3A, 1MHz, DDR Memory Termination Supply

_______________________________________________________________________________________

7

feedback voltage exceeds the external reference voltage

the PMOS switch turns off, it remains off for the pro-

short-circuit conditions, the PMOS switch remains off for

Synchronous Rectification

In a stepdown regulator without synchronous rectification,

an external Schottky diode provides a path for current to

flow when the inductor is discharging. Replacing the

Schottky diode with a low-resistance NMOS synchro-

nous switch reduces conduction losses and improves

efficiency.

The NMOS synchronous-rectifier switch turns on follow-

ing a short delay (approximately 50ns) after the PMOS

power switch turns off, thus preventing cross-conduc-

tion or “shoot-through.” In constant-off-time mode, the

synchronous-rectifier switch turns off just prior to the

PMOS power switch turning on. While both switches

are off, inductor current flows through the internal body

diode of the NMOS switch.

Current Sourcing and Sinking

By operating in a constant-off-time, pseudo-fixed-fre-

quency mode, the MAX1809 can both source and sink

current. Depending on the output current requirement,

the circuit operates in two modes. In the first mode the

output draws current and the MAX1809 behaves as a

regular buck controller, sourcing current to the output

from the input supply rail. However, when the output is

supplied by another source, the MAX1809 operates in

a second mode as a synchronous boost, taking power

from the output and returning it to the input.

Thermal Resistance

Junction-to-ambient thermal resistance,

dependent on the amount of copper area immediately

surrounding the IC leads. The MAX1809 QFN package

50°C/W with no forced airflow. The MAX1809 16-pin

thermal resistance of 80°C/W with no forced airflow.

Airflow over the board significantly reduces the junction-

to-ambient thermal resistance. For heat sinking pur-

poses, it is essential to connect the exposed backside

pad of the QFN package to a large analog ground plane.

Shutdown

Drive SHDN to a logic-level low to place the MAX1809 in

low-power shutdown mode and reduce supply current to

less than 1µA. In shutdown, all circuitry and internal

MOSFETs turn off, so the LX node becomes high imped-

for normal operation.

Power Dissipation

Power dissipation in the MAX1809 is dominated by

conduction losses in the two internal power switches.

Power dissipation due to charging and discharging the

gate capacitance of the internal switches (i.e., switch-

ing losses) is approximately:

MAX1809

IN

SHDN

EXTREF

TOFF

33

µF

10

Ω

10k

Ω

10k

Ω

(2.5V)

0.01

µF

1

µF

270

µF

2V

15m

Ω

2.2

µF

1000pF

1000pF

LX

L

PGND

GND

FB

REF

SS

2

( )

Figure 1. Typical Application Circuit