3A, 1MHz, DDR Memory Termination Supply

8

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Resistive losses in the two power switches are approxi-

mated by:

The junction-to-ambient thermal resistance required to

dissipate this amount of power is calculated by:

where:

Design Procedure

For typical applications, use the recommended compo-

nent values in Figure 1. For other applications, take the

following steps:

1) Select the desired PWM-mode switching frequency.

See Figure 4 for maximum operating frequency.

2) Select the constant off-time as a function of input

voltage, output voltage, and switching frequency.

4) Select the inductor as a function of output voltage,

off-time, and peak-to-peak inductor current.

Setting the Output Voltage

The output voltage of the MAX1809 is set by an external

voltage applied to the EXTREF pin. This can come directly

from another voltage source or external reference.

As an active termination supply in DDR applications

(see Active Bus Termination in the Applications

Information section), the output of the MAX1809 is reg-

ulated at half the DDR supply voltage. In mobile sys-

tems, the DDR supply voltage is 2.5V, and the

termination voltage is 1.25V ±40mV. To regulate to

1.25V, an external divide-by-2 resistor network is

placed across the DDR supply voltage to generate

1.25V. This 1.25V is connected to EXTREF, which sets

the output voltage of the MAX1809. When FB is directly

tied to the output (Figure 5), the output voltage range is

limited by the external reference’s input voltage limits

(see EC table). External reference may not be set within

be limited to less than 1.4V for 3.3V input voltage.

Failure to comply can cause the part to operate abnor-

mally and may cause part damage.

connecting FB to a resistor-divider between the output

voltage and ground (Figure 6). Use 50k

Ω for R1. R2 is

given by:

RR

V

V

OUT

EXTREF

21

1

=









−

Figure 2. Functional Diagram

2.2

µF

1

µF

0.01

µF

PWM LOGIC

AND

DRIVERS

SS

IN

FB

(3.0V TO 5.5V)

LX

L

CERAMIC

PGND

GND

REF

REF

SHDN

EXTREF

TIMER

CURRENT

SENSE

MAX1809