MAX1809 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1809

3A, 1MHz, DDR Memory Termination Supply

Maxim Integrated 

3A, 1MHz, DDR Memory Termination Supply

Pin Description

PIN (QFN)

1, 5, 10, 11, 12,

22, 24, 26, 28

2, 4

PIN (QSOP)

—

2, 4

3, 18, 19, 23, 25

3, 14, 16

6

7

8

9

13, backside pad,

corner tabs

14

15

16

17, 20, 21

5

6

7

8

9

10

11

12

13, 15

27

1

NAME

N.C.

IN

LX

SS

EXTREF

TOFF

FB

GND

REF

GND

VCC

PGND

SHDN

FUNCTION

No Connection. Not internally connected.

Supply Voltage Input for the Internal PMOS Power Switch. Not internally

connected. Externally connect all pins for proper operation.

Inductor Connection. Connection for the drains of the PMOS power switch

and NMOS synchronous-rectifier switch. Connect the inductor from this

node to the output filter capacitor and load. Not internally connected.

Externally connect all pins for proper operation.

Soft-Start. Connect a capacitor from SS to GND to limit inrush current

during startup.

External Reference Input. Feedback input regulates to VEXTREF. The PWM

controller remains off until EXTREF is greater than REF.

Off-Time Select Input. Sets the PMOS power switch constant-off-time.

Connect a resistor from TOFF to GND to adjust the PMOS switch off-time.

Feedback Input. Connect directly to output for fixed-voltage operation or to

a resistive-divider for adjustable operating modes.

Analog Ground. Connect exposed backside pad and corner tabs to analog

GND.

Reference Output. Bypass REF to GND with a 1µF capacitor.

Tie to GND (pin 13 QFN; pin 9 QSOP)

Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass

VCC with a 10Ω and 2.2µF low-pass filter (see Figure 1).

Power Ground. Internally connected to the internal NMOS synchronous-

rectifier switch.

Shutdown Control Input. Drive SHDN low to disable the reference, control

circuitry, and internal MOSFETs. Drive high or connect to VCC for normal

operation.

Detailed Description

The MAX1809 synchronous, current-mode, constant-

off-time, PWM DC-DC converter steps down input volt-

ages of 3V to 5.5V to an adjustable output voltage from

1.1V to VIN, as set by the voltage applied at EXTREF. It

sources and sinks up to 3A of output current. Internal

switches composed of a 90mΩ PMOS power switch

and a 70mΩ NMOS synchronous-rectifier switch

improve efficiency, reduce component count, and elim-

inate the need for an external Schottky diode across

the synchronous switch.

The MAX1809 operates in a constant-off-time mode

under all loads. A single resistor-programmable con-

stant-off-time control sets switching frequencies up to

1MHz, allowing the user to optimize performance trade-

offs in efficiency, switching noise, component size, and

cost.

When power is drawn from a regulated supply, con-

stant-off-time PWM architecture essentially provides

constant-frequency operation. This architecture has the

inherent advantage of quick response to line and load

transients. The MAX1809’s current-mode, constant-off-

time PWM architecture regulates the output voltage by

changing the PMOS switch on-time relative to the con-

stant off-time.

Constant-Off-Time Operation

In the constant-off-time architecture, the FB voltage

comparator turns the PMOS switch on at the end of

each off-time, keeping the device in continuous-con-

duction mode. The PMOS switch remains on until the

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