Datasheet

MAX16935
36V, 3.5A, 2.2MHz Step-Down Converter
with 28µA Quiescent Current
12Maxim Integrated
range in applications that require tight control on EMI.
Even though the device has an internal LS MOSFET for
fixed-frequency operation, an external Schottky diode
is still required to support the entire load range. If the
FSYNC pin is connected to GND, the skip mode is
enabled on the device.
In skip mode of operation, the converter’s switching
frequency is load dependent. At higher load current, the
switching frequency does not change and the operating
mode is similar to the FPWM mode. Skip mode helps
improve efficiency in light-load applications by allowing
the converters to turn on the high-side switch only when
the output voltage falls below a set threshold. As such,
the converters do not switch MOSFETs on and off as
often as is the case in the FPWM mode. Consequently,
the gate charge and switching losses are much lower in
skip mode.
Inductor Selection
Three key inductor parameters must be specified for
operation with the device: inductance value (L), inductor
saturation current (I
SAT
), and DC resistance (R
DCR
). To
select inductance value, the ratio of inductor peak-to-
peak AC current to DC average current (LIR) must be
selected first. A good compromise between size and loss
is a 30% peak-to-peak ripple current to average current
ratio (LIR = 0.3). The switching frequency, input voltage,
output voltage, and selected LIR then determine the
inductor value as follows:
OUT SUP OUT
SUP SW OUT
V (V V )
L
V f I LIR
−
=
where V
SUP
, V
OUT
, and I
OUT
are typical values (so that
efficiency is optimum for typical conditions). The switch-
ing frequency is set by R
FOSC
(see Figure 3).
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor RMS current requirement (I
RMS
) is
defined by the following equation:
OUT SUP OUT
RMS LOAD(MAX)
SUP
V (V V )
II
V
−
=
I
RMS
has a maximum value when the input voltage
equals twice the output voltage (V
SUP
= 2V
OUT
), so
I
RMS(MAX)
= I
LOAD(MAX)
/2.
Choose an input capacitor that exhibits less than +10NC
self-heating temperature rise at the RMS input current for
optimal long-term reliability.
The input voltage ripple is composed of DV
Q
(caused
by the capacitor discharge) and DV
ESR
(caused by the
ESR of the capacitor). Use low-ESR ceramic capacitors
with high ripple current capability at the input. Assume
the contribution from the ESR and capacitor discharge
equal to 50%. Calculate the input capacitance and ESR
required for a specified input voltage ripple using the fol-
lowing equations:
ESR
IN
L
OUT
V
ESR
I
I
2
∆
=
∆
+
where:
SUP OUT OUT
L
SUP SW
(V V ) V
I
V fL
−×
∆=
××
and:
OUT OUT
IN
Q SW SUPSW
I D(1 D) V
C and D
Vf V
×−
= =
∆×
where I
OUT
is the maximum output current and D is the
duty cycle.
Figure 3. Switching Frequency vs. R
FOSC
SWITCHING FREQUENCY vs. R
FOSC
R
FOSC
(kΩ)
SWITCHING FREQUENCY (MHz)
1027242
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
0
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