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LTM4632

4632fc

For more information www.linear.com/LTM4632

OPERATION

The LTM4632 is a dual output standalone non-isolated

switch mode DC/DC power supply for DDR-QDR4 SRAM

memory supplies and bus termination. It can deliver two

output rails which could both sink and source 3A DC cur-

rent with few external input and output ceramic capaci-

tors, plus a 10mA buffered VTTR (VREF) reference voltage

which equal to one half of VDDQIN voltage.

Two or more module outputs can be easily paralleled to

achieve a single VTT output with a higher sink and source

current capability. Up to 8 phases can be paralleled to run

simultaneously with a good current sharing guaranteed

by current mode control loop.

This module provides precisely regulated output voltage

(VOUT1) programmable via one external resistor from 0.6V

to 2.5V over 3.6V to 15V input voltage range. With INTVCC

tied to VIN, this module is able to operate from 3.3V input.

The LTM4632 has an integrated a dual constant on-time

valley current mode regulator, power MOSFETs, induc-

tor, and other supporting discrete components. The typi-

cal switching frequency is internally set to 1MHz. For

switching noise-sensitive applications, the µModule can

be externally synchronized to a clock within ±30% of the

set frequency. See the Applications Information section.

With current mode control and internal feedback loop

compensation, the LTM4632 module has sufficient sta-

bility margins and good transient performance with a wide

range of output capacitors, even with all ceramic output

capacitors.

Current mode control provides cycle-by-cycle fast cur-

rent limiting. An internal overvoltage and undervoltage

comparators pull the open-drain PGOOD output low if

the output feedback voltage exits a ±8% window around

the regulation point. Furthermore, an input overvoltage

protection been utilized by shutting down both power

MOSFETs when VIN rises above 17.5V to protect internal

devices.

Pulling the RUN pin below 1V forces the controller into

its shutdown state, turning off both power MOSFETs and

most of the internal control circuitry. At light load cur-

rents, burst mode operation can be enabled to achieve

higher efficiency compared to continuous mode (CCM) by

setting MODE pin to INTVCC. The TRACK/SS pin is used

for power supply tracking and soft-start programming.

See the Applications Information section.

APPLICATIONS INFORMATION

The typical LTM4632 application circuit is shown in

Figure 19. External component selection is primarily deter-

mined by the input voltage, the output voltage and the

maximum load current. Refer to Table 5 for specific exter-

nal capacitor requirements for a particular application.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN and VOUT step

down ratio that can be achieved for a given input voltage

due to the minimum off-time and minimum on-time limits

of the regulator. The minimum off-time limit imposes a

maximum duty cycle which can be calculated as:

DMAX = 1 – tOFF(MIN) • fSW

where tOFF(MIN) is the minimum off-time, 45ns typical for

LTM4632, and fSW is the switching frequency. Conversely

the minimum on-time limit imposes a minimum duty

cycle of the converter which can be calculated as

DMIN = tON(MIN) • fSW

where tON(MIN) is the minimum on-time, 20ns typical for

LTM4632. In the rare cases where the minimum duty

cycle is surpassed, the output voltage will still remain

in regulation, but the switching frequency will decrease

from its programmed value. Note that additional thermal

derating may be applied. See the Thermal Considerations

and Output Current Derating section in this data sheet.

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