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MAX16935
(Rev.:2014)

36V, 3.5A, 2.2MHz Step-Down Converter with 28µA Quiescent Current

Maxim Integrated 

MAX16935

36V, 3.5A, 2.2MHz Step-Down Converter

with 28µA Quiescent Current

Spread-Spectrum Option

The device has an internal spread-spectrum option to opti-

mize 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

the oscillator frequency (fOSC). The modulation signal is a

triangular 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/400kHz = 605µs.

The internal spread spectrum is disabled if the IC is

synced to an external clock. However, the IC does not

filter the input clock and passes any modulation (includ-

ing spread-spectrum) present on the driving external

clock to the SYNCOUT pin.

Automatic Slew-Rate Control on LX

The device 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 = 73.2kI. Higher frequen-

cies 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

dissipation in the device. When the junction temperature

Maxim Integrated

exceeds 175NC (typ), an internal thermal sensor shuts

down the internal bias regulator and the step-down

controller, 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 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 device 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 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.

MAX16935

FB

VOUT

RFB1

RFB2

Figure 2. Adjustable Output-Voltage Setting

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