MAX8553EEE datasheet(20/24 Pages) MAXIM

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Setting the Current Limit

Constant Current Limit

The adjustable current limit accommodates MOSFETs

with a wide range of on-resistance values. The current-

limit threshold is adjusted with an external resistor con-

nected from ILIM to GND (RILIM_). The adjustment

range is 50mV to 200mV measured across the low-side

MOSFET. The value of RILIM is calculated using the fol-

lowing formula:

where IVALLEY is the valley current limit and RDS(ON) is

the on-resistance of the low-side MOSFET. To avoid

reaching the current at a lower current than expected,

use the maximum value for RDS(ON) at elevated junc-

tion temperature. Refer to the MOSFET manufacturer’s

data sheet for maximum values.

Foldback Current Limit

Foldback current limit is used to reduce power dissipa-

tion during overload and short-circuit conditions. This is

accomplished by lowering the current-limit threshold as

the output voltage drops due to the overload.

To use foldback current limit, connect a resistor

(RFOBK) from ILIM to the output, and connect a resistor

(RILIM) from ILIM to GND (Figure 5). The values of RILIM

and RFOBK are calculated as follows:

First, select the percentage of foldback, PFB. This per-

cent corresponds to the current limit when VOUT equals

zero divided by the current limit when VOUT equals its

nominal voltage. Typical values range from 15% to

30%. To solve for the resistor values, use the following

equations:

If RILIM results in a negative number, select another low-

side MOSFET with lower RDS(ON) or increase PFB or a

combination of both for the best compromise of cost,

efficiency, and lower short-circuit power dissipation.

Adjusting the Switching Frequency

The switching frequency of the MAX8553/MAX8554 can

be lowered from the value set by FSEL by adding a

resistor voltage-divider to EN/HSD (HSD) as shown in

Figure 6. This voltage-divider lowers the voltage the IC

measures on EN/HSD (HSD), which increases the on-

time. The switching frequency with the added resistor-

divider is calculated as follows:

where K = 1.7µs and N is given in Tables 1 and 2. To set

the frequency, select a value for R2 between 10k

Ω and

100k

Ω, then calculate R1 from the following equation:

With the minimum input voltage, make sure that the

voltage present at EN/HSD (HSD) is greater than 1.5V

when the resistor-divider is used:

IN MIN

()

S ≅

ILIM

DS ON

VALLEY

FOBK

OUT

DS ON

VALLEY

××

() ×

××

()

FOBK

OUT

()

ILIM

VALLEY

DS ON

=×

()

4.5V to 28V Input, Synchronous PWM Buck Controllers

for DDR Termination and Point-of-Load Applications

______________________________________________________________________________________

MAX8553

RILIM

RFOBK

VOUT

PGND

ILIM

Figure 5. Setting the Foldback Current Limit with Two

Resistors, RILIM and RFOBK

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