lt3473a Linear Technology Corporation, lt3473a Datasheet - Page 7

• Manufacturer Part Number: lt3473a
• Description: Micropower 1a Boost Converter With Schottky And Output Disconnect
• Manufacturer: Linear Technology Corporation

I
APPLICATIO S I FOR ATIO
LOAD
500mV/DIV
500mA/DIV
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
When V
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
200mA/DIV
10V/DIV
51mA
V
1mA
V
OUT
SW
I
I
L
L
C
V
V
V
V
I
LOAD
IN
OUT
IN
OUT
is below the Burst Mode threshold voltage, B
= 3.6V
= 3.6V
= 20V
= 20V
= 50mA
Switching Waveforms
0.5V/DIV
10V/DIV
Transient Response
5V/DIV
SHDN
V
CAP
OUT
U
500µs/DIV
0.5µs/DIV
V
V
I
LOAD
IN
OUT
= 3.6V
= 20V
= 60mA
Shutdown Waveforms
U
100µs/DIV
3473 AI04
3473 AI01
W
I
LOAD
200mA/DIV
500mV/DIV
500mA/DIV
10V/DIV
55mA
V
5mA
V
OUT
SW
I
I
L
L
3473 AI07
U
V
V
I
V
V
LOAD
IN
OUT
IN
OUT
= 3.6V
= 3.6V
= 20V
= 20V
= 8mA
Switching Waveforms
Transient Response
TH
C
.
,
500µs/DIV
0.5µs/DIV
causes the FB voltage to decrease, V
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
500mA/DIV
10V/DIV
2V/DIV
SHDN
V
OUT
IL
V
V
I
SHDN 20k, 100nF
LOAD
IN
OUT
= 3.6V
3473 AI05
3473 AI02
= 20V
= 30mA
Start-Up Waveforms
I
I
200mV/DIV
200mA/DIV
LOAD
LOAD
500mV/DIV
500mA/DIV
500µs/DIV
11mA
75mA
25mA
V
1mA
V
OUT
OUT
I
I
L
L
V
V
V
V
LT3473/LT3473A
IN
OUT
IN
OUT
= 3.6V
= 3.6V
= 20V
= 20V
Transient Response
Transient Response
C
3473 AI08
increases causing A1
500µs/DIV
200µs/DIV
3473 AI03
7
3473 AI06
3473f