ltc3803es6-5 Linear Technology Corporation, ltc3803es6-5 Datasheet - Page 12

• Manufacturer Part Number: ltc3803es6-5
• Description: Constant Frequency Current Mode Flyback Dc/dc Controller In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3803-5
12
effectively reducing the current comparator threshold in
proportion to duty cycle. This stabilizes the control loop
against subharmonic oscillation. The amount of reduction
in the current comparator threshold (ΔV
calculated using the following equation:
Note: LTC3803-5 enforces 6.5% < Duty Cycle < 80%.
A good starting value for R
drop in current comparator threshold at 80% duty cycle.
Designs not needing slope compensation may replace
R
V
An internal shunt regulator allows the LTC3803-5 to be
powered through a single dropping resistor from V
V
closely decouples V
regulator can draw up to 25mA through the V
CC
CC
SL
Δ
, in conjunction with a bypass capacitor, C
with a short circuit.
SHUNT REGULATOR
V
SENSE
=
Figure 3. Powering the LTC3803-5
Via the Internal Shunt Regulator
Duty Cycle
U
V
CC
IN
73 5
R
VCC
C
to GND (see Figure 3). The shunt
VCC
. %
U
SL
– . %
6 5
V
is 5.9k, which gives a 30mV
LTC3803-5
CC
GND
38035 F03
•
W
5
μ
A R
•
SENSE
SL
U
CC
) can be
VCC
pin to
, that
IN
to
GND to drop enough voltage across R
to around 8V. For applications where V
such that the static power dissipation in R
able, using the V
power the LTC3803-5.
EXTERNAL PREREGULATOR
The circuit in Figure 4 shows another way to power the
LTC3803-5. An external series preregulator consisting of
series pass transistor Q1, Zener diode D1, and bias
resistor R
enabling the LTC3803-5.
B
brings V
Figure 4. Powering the LTC3803-5
with an External Preregulator
R
100k
B
D1
6.8V
CC
75 V
8V TO
shunt regulator is the simplest way to
CC
Q1
MMBTA42
IN
above the V
C
0.1μF
VCC
CC
V
LTC3803-5
CC
VCC
turn-on threshold,
GND
IN
to regulate V
38035 F04
is low enough
VCC
is accept-
38035fb
CC