3A, 1MHz, DDR Memory Termination Supply

_______________________________________________________________________________________

9

Programming the Switching Frequency

and Off-Time and On-Time

The MAX1809 features a programmable PWM-mode

switching frequency, which is set by the input and out-

time in PWM mode. Use the following equation to select

the off-time while sourcing current according to the

desired switching frequency in PWM mode:

where:

PMOS power switch

NMOS synchronous-rectifier switch

formula:

Ω /1.00µs)

Ω to

430k

Ω for off-times of 0.4µs to 4µs.

When sinking current, the switching frequency increas-

es due to the on-resistances of the internal switches

adding to the voltage across the inductor, reducing the

equation:

than 350ns.

Inductor Selection

The key inductor parameters must be specified: inductor

tion includes a constant, denoted as LIR, which is the

ratio of peak-to-peak inductor AC current (ripple current)

tt

VV

VV

V

ON

OFF

OUT

NMOS

IN

OUT

PMOS

=

+













−

−

t

VV

V

fV

V

V

OFF

IN

OUT

PMOS

SW

IN

PMOS

NMOS

=

()

+

()

−−

−

Figure 3. Sourcing and Sinking Capabilities of the MAX1809

SYNCHRONOUS BUCK MODE

(SOURCING CURRENT)

SYNCHRONOUS BOOST MODE

(SINKING CURRENT)

Figure 4. Maximum Recommended Operating Frequency vs.

Input Voltage

0

400

200

800

600

1200

1000

1400

2.7

3.1

3.5

4.3

4.7

5.1

5.5

3.9