MAX1960/MAX1961/MAX1962
4) The circuit parasitic inductance, L
PAR
, is calculated
by:
5) The resistor for critical dampening, R
SNUB
= 2π x
f
R
x L
PAR
. The resistor value can be adjusted up
or down to tailor the desired damping and the
peak voltage excursion.
6) The capacitor, C
SNUB
, should be at least 2 to 4
times the value of the C
PAR
to be effective.
7) The snubber circuit power loss is dissipated in the
resistor, P
RSNUB
, and can be calculated as:
where V
IN
is the input voltage, and f
S
is the
switching frequency. Choose R
SNUB
power rating
that exceeds the calculated power dissipation.
MOSFET Power Dissipation
Worst-case power dissipation occurs at duty factor
extremes. For the high-side MOSFET, the worst-case
power dissipation due to resistance occurs at minimum
input voltage (V
IN(MIN)
):
The following formula calculates switching losses for
the high-side MOSFET, but is only an approximation
and not a substitute for evaluation:
where V
IN(MAX)
is the maximum value of the input volt-
age, t
FALL
and t
RISE
are the fall and rise time of the
MOSFET, I
L(PEAK)
and I
L(VALLEY)
are the maximum
peak and valley inductor current, and f
S
is the PWM
switching frequency:
I
L(PEAK)
= I
OUT(MAX)
× (1 + 0.5 × LIR) and I
L(VALLEY)
=
I
OUT(MAX)
× (1 - 0.5 × LIR)
where LIR is the peak-to-peak inductor ripple current
divided by the load current.
The total power dissipation in the high-side MOSFET is
the sum of these two power losses:
P
D(N1)
= P
D(N1RESISTIVE)
+ P
D(N1SWITCHING)
For the low-side MOSFET, the worst-case power dissi-
pation occurs at maximum input voltage:
Applications Information
PC Board Layout Guidelines
A properly designed PC board layout is important in
any switching DC-DC converter circuit. If possible,
mount the MOSFETs, inductor, input/output capacitors,
and current-sense resistor on the top side. Connect the
ground for these devices close together on a power-
ground trace. Make all other ground connections to a
separate analog ground plane. Connect the analog
ground plane to power ground at a single point.
To help dissipate heat, place high-power components
(MOSFETs, inductor, and current-sense resistor) on a
large PC board area. Keep high-current traces short and
wide to reduce the resistance in these traces. Also make
the gate drive connections (DH and DL) short and wide,
measuring 10 to 20 squares (50mils to 100mils wide if the
MOSFET is 1in from the controller IC).
For the MAX1960/MAX1961, connect LX and PGND to
the low-side MOSFET using Kelvin sense connections.
For the MAX1962, connect CS and OUT to the current-
sense resistor using Kelvin sense connections.
Place the REF capacitor, the BST diode and capacitor,
and the charge-pump components as close as possible
to the IC. If the IC is far from the input capacitors, bypass
V
CC
to GND with a 0.1µF or greater ceramic capacitor
close to the V
CC
pin.
For an example PC board layout, see the MAX1960
evaluation kit.
