Resistors R5 and R6 set the under-voltage lockout value. The voltage at this pin must be
1.28 V before the IC will operate. A reasonable input voltage is 95 V for this pin to be
active. The value of R5 is very large, so the bias current of the UVLO pin will affect the
value needed for R6. We can consider V
IN
and R5 to be a constant current source, so we
need to subtract the bias current from the current supplied by R5 when calculating R6.
The data sheet also gives equations for calculating these resistors.
6.5 Non-isolated Flyback Example
Our next example shows the advantage of a non-isolated fl yback design for automotive
use. An automotive system can range from 11.5 V at low battery with the key off
to 15.0 V when charging a drained battery. Some systems are designed to work at a
nominal 13.6 V 0.5 V. This represents full voltage for a charged battery. Our example
implements a system that produces 13.6 V at 10 A. The output ripple target is 300 mV. The
regulation target is 400 mV. Figure 6.14 shows our circuit.
A reasonable choice for the control IC is the LT1680. This IC is designed for high
power step-up DC–DC converters using an external MOSFET switch. It provides all
of the necessary current mode PWM functions and will operate directly from the input
supply.
A reasonable switching frequency is 167 kHz. The maximum frequency of the IC is
200 kHz, but we want to stay away from effects where we have no control. The cycle
time is 6.0 s. This frequency is low enough that parasitic effects at the high power level
will be manageable. This frequency is also within the power range of reasonably priced
inductor cores.
Continuous mode operation is a reasonable selection for this design. The output current
will approximate the input current, since the input voltage range is 10/ 20% of the
output voltage range. Choosing continuous mode will allow the peak current to be only
slightly larger than twice the output current. If we set the 50% duty cycle voltage to
10.5 V input, we will have enough margin when the voltage drops to 11.0 V to maintain
control and avoid the need for slope compensation. This sets the target duty cycle for the
lowest input voltage around 40%, as a fi rst guess. We use the graph in the data sheet to
choose the 3 K timing resistor based on our maximum duty cycle. Another graph in the
data sheet gives us 2.2 nF based on the 167 kHz frequency and 3 K timing resistor.
A 60 V Schottky diode is a reasonable fi rst try at the output rectifi er. The turns ratio of the
inductor is likely to be very close to 1:1. It is a reasonable guess that the turns ratio will
be no larger than 1:2. The IRF 30CPQ060 has 60 V PRV and 30 A average current and
is a dual diode package. The peak forward current is likely to be approximately 20 A, so
this diode should fi t our requirements. Each diode will pass one-half of the total current,
170
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Chapter 6
13.6V Out
GND
V
in
GND
U1
LT1680
SL/ADJ
Ct
Iave
SS
Vc
SGND
FB
Sense
Sense
Run/SHDN
PGND
GATE
12V
in
5V
ref
IRFZ44 V
T1
1:1.93
R2
0.0037
R3
10.0 K
R4
1020
50µF
670 nF
2.2 nF
F
220 pF
220 nF
1K
100 K
18 V
10
3.0 K
30CPQ060
7 4.7 uF Ceramic
10
10 MLC
Capacitors
in Parallel
Figure 6.14 : 13.6 V non-isolated fl yback supply for automotive systems using the LT1680
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