Datasheet

6
LT1302/LT1302-5
BLOCK DIAGRA SM
W
The LT1302’s operation can best be understood by
examining the block diagram in Figure 2. The LT1302
operates in one of two modes, depending on load. With
light loads, comparator CMP1 controls the output; with
heavy loads, control is passed to error amplifier A1.
Burst Mode operation consists of monitoring the FB pin
voltage with hysteretic comparator CMP1. When the FB
voltage, related to the output voltage by external attenu-
ator R1 and R2, falls below the 1.24V reference voltage,
the oscillator is enabled. Switch Q4 alternately turns on,
causing current buildup in inductor L1, then turns off,
allowing the built-up current to flow into output capaci-
tor C3 via D1. As the output voltage increases, so does
the FB voltage; when it exceeds the reference plus
OPERATIO
U
CMP1’s hysteresis (about 5mV) CMP1 turns the oscilla-
tor off. In this mode, peak switch current is limited to
approximately 1A by A2, Q2, and Q3. Q2’s current, set at
34µA, flows through R5, causing A2’s negative input to
be 25mV lower than V
IN
. This node must fall more than
36mV below V
IN
for A2 to trip and turn off the oscillator.
The remaining 11mV is generated by Q3’s current flow-
ing through R4. Emitter-area scaling sets Q3’s collector
current to 0.625% of switch Q4’s current. When Q4’s
current is 1A, Q3’s current is 6.25mA, creating an 11mV
drop across R4 which, added to R5’s 25mV drop, is
enough to trip A2.
When the output load is increased to the point where the
1A peak current cannot support the output voltage,
1302 F03
–
+
–
+
–
+
V
OS

15mV
36mV
CMP1
OFF
V
IN
2 µA
ENABLE
A2
V
IN
SW
PGND
I
T
V
C
GND
SHDN
SHUTDOWN
SENSE
ERROR
AMPLIFIER
HYSTERETIC
COMPARATOR
DRIVER
A1
4
3
5 8
7
21
1.24V
REFERENCE
220kHz
OSCILLATOR
300Ω 3.6k
Q3
R4
1.75Ω
R5
730Ω
V
IN
BIAS
R1
315k
R2
105k
6
Q4
160X
A3
Q1 Q2
Q5
Figure 3. LT1302-5 Block Diagram