5

LT1300

TEST CIRCUITS

Oscillator Test Circuit

20

µs/DIV

flow through R2 into Q3’s collector. Q3’s current causes

a 10.4mV drop in R2 so that only an additional 7.6mV is

required across R1 to turn off the switch. This corre-

sponds to a 400mA switch current as shown in Figure 2,

loses in Q1, L1, C1 and D1. Efficiency can be increased by

doing this provided that the accompanying reduction in

full load output current is acceptable. Lower peak currents

also extend alkaline battery life due to the alkaline cell’s

high internal impedance. Typical operating waveforms are

shown in Figure 3.

Operation of the LT1300 is best understood by referring to

the Block Diagram in Figure 1. When A1’s negative input,

related to the Sense pin voltage by the appropriate resis-

tor-divider ratio, is higher that the 1.25V reference voltage,

A1’s output is low. A2, A3 and the oscillator are turned off,

drawing no current. Only the reference and A1 consume

current, typically 120

µA. When the voltage at A1’s nega-

tive input decreases below 1.25V, overcoming A1’s 6mV

hysteresis, A1’s output goes high, enabling the oscillator,

current comparator A2, and driver A3. Quiescent current

increases to 2mA as the device prepares for high current

switching. Q1 then turns on in a controlled saturation for

(nominally) 5.3

µs or until current comparator A2 trips,

whichever comes first. After a fixed off-time of (nominally)

1.2

µs, Q1 turns on again. The LT1300’s switching causes

current to alternately build up in L1 and dump into capaci-

tor C2 via D1, increasing the output voltage. When the

output is high enough to cause A1’s output to go to low,

switching action ceases. C2 is left to supply current to the

high, and the entire cycle repeats.

If switch current reaches 1A, causing A2 to trip, switch on-

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

continuous mode operation during bursts. Current com-

parator A2 monitors the voltage across 3

Ω resistor R1

which is directly related to inductor L1’s 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 inductor current, A2’s output

goes high, truncating the on-time portion of the oscillator

cycle and increasing off-time to about 2

µs as shown in

Figure 2, trace A. This programmed peak current can be

Figure 3. Burst Mode Operation in Action

TRACE B

500mA/DIV

GROUNDED

LT1300 F2

TRACE A

500mA/DIV

OPEN

20mV/DIV

AC COUPLED

5V/DIV

IA/DIV

20

µS/DIV

LT1300 F2

2V

100µF

SEL

SENSE

GND

PGND

SHDN

SW

LT1300

100

Ω

5V