and:
where I
PEAK
is the peak inductor current (see the
Inductor Selection
section). For ceramic capacitors, the
output voltage ripple is typically dominated by V
RIPPLE(C)
.
The voltage rating and temperature characteristics of
the output capacitor must also be considered.
Input Capacitor Selection
The input capacitor (C
IN
) reduces the current peaks
drawn from the input supply and reduces noise injec-
tion into the IC. Two 4.7µF ceramic capacitors are used
in the typical operating circuit in Figure 1 because of
the high source impedance seen in typical lab setups.
Actual applications usually have much lower source
impedance since the step-up regulator often runs
directly from the output of another regulated supply.
Typically, C
IN
can be reduced below the values used in
Figure 1. Ensure a low-noise supply at IN by using ade-
quate C
IN
. Alternatively, greater voltage variation can
be tolerated on C
IN
if IN is decoupled from C
IN
using
an RC lowpass filter (see Figure 1).
Rectifier Diode Selection
The MAX17112 high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. The diode should be rated to
handle the output voltage and the peak switch current.
Make sure that the diode’s peak current rating is at least
I
PEAK
calculated in the
Inductor Selection
section and
that its breakdown voltage exceeds the output voltage.
Output Voltage Selection
The MAX17112 operates with an adjustable output from
V
IN
to 20V. Connect a resistive voltage-divider from the
output (V
MAIN
) to GND with the center tap connected to
FB (see Figure 1). Select R3 in the 10kΩ to 50kΩ range.
Calculate R4 with the following equation:
where V
FB
, the step-up regulator’s feedback set point,
is 1.24V (typ). Place R3 and R4 as close as possible to
the IC.
Loop Compensation
Choose R
COMP
to set the high-frequency integrator
gain for fast-transient response. Choose C
COMP
to set
the integrator zero to maintain loop stability.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
To further optimize transient response, vary R
COMP
in
20% steps and C
COMP
in 50% steps while observing
transient response waveforms.
Soft-Start Capacitor
The soft-start capacitor should be large enough so that
it does not reach final value before the output has
reached regulation. Calculate C
SS
to be:
where C
OUT
is the total output capacitance including
any bypass capacitor on the output bus, V
OUT
is the
maximum output voltage, I
INRUSH
is the peak inrush
current allowed, I
OUT
is the maximum output current
during power-up, and V
IN
is the minimum input voltage.
The load must wait for the soft-start cycle to finish
before drawing a significant amount of load current.
The soft-start duration after which the load can begin to
draw maximum load current is:
PCB Layout and Grounding
Careful PCB layout is important for proper operation.
Use the following guidelines for good PCB layout:
1) Minimize the area of high-current loops by placing
the inductor, output diode, and output capacitors
near the input capacitors and near the LX and GND
pins. The high-current input loop goes from the pos-
itive terminal of the input capacitor to the inductor,
to the IC’s LX pin, out of GND, and to the input
capacitor’s negative terminal. The high-current out-
put loop is from the positive terminal of the input
capacitor to the inductor, to the output diode (D1),
to the positive terminal of the output capacitors,
reconnecting between the output capacitor and
input capacitor ground terminals. Connect these
loop components with short, wide connections.
Avoid using vias in the high-current paths. If vias
are unavoidable, use many vias in parallel to
reduce resistance and inductance.
