Datasheet

7
LT3473/LT3473A
3473f
Operation
The LT3473 combines a current mode, fixed frequency
PWM architecture with Burst Mode micropower operation
to maintain high efficiency at light loads. Operation can
best be understood by referring to the Block Diagram.
The reference of the part is determined by the lower of the
internal 1.25V bandgap reference and the voltage at the
CTRL pin. The error amplifier compares voltage at the FB
pin with the reference and generates an error signal V
C
.
When V
C
is below the Burst Mode threshold voltage, B
TH
,
the hysteretic comparator, A1, shuts off the power section
leaving only the low power circuitry running. Total current
consumption in this state is minimized. As output loading
causes the FB voltage to decrease, V
C
increases causing A1
to enable the power section circuitry. The chip starts switch-
ing. If the load is light, the output voltage (and FB voltage)
will increase until A1 turns off the power section. The output
voltage starts to fall again. This cycle repeats and gener-
ates low frequency ripple at the output. This Burst Mode
operation keeps the output regulated and reduces average
current into the IC, resulting in high efficiency at light load.
If the output load increases sufficiently, A1’s output remains
high, resulting in continuous operation.
At the start of each oscillator cycle, the SR latch is set,
turning on the power switch Q1. A voltage proportional to
the switch current is added to a stabilizing ramp and the
APPLICATIO S I FOR ATIO
WUUU
I
L
200mA/DIV
V
IN
= 3.6V
V
OUT
= 20V
I
LOAD
= 50mA
0.5µs/DIV
3473 AI01
V
SW
10V/DIV
I
L
200mA/DIV
V
IN
= 3.6V
V
OUT
= 20V
I
LOAD
= 8mA
0.5µs/DIV
3473 AI02
V
SW
10V/DIV
V
OUT
200mV/DIV
V
IN
= 3.6V
V
OUT
= 20V
500µs/DIV
3473 AI03
I
L
200mA/DIV
11mA
1mA
I
LOAD
Switching Waveforms Switching Waveforms Transient Response
Transient Response Transient Response
V
OUT
500mV/DIV
V
IN
= 3.6V
V
OUT
= 20V
500µs/DIV
3473 AI04
I
L
500mA/DIV
51mA
1mA
I
LOAD
V
OUT
500mV/DIV
V
IN
= 3.6V
V
OUT
= 20V
500µs/DIV
3473 AI05
I
L
500mA/DIV
I
LOAD
55mA
5mA
Transient Response
V
OUT
500mV/DIV
V
IN
= 3.6V
V
OUT
= 20V
200µs/DIV
3473 AI06
I
L
500mA/DIV
I
LOAD
75mA
25mA
V
OUT
10V/DIV
V
IN
= 3.6V
V
OUT
= 20V
I
LOAD
= 60mA
100µs/DIV
3473 AI07
CAP
0.5V/DIV
SHDN
5V/DIV
V
OUT
10V/DIV
V
IN
= 3.6V
V
OUT
= 20V
I
LOAD
= 30mA
SHDN 20k, 100nF
500µs/DIV
3473 AI08
IL
500mA/DIV
SHDN
2V/DIV
Shutdown Waveforms Start-Up Waveforms