MAX5098A
Dual, 2.2MHz, Automotive Buck or Boost
Converter with 80V Load-Dump Protection
18 ______________________________________________________________________________________
Applications Information
Setting the Switching Frequency
The controller generates the clock signal by dividing
down the internal oscillator f
OSC
or the SYNC input sig-
nal when driven by an external oscillator. The switching
frequency equals half the internal oscillator frequency
(f
SW
= f
OSC
/2). The internal oscillator frequency is set
by a resistor (R
OSC
) connected from OSC to SGND. To
find R
OSC
for each converter switching frequency f
SW
,
use the formulas:
A rising clock edge on SYNC is interpreted as a syn-
chronization input. If the SYNC signal is lost, the inter-
nal oscillator takes control of the switching rate,
returning the switching frequency to that set by R
OSC
.
When an external synchronization signal is used, R
OSC
must be selected such that f
SW
= 1/2 f
SYNC
. When
f
SYNC
clock signal is applied, f
CKO
equals f
SYNC
wave-
form, phase shifted by 180°. If the MAX5098A is run-
ning without external synchronization, f
CKO
equals the
internal oscillator frequency f
OSC
.
Buck Converter
Effective Input Voltage Range
Although the MAX5098A converter can operate from
input supplies ranging from 5.2V to 19V, the input volt-
age range can be effectively limited by the MAX5098A
duty-cycle limitations for a given output voltage. The
maximum input voltage is limited by the minimum on-
time (t
ON(MIN)
):
where t
ON(MIN)
is 100ns. The minimum input voltage is
limited by the maximum duty cycle (D
MAX
= 0.82):
where V
DROP1
is the total parasitic voltage drops in the
inductor discharge path, which includes the forward
voltage drop (V
D
) of the rectifier, the series resistance
of the inductor, and the PCB resistance. V
DROP2
is the
total resistance in the charging path that includes the
on-resistance of the high-side switch, the series resis-
tance of the inductor, and the PCB resistance.
Setting the Output Voltage
For 0.8V or greater output voltages, connect a voltage-
divider from OUT_ to FB_ to SGND (Figure 3). Select R
B
(FB_ to SGND resistor) to between 1kΩ and 20kΩ.
Calculate R
A
(OUT_ to FB_ resistor) with the following
equation:
where V
FB_
= 0.8V (see the
Electrical Characteristics
table) and V
OUT_
can range from V
FB_
to 28V (boost
operation).
For output voltages below 0.8V, set the MAX5098A out-
put voltage by connecting a voltage-divider from OUT_
to FB_ to BYPASS (Figure 3). Select R
C
(FB_ to BYPASS
resistor) in the 50kΩ range. Calculate R
A
with the fol-
lowing equation:
where V
FB_
= 0.8V, V
BYPASS
= 2V (see the
Electrical
Characteristics
table), and V
OUT_
can range from 0V to
V
FB_
.
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