MAX15004_11 Datasheet, PDF (13/27 Pages) Maxim Integrated Products – 4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers

4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
(t_DISCHARGE). See Figures 3a and 3b for a coarse selec-
tion of capacitor values for a given switching frequency
and maximum duty cycle and then use the following
equations to calculate the resistor value to fine-tune the
switching frequency and verify the worst-case maximum
duty cycle.
t _CHARGE
=
D_MAX
f_OSC
RT = ^{t CHARGE}
0.7 × CT
t _DISCHARGE
=
(1.33
2.25(V) × RT × CT
× 10 ^−3(A) × RT) − 3.375(V)
f_OSC
=
⎧
⎪⎪
⎨
⎪
⎩⎪
t _CHARGE
t _CHARGE
1
+ t_DISCHARGE
1
+ t_DISCHARGE
...................Use
.......Use
+ 160ns
This Equation If f_OSC
This Equation If f_OSC
≤
>
500kHz
500kHz
where f_OSC is the oscillator frequency, RT is a resistor
connected from RTCT to REG5, and CT is a capacitor
connected from RTCT to SGND. Verify that the oscilla-
tor frequency value meets the target. Above calcula-
tions could be repeated to fine-tune the switching
frequency.
The MAX15004A/B is a 50% maximum duty-cycle part,
while the MAX15005A/B is 100% maximum duty-cycle
part.
f_OUT
=
1
2
f_OSC
for the MAX15004A/B and
f_OUT = f_OSC
for the MAX15005A/B.
The MAX15004A/B/MAX15005A/B can be synchronized
using an external clock at the SYNC input. For proper
frequency synchronization, SYNC’s input frequency must
be at least 102% of the programmed internal oscillator
frequency. Connect SYNC to SGND when not using an
external clock. A rising clock edge on SYNC is interpret-
ed as a synchronization input. If the SYNC signal is lost,
the internal oscillator takes control of the switching rate,
returning the switching frequency to that set by RC net-
work connected to RTCT. This maintains output regula-
tion even with intermittent SYNC signals.
RTCT
CLKINT
SYNC
OUT
D = 50%
MAX15004A/B (DMAX = 50%)
WITH SYNC
INPUT
WITHOUT
SYNC INPUT
D = 50%
RTCT
CLKINT
SYNC
OUT
MAX15005A/B (DMAX = 81%)
WITH SYNC
INPUT
WITHOUT
SYNC INPUT
D = 81.25%
D = 80%
Figure 2. Timing Diagram for Internal Oscillator vs. External SYNC and DMAX Behavior
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