LT3473 Просмотр технического описания (PDF) - Linear Technology
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LT3473 Datasheet PDF : 12 Pages
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LT3473/LT3473A
APPLICATIO S I FOR ATIO
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 VC.
When VC is below the Burst Mode threshold voltage, BTH,
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, VC 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
Switching Waveforms
Switching Waveforms
Transient Response
IL
200mA/DIV
VSW
10V/DIV
VIN = 3.6V
VOUT = 20V
ILOAD = 50mA
0.5µs/DIV
IL
200mA/DIV
VSW
10V/DIV
3473 AI01
VIN = 3.6V
VOUT = 20V
ILOAD = 8mA
0.5µs/DIV
VOUT
200mV/DIV
IL
200mA/DIV
3473 AI02
11mA
ILOAD
1mA
VIN = 3.6V
VOUT = 20V
500µs/DIV
3473 AI03
Transient Response
Transient Response
Transient Response
VOUT
500mV/DIV
IL
500mA/DIV
51mA
ILOAD 1mA
VIN = 3.6V
VOUT = 20V
500µs/DIV
VOUT
500mV/DIV
IL
500mA/DIV
55mA
ILOAD
5mA
3473 AI04
VIN = 3.6V
VOUT = 20V
500µs/DIV
VOUT
500mV/DIV
IL
500mA/DIV
3473 AI05
75mA
ILOAD 25mA
VIN = 3.6V
VOUT = 20V
200µs/DIV
3473 AI06
Shutdown Waveforms
Start-Up Waveforms
VOUT
10V/DIV
CAP
0.5V/DIV
SHDN
5V/DIV
VIN = 3.6V
VOUT = 20V
ILOAD = 60mA
100µs/DIV
3473 AI07
VOUT
10V/DIV
IL
500mA/DIV
SHDN
2V/DIV
VIN = 3.6V
500µs/DIV
VOUT = 20V
ILOAD = 30mA
SHDN 20k, 100nF
3473 AI08
3473f
7
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