LT8610AB datasheet(22/28 Pages) AD

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LT8638S

Rev. 0

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APPLICATIONS INFORMATION

gm = 12S

gm = 1.4mS

CURRENT MODE

POWER STAGE

LT8638S

500k

0.6V

8638S F05

–

CPL

COUT

OUTPUT

Figure 5. Model for Loop Response

Table 2 provides a guidance for the compensation val-

ues of several typical applications. Slight tweaks to these

values may be required depending on the specific appli-

cation. All applications were using R1 = 100k

Table 2. Compensation Values

VOUT

fSW

COUT

CPL

3.3V

400k

820pF

8.87k

47μF ×3

33pF

3.3V

220pF

12.1k

47μF ×2

15pF

400k

820pF

9.31k

47μF ×3

33pF

220pF

13.7k

47μF

10pF

Output Voltage Tracking and Soft-Start

The LT8638S allows the user to program its output

voltage ramp rate by means of the SS pin. An internal

2µA pulls up the SS pin to INTVCC. Putting an external

capacitor on SS enables soft starting the output to prevent

current surge on the input supply. During the soft-start

ramp the output voltage will proportionally track the SS

pin voltage. For output tracking applications, SS can be

externally driven by another voltage source. From 0V to

1V, the SS voltage will override the internal 0.6V refer-

ence input to the error amplifier, thus regulating the FB

pin voltage to a function of the SS pin. See plot in the

Typical Performance Characteristics section. When SS is

above 1V, tracking is disabled and the feedback voltage

will regulate to the internal reference voltage. The SS pin

may be left floating if the function is not needed.

An active pull-down circuit is connected to the SS pin

which will discharge the external soft-start capacitor in

the case of fault conditions and restart the ramp when the

faults are cleared. Fault conditions that clear the soft-start

capacitor are the EN/UV pin transitioning low, VIN voltage

falling too low, or thermal shutdown.

Multiphase Operation

For output loads that demand more current, multiple

LT8638S devices can be connected in parallel to the

same output. To do this, the VC and FB pins are con-

nected together, and each LT8638S’s SW node is con-

nected to the common output through its own inductor.

The CLKOUT signal can be connected to the SYNC/MODE

pin of the following LT8638S to line up both the frequency

and the phase of the entire system. Tying the PHMODE

pin to GND, INTVCC, or floating the pin generates a phase

difference between the LT8638S device’s internal clock

and CLKOUT of 180 degrees, 90 degrees, or 120 degrees

respectively, which corresponds to 2-phase, 4-phase, or

3-phase operation. A total of 12 phases can be paralleled

to run simultaneously with interleaving phase shift with

respect to each other by programming the PHMODE pin

of each LT8638S to different voltage levels. During FCM,

Spread Spectrum, and Synchronization modes, all devices

will operate at the same frequency. Figure 6 shows a

2-phase application where two LT8638Ss are paralleled

to get one output capable of up to 20A.

8638S F06

CPL

COUT

VOUT

20A

LT8638S

CLKOUT

PHMODE

LT8638S

SYNC/MODE

Figure 6. Paralleling Two LT8638S devices

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