MAX17536ATP datasheet(13/19 Pages) MAXIM

13 / 19 page

Addalocalbypassingcapof0.1μFontheEXTVCCpin

toSGND.Also,adda4.7Ωresistorfrombuckconverter

output node to the EXTVCC pin to limit VCC bypass cap

discharge current and to protect the EXTVCC pin from

reaching its absolute maximum rating (-0.3V) during out-

put short-circuit condition.In applications where the buck

converter output is connected to the EXTVCC pin, if the

output is shorted to ground, the transfer from EXTVCC

LDOtoINLDOhappensseamlessly,withoutanyimpact

on the normal functionality. Connect EXTVCC pin to

SGNDwhenthepinisnotbeingused.

Setting the Switching Frequency (RT)

The switching frequency of the MAX17536 can be

programmedfrom100kHzto2.2MHzbyusingaresistor

connected from RT to SGND. The switching frequency

(fSW)isrelatedtotheresistorconnectedattheRTpin

(RRT) by the following equation:

19 10

1.7

−

≅

whereRRTisinkΩandfSWisinkHz.LeavingtheRTpin

open causes the device to operate at the default switching

frequencyof450kHz.SeeTable 1forRTresistorvalues

for a few common switching 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)

DCR(MAX)

DS-ONL(MAX)

IN(MIN)

SW(MAX)

OFF-MIN(MAX)

OUT(MAX)

DS-ONH(MAX)

DS-ONL(MAX)

OUT

IN(MAX)

SW(MAX)

ON-MIN(MAX)

)

+×

−×

×−

where:

VOUT = Steady-state output voltage

IOUT(MAX) = Maximum load current

RDCR(MAX) = Worst-case DC resistance of the inductor

fSW(MAX) = Maximum switching frequency

tOFF-MIN(MAX) = Worst-case minimum switch off-time

(160ns)

tON-MIN(MAX) = Worst-case minimum switch on-time

(160ns)

RDS-ONH(MAX) = Worst-case on-state resistances and

high-side internal MOSFET,

RDS-ONL(MAX) = Worst-case on-state resistances and

low-side external MOSFET.

External Frequency Synchronization

Theinternaloscillatorofthedevicecanbesynchronized

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

programmedbytheRTresistor.Whenanexternalclock

is applied to MODE/SYNC pin, the internal oscillator

frequency changes to external clock frequency (from

originalfrequencybasedonRTsetting)afterdetecting16

external clock edges. The converter will operate in PWM

mode during synchronization operation. When MODE/

SYNCisfloatedforPFMmode,internal300kΩpull-down

resistor on this pin pulls the node below VIL of the SYNC

threshold and maintains the part in PFM mode. When the

external clock is applied on the fly, the mode of operation

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

When the external clock is removed on the fly, the internal

oscillatorfrequencychangestotheRTsetfrequencyand

the converter continues to operate in PWM mode. The

minimum external clock pulse-width high should be greater

than 22ns. See the Mode Selection (MODE) section in the

Electrical Characteristics table for details.

DL-to-LX Short Detection

In the MAX17536, the DL and LX pins are adjacent to

each other. To prevent damage to the low-side exter-

nal FET in case DL pin is shorted to the LX pins, DL

to LX_ short-detection feature has been implemented.

If the device detects that the DL pin is shorted to the

LX pins before startup, the startup sequence is not be

initiated and output voltage is not soft-started.

Overcurrent Protection/Hiccup Mode

The device is provided with a robust overcurrent-

protection scheme that protects the device under overload

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

current limit turns off the high-side MOSFET when-

ever the high-side switch current exceeds an internal

limit. A runaway-current limit on the high-side switch

current protects the device under high input voltage,

short-circuit conditions when there is insufficient output

voltage available to restore the inductor current that was

built up during the on-period of the step-down converter.

Table 1. Switching Frequency vs. RT

Resistor

SWITCHING FREQUENCY (kHz)

RT RESISTOR (kΩ)

450

OPEN

100

200

93.1

2200

6.98

Maxim Integrated │ 13

MAX17536

4.5V to 60V, 4A, High-Efficiency, Synchronous

Step-Down DC-DC Converter

with Internal Compensation

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