LT1303CS8-5 View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LT1303CS8-5

Micropower High Efficiency DC/DC Converters with Low-Battery Detector Adjustable and Fixed 5V

Linear Technology 

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LT1303/LT1303-5

BLOCK DIAGRA S

FB

4

6 VIN

SW

7

R1

474k

R2

156k

GND

1

REFERENCE

1.24V

HYSTERETIC

COMPARATOR

–

C1

+

CURRENT

COMPARATOR

C2

OFF

OSCILLATOR

A3

DRIVER

18mV

R1

3Ω

Q2

1×

Q1

160×

–

C3

+

LBI

5

LBO

2

SHUTDOWN

3

PGND

8

Figure 3. LT1303-5 Block Diagram

LT1303 BD02

U

OPERATION

Operation of the LT1303 is best understood by referring to

the Block Diagram in Figure 2. When C1’s negative input,

related to the output voltage by the appropriate resistor-

divider ratio, is higher than the 1.24V reference voltage,

C1’s output is low. C2, A3 and the oscillator are turned off,

drawing no current. Only the reference and C1 consume

current, typically 140µA. When C1’s negative input drops

below 1.24V and overcomes C1’s 6mV hysteresis, C1’s

output goes high, enabling the oscillator, current compara-

tor C2 and driver A3. Quiescent current increases to 2mA

as the device goes into active switching mode. Q1 then

turns on in controlled saturation for nominally 6µs or until

current comparator C2 trips, whichever comes first. The

switch then turns off for approximately 1.5µs, then turns on

again. The LT1303’s switching causes current to alter-

nately build up in L1 and dump into output capacitor C4 via

D1, increasing the output voltage. When the output is high

enough to cause C1’s output to go high, switching action

ceases. Capacitor C4 is left to supply current to the load

until VOUT decreases enough to force C1’s output high, and

the entire cycle repeats. Figure 4 details relevant wave-

forms. C1’s cycling causes low-to-mid-frequency ripple

voltage on the output. Ripple can be reduced by making the

output capacitor large. The 100µF unit specified results in

ripple of 50mV to 100mV on the 5V output. A 220µF

capacitor will decrease ripple by approximately 50%.

VOUT

100mV/DIV

AC COUPLED

VSW

5V/DIV

IL

1A/DIV

20µs/DIV

LT1303 F04

Figure 4. Burst Mode Operation in Action

If switch current reaches 1A, causing C2 to trip, switch on-

time is reduced and off-time increases slightly. This allows

continuous operation during bursts. C2 monitors the

voltage across 3Ω resistor R1 which is directly related to

the switch current. Q2’s collector current is set by the

emitter-area ratio to 0.6% of Q1’s collector current. When

R1’s voltage drop exceeds 18mV, corresponding to 1A

switch current, C2’s output goes high, truncating the on-

time portion of the oscillator cycle and increasing off-time

6

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