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LTM4615

4615f

OPERATION

LTM4615 POWER MODULE DESCRIPTION

Dual Switching Regulator Section

The LTM4615 is a standalone dual nonisolated switching

mode DC/DC power supply with an additional onboard 1.5A

VLDO. It can deliver up to 4A of DC output current for each

channel with few external input and output capacitors. This

module provides two precisely regulated output voltages

programmable via one external resistor for each channel

from 0.8V DC to 5V DC over a 2.375V to 5.5V input voltage.

The VLDO is an independent 1.5A linear regulator that can

be powered from either switching converter. The typical

application schematic is shown in Figure 12.

The LTM4615 has two integrated constant frequency cur-

rent mode regulators, with built-in power MOSFETs with

fast switching speed. The typical switching frequency is

1.25MHz. With current mode control and internal feedback

loop compensation, these switching regulators have suf-

ﬁcient stability margins and good transient performance

under a wide range of operating conditions, and with a

wide range of output capacitors, even all ceramic output

capacitors.

Current mode control provides cycle-by-cycle fast current

limit. Besides, current limiting is provided in an overcurrent

condition with thermal shutdown. In addition, internal

overvoltage and undervoltage comparators pull the open-

drain PGOOD outputs low if the particular output feedback

voltage exits a ±7.5% window around the regulation point.

Furthermore, in an overvoltage condition, internal top FET,

M1, is turned off and bottom FET, M2, is turned on and

held on until the overvoltage condition clears, or current

limit is exceeded.

Pulling each speciﬁc RUN pin below 0.8V forces the spe-

ciﬁc regulator controller into its shutdown state, turning

off both M1 and M2 for each power stage. At low load

current, each regulator works in continuous current mode

by default to achieve minimum output voltage ripple.

The TRACK/SS pins are used for power supply tracking

and soft-start programming for each speciﬁc regulator.

See the Applications Information section.

The LTM4615 is internally compensated to be stable over

the operating conditions. Table 4 provides a guideline for

input and output capacitance for several operating condi-

tions. The Linear Technology μModule Power Design Tool

will be provided for transient and stability analysis.

The FB pins are used to program the speciﬁc output volt-

age with a single resistor to ground.

VLDO Section

The VLDO (very low dropout) linear regulator operates

from a 1.14V to 3.5V input. The VLDO uses an internal

NMOS transistor as the pass device in a source-follower

conﬁguration. The BOOST3 pin is the output of an inter-

nal boost converter that supplies the higher supply drive

to the pass device for low dropout enhancement. The

internal boost converter operates on very low current,

thus optimizing high efﬁciency for the VLDO in close to

dropout operation.

An undervoltage lockout comparator on the LDO ensures

that the boost voltage is greater than 4.2V before enabling

the LDO, otherwise the LDO is disabled.

The LDO provides a high accuracy output capable of supply

1.5A of output current with a typical drop out of 100mV.

A single ceramic 10μF capacitor is all that is required

for output capacitor bypassing. A low reference voltage

allows the VLDO to have lower output voltages than the

commonly available LDO.

The device also includes current limit and thermal over-

load protection. The NMOS follower architecture has fast

transient response without the traditional high drive cur-

rents in dropout. The VLDO includes a soft-start feature

to prevent excessive current on the input during start-up.

When the VLDO is enabled, the soft-start circuitry gradu-

ally increases the 0.4V reference voltage over a period of

approximately 200μs.

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