LTC3803-5_15 Datasheet, PDF (11/16 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803-5
APPLICATIONS INFORMATION
to duty cycle. This stabilizes the control loop against
subharmonic oscillation. The amount of reduction in the
current comparator threshold (ΔV_SENSE) can be calculated
using the following equation:
ΔV_SENSE
=
Duty
Cycle –
73.5%
6.5%
•
5μA
•
R_SL
Note: LTC3803-5 enforces 6.5% < Duty Cycle < 80%.
A good starting value for R_SL is 5.9k, which gives a 30mV
drop in current comparator threshold at 80% duty cycle.
Designs not needing slope compensation may replace
R_SL with a short circuit.
V_CC SHUNT REGULATOR
An internal shunt regulator allows the LTC3803-5 to be
powered through a single dropping resistor from V_IN to
V_CC, in conjunction with a bypass capacitor, C_VCC, that
closely decouples V_CC to GND (see Figure 3). The shunt
regulator can draw up to 25mA through the V_CC pin to
V_IN
R_VCC
C_VCC
LTC3803-5
V_CC
GND
GND to drop enough voltage across R_VCC to regulate
V_CC to around 8.1V. For applications where V_IN is low
enough such that the static power dissipation in R_VCC is
acceptable, using the V_CC shunt regulator is the simplest
way to 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_B brings V_CC above the V_CC turn-on threshold,
enabling the LTC3803-5.
8V TO
75 V_IN
R_B
100k
D1
6.8V
Q1
MMBTA42
C_VCC
0.1μF
LTC3803-5
V_CC
GND
38035 F04
Figure 4. Powering the LTC3803-5
with an External Preregulator
38035 F03
Figure 3. Powering the LTC3803-5
Via the Internal Shunt Regulator
38035fd
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