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MAX16935CAUES/V

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

Maxim Integrated 

MAX16935/MAX16939

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

with 28µA Quiescent Current

clock within one cycle. When the external clock signal

at FSYNC is absent for more than two clock cycles, the

devices revert back to the internal clock.

System Enable (EN)

An enable control input (EN) activates the device from its

low-power shutdown mode. EN is compatible with inputs

from automotive battery level down to 3.5V. The high

voltage compatibility allows EN to be connected to SUP,

KEY/KL30, or the inhibit pin (INH) of a CAN transceiver.

EN turns on the internal regulator. Once VBIAS is above

the internal lockout threshold, VUVL = 3.15V (typ), the

controller activates and the output voltage ramps up

within 8ms.

A logic-low at EN shuts down the device. During

shutdown, the internal linear regulator and gate drivers

turn off. Shutdown is the lowest power state and reduces

the quiescent current to 5FA (typ). Drive EN high to bring

the device out of shutdown.

Spread-Spectrum Option

The devices have an internal spread-spectrum option

to optimize EMI performance. This is factory set and the

S-version of the device should be ordered. For spread-

spectrum-enabled devices, 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 device is

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

filter 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 devices have automatic slew-rate adjustment that

optimizes the rise times on the internal HSFET gate drive

to minimize EMI. The device 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 fre-

quency. For example, a 400kHz switching frequency is set

with RFOSC = 73.2kI. 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

dissipation in the device. When the junction temperature

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.

MAX16935

MAX16939

FB

VOUT

RFB1

RFB2

Figure 2. Adjustable Output-Voltage Setting

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