LTC3828 datasheet(14/32 Pages) LINER

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LTC3828

3828f

APPLICATIO S I FOR ATIO

Figure 5. Setup for Coincident and Ratiometric Tracking

VOUT2

(5a) Coincident Tracking Setup

VOSENSE1

TRCKSS2

VOSENSE2

VOUT1

VOUT2

3828 F05

(5b) Ratiometric Tracking Setup

VOSENSE1

TRCKSS2

VOSENSE2

VOUT1

Figure 6. Equivalent Input Circuit of

Error Amplifier of Channel 2

Figure 7. Alternative Setup for Coincident Tracking

–

TRCKSS

0.8V

VOSENSE

3828 F06

VOUT1

VOUT2

TO TRCKSS2 PIN

TO VOSENSE1 PIN

TO VOSENSE2 PIN

3828 F07

OUT

20 8

40 8

⎛

⎝

⎜

⎞

⎠

⎟

–,

–

OUT

30 8

50 8

⎛

⎝

⎜

⎞

⎠

⎟

–,

–

By selecting different resistors, the LTC3828 can achieve

different modes of tracking including the two in Figure 4.

Figure 6 helps to explaining the tracking function. At the

input stage of an error amplifier, two diodes are used to

clamp the equivalent reference voltage and an additional

diode is used to match the shifted common mode voltage.

The top two current sources are of the same value. When

the TRCKSS voltage is low, switch S1 is on and VOSENSE

follows the TRCKSS voltage. When the TRCKSS voltage

is close to 0.8V, the reference voltage, switch S1, is off

and VOSENSE follows the reference voltage. The regulation

for both channels’ outputs are not affected by the tracking

mode. In the ratiometric tracking mode, the two channels

do not exhibit cross talk.

The number of resistors in Figure 5a can be further

reduced with the scheme in Figure 7.

In a system that requires more than two tracked supplies,

multiple LTC3828s can be daisy-chained through the

TRCKSS1 pin. TRCKSS1 clamps channel 1’s reference in

the same manner TRCKSS2 clamps channel 2. To elimi-

nate the possibility of multiple LTC3828s coming on at

different times, only the master LTC3828’s TRCKSS1 pin

should be connected to a soft-start capacitor. Figure 8

shows the circuit with four outputs. Three of them are

programmed in the coincident mode while the fourth one

tracks ratiometrically. If output tracking is not needed, the

TRCKSS pins are used as soft start-up pins. The capaci-

tors connected to those pins set the soft-start ramping up

speed.

Figure 15 is a basic LTC3828 application circuit. External

component selection is driven by the load requirement,

and begins with the selection of RSENSE and the inductor

value. Next, the power MOSFETs and D1 are selected.

Finally, CIN and COUT are selected. The circuit shown in

Figure 15 is configured for operation up to an input

voltage of 28V (limited by the external MOSFETs).

RSENSE Selection For Output Current

RSENSE is chosen based on the required output current.

The current comparator has a maximum threshold of

75mV/RSENSE and an input common mode range of SGND

to 1.1(INTVCC). The current comparator threshold sets the

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