LT1767EMS8E-3.3-PBF Ver la hoja de datos (PDF) - Linear Technology

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LT1767EMS8E-3.3-PBF

Monolithic 1.5A, 1.25MHz Step-Down Switching Regulators

Linear Technology 

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

APPLICATIONS INFORMATION

CATCH DIODE

The suggested catch diode (D1) is a UPS120 Schottky, or

its Motorola equivalent, MBRM120LTI/MBRM130LTI. It is

rated at 2A average forward current and 20V/30V reverse

voltage. Typical forward voltage is 0.5V at 1A. The diode

conducts current only during switch off time. Peak reverse

voltage is equal to regulator input voltage. Average forward

current in normal operation can be calculated from:

( ) ID(AVG) = IOUT

VIN − VOUT

VIN

Shutdown and Undervoltage Lockout

Figure 4 shows how to add undervoltage lockout (UVLO)

to the LT1767. 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.

BOOST PIN

For most applications, the boost components are a 0.1µF

capacitor and a CMDSH-3 diode. The anode is typically

connected to the regulated output voltage to generate

a voltage approximately VOUT above VIN to drive the

output stage. The output driver requires at least 2.7V of

headroom throughout the on period to keep the switch

fully saturated. However, the output stage discharges the

boost capacitor during the on time. 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 2x 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, an independent supply can be used with a local

bypass capacitor.

A 0.1µF 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

700ns on-time, 50mA boost current, and 0.7V discharge

ripple. This value is then guard banded by 2x for secondary

factors such as capacitor tolerance, ESR and temperature

effects. 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.

LT1767

VIN

IN

7µA

1.33V

R1

3µA

SHDN

C1 R2

GND

OUTPUT

VSW

VCC

+

1767 F04

Figure 4. Undervoltage Lockout

An internal comparator will force the part into shutdown

below the minimum VIN of 2.6V. This feature can be

used to prevent excessive discharge of battery-operated

systems. If an adjustable UVLO threshold is required, the

shutdown pin can be used. The threshold voltage of the

shutdown pin comparator is 1.33V. A 3µA internal current

source defaults the open pin condition to be operating (see

Typical Performance Graphs). Current hysteresis is added

above the SHDN threshold. This can be used to set voltage

hysteresis of the UVLO using the following:

R1= VH − VL

7µA

R2 =

(VH

−

1.33V

1.33V)

+

3µA

R1

VH – Turn-on threshold

VL – Turn-off threshold

For more information www.linear.com/LT1767

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