LTM4619 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTM4619

Dual, 26VIN, 4A DC/DC µModule Regulator

Linear Technology 

LTM4619

APPLICATIONS INFORMATION

Soft-Start and Tracking

The LTM4619 has the ability to either soft-start by itself

with a capacitor or track the output of another channel or

external supply. When one particular channel is configured

to soft-start by itself, a capacitor should be connected to

its TK/SS pin. This channel is in the shutdown state if its

RUN pin voltage is below 1.2V. Its TK/SS pin is actively

pulled to ground in this shutdown state.

Once the RUN pin voltage is above 1.2V, the channel pow-

ers up. A soft-start current of 1.3μA then starts to charge

its soft-start capacitor. Note that soft-start or tracking is

achieved not by limiting the maximum output current of

the controller but by controlling the output ramp voltage

according to the ramp rate on the TK/SS pin. Current

foldback is disabled during this phase to ensure smooth

soft-start or tracking. The soft-start or tracking range is

defined to be the voltage range from 0V to 0.8V on the

TK/SS pin. The total soft-start time can be calculated as:

tSOFT-START

=

0.8V • CSS

1.3µA

Output voltage tracking can be programmed externally

using the TK/SS pin. The master channel is divided down

with an external resistor divider that is the same as the

slave channel’s feedback divider to implement coincident

tracking. The LTM4619 uses an accurate 60.4k resistor

internally for the top feedback resistor. Figure 3 shows an

example of coincident tracking. Figure 4 shows the output

voltages with coincident tracking.

VSLAVE

=

⎛

⎝⎜

1+

R1⎞

R2 ⎠⎟

•

VTRACK

VTRACK is the track ramp applied to the slave’s TK/SS2

pin. VTRACK has a control range of 0V to 0.8V. 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.

Ratiometric modes of tracking can be achieved by select-

ing different divider resistors values to change the output

tracking ratio. The master output must be greater than the

slave output for the tracking to work. Master and slave

data inputs can be used to implement the correct resistors

values for coincident or ratio tracking.

VIN

5.5V TO

28V

VOUT1

3.3V

CIN

R3

19.1k

MODE/PLLIN INTVCC

VIN

FREQ/PLLFLTR

C2

22pF

VFB1

VFB2

COMP1

COMP2

VOUT1 LTM4619 VOUT2

COUT1

C1

0.1μF

TK/SS1

RUN1

TK/SS2

RUN2

PGOOD

SGND

EXTVCC

PGND

4619 F03

C3

22pF

R4

28k

VOUT2

2.5V

COUT2

VOUT1

R1

60.4k

R2

28k

OUTPUT

VOLTAGE

MASTER OUTPUT

SLAVE OUTPUT

TIME

4619 F04

Figure 3. Example of Coincident Tracking

Figure 4. Coincident Tracking

4619f

11

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