Datasheet

MAX16936
36V, 220kHz to 2.2MHz Step-Down Converter
with 28µA Quiescent Current
12Maxim Integrated
Applications Information
Setting the Output Voltage
Connect FB to BIAS for a fixed +5V/+3.3 output voltage.
To set the output to other voltages between 1V and 10V,
connect a resistive divider from output (OUT) to FB to
AGND (Figure 2). Use the following formula to determine
the R
FB2
of the resistive divider network:
R
FB2
= R
TOTAL
x V
FB
/V
OUT
where V
FB
= 1V, R
TOTAL
= selected total resistance of
R
FB1
, R
FB2
in ω, and V
OUT
is the desired output in volts.
Calculate R
FB1
(OUT to FB resistor) with the following
equation:
OUT
FB1 FB2
FB
V
RR 1
V


= −





where V
FB
= 1V (see the Electrical Characteristics table).
FPWM/Skip Modes
The MAX16936 offers a pin selectable skip mode or fixed-
frequency PWM mode option. The IC has an internal LS
MOSFET that turns on when the FSYNC pin is connected to
V
BIAS
or if there is a clock present on the FSYNC pin. This
enables the fixed-frequency-forced PWM mode operation
over the entire load range. This option allows the user to
maintain fixed frequency over the entire load range in appli-
cations that require tight control on EMI. Even though the
MAX16936 has an internal LS MOSFET for fixed-frequency
operation, an external Schottky diode is still required to sup-
port the entire load range. If the FSYNC pin is connected
to GND, the skip mode is enabled on the MAX16936.
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 switching
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
Figure 3. Switching Frequency vs. R
FOSC
SWITCHING FREQUENCY vs. R
FOSC
MAX16936 toc08
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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