MAXM17536 datasheet(16/22 Pages) MAXIM

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Setting the Switching Frequency (RT)

The switching frequency of the MAXM17536 can be

programmed from 100kHz to 2.2MHz by using a resistor

connected from RT to SGND. The switching frequency

(fSW) is related to the resistor connected at the RT pin

(RRT) by the following equation:

RRT

≅ 19 ×103f

− 1.7

where RRT is in kΩ and fSW is in kHz. Leaving the RT pin

open causes the device to operate at the default switching

frequency of 450kHz. See the Electrical Characteristics

table for RT resistor value recommendations for a few

common frequencies.

Operating Input-Voltage Range

The minimum and maximum operating input voltages for

a given output voltage should be calculated as follows:

OUT

OUT(MAX)

IN(MIN)

OUT(MAX)

SW(MAX)

0.076)

0.04)

1 (f

230 10 )

−

+×

−

××

VIN(MAX)

VOUT

fSW(MAX) × tON(MIN)

where,

VOUT = Steady-state output voltage,

IOUT(MAX) = Maximum load current,

fSW(MAX) = Maximum switching frequency,

tON(MIN) = Worst-case minimum switch on-time (160ns).

Also, for duty cycle > 0.5:

IN(MIN)

OUT

(4.04 V

) (35 10

)

−

−×

where fSW is the switching frequency in Hz.

Choose the greater of the two VIN(MIN) values obtained from

the above equations as the minimum operating input voltage.

The Component Selection Table, Table 1 provides the

operating input-voltage range and the optimum switching-

frequency range for the different selected output voltages.

External Frequency Synchronization (SYNC)

The internal oscillator of the MAXM17536 can be

synchronized to an external clock signal on the MODE/

SYNC pin. The external synchronization clock frequency

must be between 1.1 x fSW and 1.4 x fSW, where fSW is the

frequency programmed by the RT resistor. When an

external clock is applied to the MODE/SYNC pin, the

internal oscillator frequency changes to the external

clock frequency (from the original frequency based on

the RT setting) after detecting 16 external clock edges.

The converter operates in PWM mode during synchro-

nization operation. When the external clock is applied

to the MODE/SYNC pin, the mode of operation changes

to PWM from the initial state of PFM/DCM. When the

external clock is removed on-the-fly then the internal

oscillator frequency changes to the RT set frequency and

the converter still continues to operate in PWM mode.

The minimum external clock pulse-width high should be

greater than 50ns. See the MODE/SYNC section in the

Electrical Characteristics table for details.

DL-to-OUT Short Detection

In the MAXM17536, DL and OUT pins are adjacent to

each other. To prevent damage to the low-side FET in

case the DL pin is shorted to the OUT pins, the DL-to-

OUT short detection feature has been implemented. If the

MAXM17536 detects that the DL pin is shorted to the OUT

pins before startup, the startup sequence is not initiated

and output voltage is not soft-started.

Overcurrent Protection

The MAXM17536 is provided with a robust overcurrent

protection (OCP) scheme that protects the modules under

overload and output short-circuit conditions. A cycle-by-

cycle peak current limit turns off the high-side MOSFET

whenever the high-side switch current exceeds an internal

limit of 7.8A (typ). The module enters hiccup mode of

operation, either if one occurrence of the runaway current

limit 8.8A (typ), or if the FB node goes below 64.5% of its

nominal regulation threshold after soft-start is complete.

In hiccup mode, the module is protected by suspending

switching for a hiccup timeout period of 32,768 switching

cycles. Once the hiccup timeout period expires, soft-start

is attempted again. Hiccup mode of operation ensures low

power dissipation under output overload or short-circuit

conditions. Note that when soft-start is attempted under

overload condition, if feedback voltage does not exceed

64.5% of desired output voltage, the device switches at

half the programmed switching frequency.

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