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Fabricante

LTM4620AMPY

Dual 13A or Single 26A DC/DC µModule Regulator

Linear Technology 

LTM4620A

APPLICATIONS INFORMATION

900

800

700

600

500

400

300

200

100

0

0

0.5

1

1.5

2

2.5

fSET PIN VOLTAGE (V)

4620A F05

Figure 5. Operating Frequency vs fSET Pin Voltage

The LTM4620A switching frequency can be set with an exter-

nal resistor from the fSET pin to SGND. An accurate 10µA

current source into the resistor will set a voltage that pro-

grams the frequency or a DC voltage can be applied. Figure 5

shows a graph of frequency setting verses programming

voltage. An external clock can be applied to the MODE/

PLLIN pin from 0V to INTVCC over a frequency range of

400kHz to 780kHz. The clock input high threshold is 1.6V

and the clock input low threshold is 1V. The LTM4620A

has the PLL loop filter components on board. The fre-

quency setting resistor should always be present to set

the initial switching frequency before locking to an exter-

nal clock. Both regulators will operate in continuous

mode while being externally clock.

The output of the PLL phase detector has a pair of com-

plementary current sources that charge and discharge

the internal filter network. When the external clock is

applied then the fSET frequency resistor is disconnected

with an internal switch, and the current sources control

the frequency adjustment to lock to the incoming external

clock. When no external clock is applied, then the internal

switch is on, thus connecting the external fSET frequency

set resistor for free run operation.

Minimum On-Time

Minimum on-time tON is the smallest time duration that

the LTM4620A 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 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 master regulator’s out-

put is divided down with an external resistor divider that

is the same as the slave regulator’s feedback divider to

implement coincident tracking. The LTM4620A uses an

accurate 60.4k resistor internally for the top feedback

resistor for each channel. Figure 6 shows an example of

coincident tracking. Equations:

SLAVE

=





1+

60.4k

RTA





•

VTRACK

VTRACK is the track ramp applied to the slave’s track pin.

VTRACK has a control range of 0V to 0.6V, or the internal

reference voltage. When the master’s output is divided

down with the same resistor values used to set the slave’s

output, then the slave will coincident track with the master

until it reaches its final value. The master will continue to

its final value from the slave’s regulation point. Voltage

tracking is disabled when VTRACK is more than 0.6V. RTA

in Figure 6 will be equal to the RFB for coincident tracking.

Figure 7 shows the coincident tracking waveforms.

For more information www.linear.com/LTM4620A

4620afc

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