This algorithm results in a nearly constant switching fre-

quency despite the lack of a fixed-frequency clock

generator. The actual switching frequency, which is

given by the following equation, varies slightly due to

the voltage drop across the on-resistance of the

MOSFETs and the DC resistance of the output inductor:

where D is the duty cycle:

resistance of the output inductor. The ideal switching fre-

quency increases for positive (sourcing) load current

and decreases for negative (sinking) load current, due to

the changing voltage drop across the low-side MOSFET,

which changes the inductor-current discharge ramp

rate. The on-times guaranteed in the Electrical

Characteristics are also influenced by switching delays

caused by the loading effect of the external power

MOSFETs.

The switching frequency can also be adjusted to a

value other than the preset frequencies by adding a

resistor voltage-divider at HSD. See the Adjusting the

Switching Frequency section.

VTTR Reference (MAX8553 Only)

The MAX8553’s VTTR output is capable of sourcing or

sinking up to 25mA of current. The VTTR output voltage

is one-half of the voltage applied to REFIN. Bypass

VTTR with at least a 1µF ceramic capacitor.

Voltage Reference

The voltage at REF is nominally 2.00V. Connect a 0.22µF

ceramic bypass capacitor between REF and GND.

EN and HSD (MAX8554 Only)

EN is a logic input used to enable or shut down the

MAX8554. Drive EN high or connect to V+ to enable the

output. Drive EN low to place the MAX8554 in low-

power shutdown mode, reducing input current to less

than 5µA (typ).

HSD senses the input voltage at the drain of the high-side

MOSFET, which is used to set the high-side MOSFET on-

time. For normal operation, connect HSD to the drain of

the high-side MOSFET.

EN/HSD Function (MAX8553 Only)

In order to reduce pin count and package size, the

MAX8553 features a dual-function input pin, EN/HSD.

When EN/HSD is pulled to ground, the internal circuitry

powers off, reducing current consumption to less than

5µA (typ). To enable normal operation, connect

EN/HSD to the drain of the high-side MOSFET through

a 5.1k

becomes an input that monitors the high-side MOSFET

drain voltage (converter input voltage) and uses that

measurement to calculate the appropriate on-time for the

converter. If the enable function is not used, connect

EN/HSD directly to the high-side MOSFET drain.

D

VI

R

R

VI

R

R

OUT

O

DSONL

DC

HSD

O

DSONL

DSONH

=

++

()

+

()

-

f

D

tK

N

S

ON

=≅

×

1

4.5V to 28V Input, Synchronous PWM Buck Controllers

for DDR Termination and Point-of-Load Applications

16

______________________________________________________________________________________

FSEL CONNECTED TO

N

FREQUENCY (kHz)

CONDITION

Ground

1.07

0.91

550

REF

1.33

1.15

400

Floating

2.00

1.70

300

VL

3.00

2.55

200

FSEL CONNECTED TO

N

FREQUENCY (kHz)

CONDITION

Ground

1.07

0.37

550

REF

1.33

0.49

400

Floating

2.00

0.71

300

VL

3.00

1.02

200