Datasheet

LTC3803-5
11
38035fd
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 – 6.5%
73.5%
•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
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.
Figure 3. Powering the LTC3803-5
Via the Internal Shunt Regulator
Figure 4. Powering the LTC3803-5
with an External Preregulator
LTC3803-5
V
CC
R
VCC
C
VCC
38035 F03
V
IN
GND
LTC3803-5
V
CC
R
B
100k
D1
6.8V
Q1
MMBTA42
C
VCC
0.1μF
38035 F04
8V TO
75 V
IN
GND