MAX1809 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1809

3A, 1MHz, DDR Memory Termination Supply

Maxim Integrated 

3A, 1MHz, DDR Memory Termination Supply

VIN

IN

LX

33µF

10Ω

MAX1809 PGND

VCC

GND

2.2µF

SHDN

FB

VDDQ

(2.5V)

VSET EXTREF

REF

10kΩ

1000pF

TOFF

SS

RTOFF

10kΩ

VSSQ

1000pF

Figure 1. Typical Application Circuit

L

RDROOP

( ) VOUT =

VDDQ

2

270µF

2V

15mΩ

1µF

0.01µF

FOR VIN = 5V: L = 1µH, RTOFF = 130kΩ

FOR VIN = 3.3V: L = 0.68µH, RTOFF = 73.2kΩ

feedback voltage exceeds the external reference voltage

(VEXTREF) or the positive current limit is reached. When

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

grammed off-time (tOFF). To control the current under

short-circuit conditions, the PMOS switch remains off for

approximately 4 ✕ tOFF when VFB < VEXTREF / 4.

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, θJA, is highly

dependent on the amount of copper area immediately

surrounding the IC leads. The MAX1809 QFN package

has 1in2 of copper area and a thermal resistance of

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

QSOP evaluation kit has 0.5in2 of copper area and a

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-

ance. Drive SHDN to a logic-level high or connect to VCC

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:

PD(CAP) = C ✕ VIN2 ✕ fSW

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