LTM4608 datasheet(10/24 Pages) LINER

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LTM4608

4608f

OPERATION

Current mode control provides cycle-by-cycle fast current

limit and thermal shutdown in an overcurrent condition.

Internal overvoltage and undervoltage comparators pull the

open-drain PGOOD output low if the output feedback volt-

age exits a ±10% window around the regulation point.

Pulling the RUN pin below 1.3V forces the controller into

its shutdown state, by turning off both M1 and M2 at low

load current. The TRACK pin is used for programming the

output voltage ramp and voltage tracking during start-up.

See Applications Information.

The LTM4608 is internally compensated to be stable over

all operating conditions. Table 3 provides a guideline for

input and output capacitances for several operating con-

ditions. The Linear Technology μModule Power Design

Tool is provided for transient and stability analysis. The

FB pin is used to program the output voltage with a single

external resistor to ground.

The typical LTM4608 application circuit is shown in

Figure 18. External component selection is primarily

determined by the maximum load current and output

voltage. Refer to Table 3 for speciﬁc external capacitor

requirements for a particular application.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN to VOUT step-

down ratio that can be achieved for a given input voltage.

The LTM4608 is 100% duty cycle, but the VIN to VOUT

minimum drop out is still shown as a function of its load

current. For 5V input, all outputs can deliver 8A. For 3.3V

input, all outputs can deliver 8A, except 2.5V which is

limited to 6A.

Output Voltage Programming

The PWM controller has an internal 0.596V reference

voltage. As shown in the Block Diagram, a 10kΩ/0.5%

internal feedback resistor connects VOUT and FB pins

together. The output voltage will default to 0.596V with

no feedback resistor. Adding a resistor RFB from FB pin

to GND programs the output voltage:

Multiphase operation can be easily employed with the

synchronization and phase mode controls. Up to 12 phases

can be cascaded to run simultaneously with respect to each

other by programming the PHMODE pin to different levels.

The LTM4608 has clock in and clock out for poly phasing

multiple devices or frequency synchronization.

High efﬁciency at light loads can be accomplished with

selectable Burst Mode operation using the MODE pin. These

light load features will accommodate battery operation.

Efﬁciency graphs are provided for light load operation in

the Typial Performance Characteristics.

Output voltage margining is supported, and can be pro-

gramed from ±5% to ±15% using the MGN and BSEL pins.

The PGOOD pin is disabled during margining

APPLICATIONS INFORMATION

OUT

0 596

.•

Table 2. RFB Resistor vs Output Voltage

VOUT

0.596V

1.2V

1.5V

1.8V

2.5V

3.3V

RFB

Open

10k

6.65k

4.87k

3.09k

2.21k

Input Capacitors

The LTM4608 module should be connected to a low AC

impedance DC source. Three 10μF ceramic capacitors

are included inside the module. Additional input capaci-

tors are only needed if a large load step is required up to

the 4A level. A 47μF to 100μF surface mount aluminum

electrolytic bulk capacitor can be used for more input bulk

capacitance. This bulk input capacitor is only needed if

the input source impedance is compromised by long in-

ductive leads, traces or not enough source capacitance.

If low impedance power planes are used, then this 47μF

capacitor is not needed.

For a buck converter, the switching duty-cycle can be

estimated as:

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