MAX16936 データシートの表示（PDF） - Maxim Integrated
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MAX16936 Datasheet PDF : 17 Pages
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MAX16936
36V, 220kHz to 2.2MHz Step-Down Converter
with 28µA Quiescent Current
Spread-Spectrum Option
The MAX16936 has an internal spread-spectrum option
to optimize EMI performance. This is factory set and
the S-version of the IC should be ordered. For spread-
spectrum-enabled ICs, the operating frequency is varied
±6% centered on FOSC. The modulation signal is a trian-
gular wave with a period of 110µs at 2.2MHz. Therefore,
FOSC will ramp down 6% and back to 2.2MHz in 110µs
and also ramp up 6% and back to 2.2MHz in 110µs. The
cycle repeats.
For operations at FOSC values other than 2.2MHz, the
modulation signal scales proportionally, e.g., at 400kHz,
the 110µs modulation period increases to 110µs x
2.2MHz/400MHz = 550µs.
The internal spread spectrum is disabled if the IC is
synced to an external clock. However, the IC does not fil-
ter the input clock and passes any modulation (including
spread-spectrum) present on the driving external clock
to the SYNCOUT pin.
Automatic Slew-Rate Control on LX
The MAX16936 has automatic slew-rate adjustment
that optimizes the rise times on the internal HSFET gate
drive to minimize EMI. The IC detects the internal clock
frequency and adjusts the slew rate accordingly. When
the user selects the external frequency setting resistor
RFOSC such that the frequency is > 1.1MHz, the HSFET
is turned on in 4ns (typ). When the frequency is < 1.1MHz
the HSFET is turned on in 8ns (typ). This slew-rate control
optimizes the rise time on LX node externally to minimize
EMI while maintaining good efficiency.
Internal Oscillator (FOSC)
The switching frequency, fSW, is set by a resistor (RFOSC)
connected from FOSC to AGND. See Figure 3 to select the
correct RFOSC value for the desired switching frequency.
For example, a 400kHz switching frequency is set with
RFOSC = 732kI. Higher frequencies allow designs with
lower inductor values and less output capacitance.
Consequently, peak currents and I2R losses are lower
at higher switching frequencies, but core losses, gate
charge currents, and switching losses increase.
Synchronizing Output (SYNCOUT)
SYNCOUT is an open-drain output that outputs a 180N
out-of-phase signal relative to the internal oscillator.
Overtemperature Protection
Thermal-overload protection limits the total power dis-
sipation in the device. When the junction temperature
exceeds 175NC (typ), an internal thermal sensor shuts
Maxim Integrated
down the internal bias regulator and the step-down con-
troller, allowing the device to cool. The thermal sensor
turns on the device again after the junction temperature
cools by 15NC.
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 RFB2 of the resistive divider network:
RFB2 = RTOTAL x VFB/VOUT
where VFB = 1V, RTOTAL = selected total resistance of
RFB1, RFB2 in ω, and VOUT is the desired output in volts.
Calculate RFB1 (OUT to FB resistor) with the following
equation:
= RFB1
RFB2
VOUT
VFB
−
1
where VFB = 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 VBIAS 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 applications that require tight
control on EMI. Even though the MAX16936 has an inter-
nal LS MOSFET for fixed-frequency operation, an exter-
nal 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 MAX16936.
MAX16936
FB
VOUT
RFB1
RFB2
Figure 2. Adjustable Output Voltage Setting
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