LTM4680 datasheet(16/36 Pages) AD

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LTM4650-2

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 8. Example of Output Tracking Application Circuit

Minimum On-Time

Minimum on-time tON is the smallest time duration that the

LTM4650-2 is capable of turning on the top MOSFET on

either channel. It is determined by internal timing delays,

and the gate charge required turning on the top MOSFET.

Low-duty cycle applications may approach this minimum

on-time limit, and care should be taken to ensure that:

VOUT

VIN • FREQ

> tON(MIN)

If the duty cycle falls below what can be accommodated

by the minimum on time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the output ripple and current will increase. The on-time

can be increased by lowering the switching frequency. A

good rule of thumb is to keep on time longer than 110ns.

Output Voltage Tracking

Output voltage tracking can be programmed externally

using the TRACK pins. The output can be tracked up and

down with another regulator. The main regulator’s output

is divided down with an external resistor divider that is

the same as the subordinate regulator’s feedback divider

to implement coincident tracking. The LTM4650-2 uses

an accurate 60.4k resistor internally for the top feedback

resistor for each channel. Figure 8 shows an example of

coincident tracking. The equation:

SUBORDINATE = 1+

60.4k

RTA

• VTRACK

VTRACK is the track ramp applied to the subordinate’s

track pin. VTRACK has a control range of 0V to 0.6V, or

the internal reference voltage. When the main’s output is

divided down with the same resistor values used to set the

subordinate’s output, then the subordinate will coincident

46502 F08

LTM4650-2

VIN

TEMP

RUN1

RUN2

TRACK1

TRACK2

fSET

470µF

6.3V

RFB

60.4k

10k

100µF

6.3V

PHASMD

VOUT1

VOUTS1

SW1

VFB1

VFB2

COMP1

COMP2

VOUTS2

VOUT2

SW2

PGOOD2

MODE_PLLIN CLKOUT INTVCC EXTVCC PGOOD1

PGOOD

INTVCC

SGND

GND

1.5V

MAIN

RAMP TIME

tSOFTSTART = (CSS/1.3µA) • 0.6

DIFFP

DIFFN DIFFOUT

40.2k

4.02k

2200pF

PGOOD

SUBORDINATE

VOUT1

(MAIN)

1.5V, 25A

470µF

6.3V

100µF

6.3V

4.02k

2200pF

121k

RTB

60.4k

4V TO 15V INTERMEDIATE BUS

100k

CSS

0.1µF

22µF

25V

RTA

60.4k

22µF

25V

22µF

25V

22µF

25V

C10

4.7µF

10k

INTVCC

VOUT2

(SUBORDINATE)

1.2V, 25A

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