LT1961EMS8E 데이터 시트보기 (PDF) - Linear Technology

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LT1961EMS8E

1.5A, 1.25MHz Step-Up Switching Regulator

Linear Technology 

LT1961

APPLICATIONS INFORMATION

CATCH DIODE

The suggested catch diode (D1) is a UPS120 or 1N5818

Schottky. It is rated at 1A 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 output

voltage. Average forward current in normal operation is

equal to output current.

SHUTDOWN AND UNDERVOLTAGE LOCKOUT

Figure 4 shows how to add undervoltage lockout (UVLO)

to the LT1961. 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.

INPUT

LT1961

IN

R1

SHDN

7μA

1.35V

3μA

C1

R2

GND

VCC

1961 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 sys-

tems. If an adjustable UVLO threshold is required, the

shutdown pin can be used. The threshold voltage of the

shutdown pin comparator is 1.35V. 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 =

1.35V

VH − 1.35V

+ 3μA

R1

VH – Turn-on threshold

VL – Turn-off threshold

Example: switching should not start until the input is

above 4.75V and is to stop if the input falls below 3.75V.

VH = 4.75V

VL = 3.75V

R1= 4.75V − 3.75V = 143k

7μA

( ) R2 =

1.35V

4.75V − 1.35V

= 50.4k

+ 3μA

143k

Keep the connections from the resistors to the SHDN pin

short and make sure that the interplane or surface capaci-

tance to the switching nodes are minimized. If high resis-

tor values are used, the SHDN pin should be bypassed with

a 1nF capacitor to prevent coupling problems from the

switch node.

1961fa

9

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