6

LT1303/LT1303-5

I

DAGRA

BLOCK

S

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 3

Ω 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

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 140

µA. 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 6

µs or until

current comparator C2 trips, whichever comes first. The

switch then turns off for approximately 1.5

µs,thenturnson

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

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 100

µF unit specified results in

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

µF

capacitor will decrease ripple by approximately 50%.

–

+

–

+

6

7

8

3

2

5

1

4

R1

3

Ω

Q2

1

×

Q1

160

×

SHUTDOWN

LBO

LBI

GND

PGND

R1

474k

R2

156k

C3

C2

C1

FB

SW

OFF

OSCILLATOR

A3

DRIVER

CURRENT

COMPARATOR

HYSTERETIC

COMPARATOR

REFERENCE

1.24V

18mV

LT1303 BD02

Figure 3. LT1303-5 Block Diagram

20

µs/DIV

100mV/DIV

AC COUPLED

LT1303 F04

Figure 4. Burst Mode Operation in Action

1A/DIV

5V/DIV