VIN − 2.38V)

− RLO(5.5μA)

VIN = Minimum input voltage

RFB

INPUT

RHI

C2

RLO

LT1766

IN

SHDN

2.38V

+

5.5μA

–

+

0.4V

–

GND

VSW

STANDBY

TOTAL

SHUTDOWN

L1

OUTPUT

+

C1

Figure 4. Undervoltage Lockout

1766 F04

1766fc

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