5
LT1300
TEST CIRCUIT
Oscillator Test Circuit
OPERATION
U
20µs/DIV
reduced by tying the I
LIM
pin to ground, causing 15µA to
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,
trace B. The reduced peak switch current reduces I
2
R
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 120µA. 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) 5.3µs or until current comparator A2 trips,
whichever comes first. After a fixed off-time of (nominally)
1.2µs, 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
load until V
OUT
decreases enough to force A1’s output
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 3Ω 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 2µs as shown in
Figure 2, trace A. This programmed peak current can be
Figure 3. Burst Mode Operation in Action
TRACE B
500mA/DIV
I
LIM
PIN
GROUNDED
LT1300 F2
Figure 2. Switch Pin Current With I
LIM
Floating or Grounded
TRACE A
500mA/DIV
I
LIM
PIN
OPEN
V
OUT
20mV/DIV
AC COUPLED
V
SW
5V/DIV
I
SW
IA/DIV
20µS/DIV
LT1300 F2
2V
100µF
V
IN
SEL
SENSE
GND PGND
SHDN
SW
LT1300
I
L
100Ω
5V
f
OUT
