LTC3803-5 Datasheet, PDF (10/12 Pages) Linear Technology – Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT

LTC3803-5
APPLICATIO S I FOR ATIO
CURRENT SENSE RESISTOR CONSIDERATIONS
The external current sense resistor (R_SENSE in Figure 2)
allows the user to optimize the current limit behavior for
the particular application. As the current sense resistor is
varied from several ohms down to tens of milliohms, peak
switch current goes from a fraction of an ampere to several
amperes. Care must be taken to ensure proper circuit
operation, especially with small current sense resistor
values.
For example, a peak switch current of 5A requires a sense
resistor of 0.020Ω. Note that the instantaneous peak
power in the sense resistor is 0.5W and it must be rated
accordingly. The LTC3803-5 has only a single sense line
to this resistor. Therefore, any parasitic resistance in the
ground side connection of the sense resistor will increase
its apparent value. In the case of a 0.020Ω sense resistor,
one milliohm of parasitic resistance will cause a 5%
reduction in peak switch current. So the resistance of
printed circuit copper traces and vias cannot necessarily
be ignored.
PROGRAMMABLE SLOPE COMPENSATION
The LTC3803-5 injects a ramping current through its
SENSE pin into an external slope compensation resistor
(R_SL in Figure 2). This current ramp starts at zero right
after the NGATE pin has been high for the LTC3803-5’s
minimum duty cycle of 6.5%. The current rises linearly
towards a peak of 5µA at the maximum duty cycle of 80%,
shutting off once the NGATE pin goes low. A series resistor
(R_SL) connecting the SENSE pin to the current sense
resistor (R_SENSE) thus develops a ramping voltage drop.
From the perspective of the SENSE pin, this ramping
voltage adds to the voltage across the sense resistor,
effectively reducing the current comparator threshold in
proportion to duty cycle. This stabilizes the control loop
V_IN
R_VCC
C_VCC
LTC3803-5
V_CC
GND
38035 F03
Figure 3. Powering the LTC3803-5
Via the Internal Shunt Regulator
10
against subharmonic oscillation. The amount of reduction
in the current comparator threshold (∆V_SENSE) can be
calculated using the following equation:
Duty Cycle – 6.5%
∆V_SENSE =
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 8V. For applications where V_IN is low enough
such that the static power dissipation in R_VCC is accept-
able, 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 resis-
tor R_B brings V_CC above the V_CC turn-on threshold, en-
abling 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
38035f