LTM4657 datasheet(8/24 Pages) AD

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LTM4705

Rev. 0

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OPERATION

APPLICATIONS INFORMATION

The LTM4705 is a dual output standalone nonisolated

switch mode DC/DC power supply. It can deliver two 5A

DC outputs with few external input and output ceramic

capacitors. This module provides dual precisely regulated

output voltages programmable via two external resistors

from 0.6V to 5.5V over 3.1V to 20V input voltage range.

Both channels share the same clock and run 180° out

of phase. The typical application schematic is shown

in Figure 23.

The LTM4705 contains an integrated controlled on-time

valley current mode regulator, power MOSFETs, inductor

and other supporting discrete components. The default

switching frequency is 1MHz. For output voltages between

2.5V and 5V, an external resistor is required between

FREQ and GND pins to set the operating frequency to

higher frequency to optimize inductor current ripple. For

switching noise-sensitive applications, the µModule can

be externally synchronized to a clock. See the Applications

Information section.

With current mode control and internal feedback loop

compensation, the LTM4705 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 current

limiting. An internal overvoltage and undervoltage com-

parators pull the open-drain PGOOD output low if the out-

put feedback voltage exits a ±8% window around the regu-

lation point. Furthermore, the input overvoltage protection

will be activated by shutting down both power MOSFETs

when VIN rises above 22.5V to protect internal devices.

Multiphase operation can be easily employed by con-

necting the SYNC pin to an external oscillator. Multiple

LTM4705s can be paralleled to run simultaneously with

good current sharing guaranteed by the current mode

control loop.

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 currents,

burst mode operation can be enabled to achieve higher

efficiency compared to continuous current 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.

The typical LTM4705 application circuit is shown in

Figure 23. External component selection is primarily deter-

mined by the input voltage, the output voltage and the

maximum load current. Refer to Table 8 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 by Equation 1.

DMAX = 1 – tOFF(MIN) • fSW

(1)

where tOFF(MIN) is the minimum off-time, 100ns typical for

LTM4705, and fSW is the switching frequency. Conversely

the minimum on-time limit imposes a minimum duty cycle

of the converter which can be calculated by Equation 2.

DMIN = tON(MIN) • fSW

(2)

where tON(MIN) is the minimum on-time, 30ns typical for

LTM4705. 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.

Output Voltage Programming

The PWM controller has an internal 0.6V reference volt-

age. As shown in the Block Diagram, a 60.4k 0.5% internal

feedback resistor connects VOUT and FB pins together.

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