MAX17543 datasheet(13/19 Pages) MAXIM

13 / 19 page

R-C network has to be connected in parallel to the resistor

connected from RT to SGND, as shown in Figure 1. The

values of the components R8 and C13 are 90.9kΩ and

220pF, respectively.

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

IN(MIN)

SW(MAX)

OFF(MAX)

OUT(MAX)

0.15))

1- (f

)

0.175)

+×

OUT

IN(MAX)

SW(MAX)

ON(MIN)

)

where VOUT is the steady-state output voltage, IOUT (MAX)

is the maximum load current, RDCR is the DC resistance

of the inductor, fSW(MAX) is the maximum switching

frequency, tOFF-MAX is the worst-case minimum switch

off-time (160ns), and tON-MIN is the worst-case minimum

switch on-time (135ns).

External Frequency Synchronization (SYNC)

The internal oscillator of the device can be synchronized

to an external clock signal on the 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. The minimum external

clock pulse-width high should be greater than 50ns. See

the RT AND SYNC section of the Electrical Characteristics

table for details.

Overcurrent Protection/Hiccup Mode

The MAX17543 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 whenever

the high-side switch current exceeds an internal limit

of 3.7A (typ). A runaway current limit on the high-side

switch current at 4.3A (typ) 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. One occurrence of runaway current

limit triggers a hiccup mode. In addition, if, due to a fault

condition, feedback voltage drops to 0.58V (typ) any time

after soft-start is complete, hiccup mode is triggered. In

hiccup mode, the converter is protected by suspending

switching for a hiccup timeout period of 32,768 clock

cycles. Once the hiccup timeout period expires, soft-

start is attempted again. Note that when soft-start is

attempted under an overload condition, if the feedback

voltage does not exceed 0.58V, the device switches at

half the programmed switching frequency. Hiccup mode

of operation ensures low power dissipation under output

short-circuit conditions.

RESET Output

The device includes a RESET comparator to monitor the

output voltage. The open-drain RESET output requires

an external pullup resistor. RESET goes high (high

impedance) 1024 switching cycles after the regulator

output increases above 95% of the designed nominal

regulated voltage. RESET goes low when the regulator

output voltage drops to below 92% of the nominal

regulated voltage. RESET also goes low during thermal

shutdown.

Prebiased Output

When the device starts into a prebiased output, both the

high-side and low-side switches are turned off so that the

converter does not sink current from the output. High-

side and low-side switches do not start switching until

the PWM comparator commands the first PWM pulse, at

which point switching commences. The output voltage is

then smoothly ramped up to the target value in alignment

with the internal reference.

Table 1. Switching Frequency vs. RT

Resistor

Figure 1. Setting the Switching Frequency

SWITCHING FREQUENCY (kHz)

RT RESISTOR (kΩ)

500

Open

100

210

200

102

400

49.9

1000

19.1

2200

8.06

C13

MAX17543

4.5V–42V, 2.5A, High-Efficiency,

Synchronous Step-Down DC-DC Converter

with Internal Compensation

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