LT8640 datasheet(20/26 Pages) AD

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reference input to the error amplifier, thus regulating the

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

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.

Output Power Good

When the LT8642-1’s output voltage is within the ±8%

window of the regulation point, the output voltage is

considered good and the open-drain PG pin goes high

impedance and is typically pulled high with an external

resistor. Otherwise, the internal pull-down device will pull

the PG pin low. To prevent glitching both the upper and

lower thresholds include 0.4% of hysteresis. PG is valid

when VIN is above 2.8V.

The PG pin is also actively pulled low during several fault

conditions: EN/UV pin is below 0.99V, INTVCC has fallen

too low, VIN is too low, or thermal shutdown.

Paralleling

To increase the possible output current, two LT8642-1s

can be connected in parallel to the same output. To do

this, the VC and FB pins are connected together, and each

LT8642-1’s SW node is connected to the common out-

put through its own inductor. The CLKOUT pin of one

LT8642-1 should be connected to the SYNC/MODE pin

of the second LT8642-1 to have both devices operate

in the same mode. During FCM, Spread Spectrum, and

Synchronization modes, both devices will operate at the

same frequency and phase. Figure 6 shows an application

where two LT8642-1s are paralleled to get one output

capable of up to 20A.

LT8642-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Table 2 provides a guideline for the compensation values

of several typical applications. Slight tweaks to these val-

ues may be required depending on the specific applica-

tion. All applications were using R1 = 100k.

Table 2. Compensation Values

VOUT

fSW

CPL

COUT

1.0V

1MHz

470pF

7.68k

47pF

2× 220μF

1.0V

2MHz

330pF

10.2k

47pF

2× 220μF

1.2V

1MHz

680pF

10.2k

47pF

2× 220μF

1.2V

2MHz

150pF

11k

47pF

2× 220μF

3.3V

1MHz

470pF

15k

18pF

2× 47μF

3.3V

2MHz

220pF

6.98k

39pF

2× 47μF

Output Voltage Tracking and Soft-Start

The LT8642-1 allows the user to program its output

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

1.9µA pulls up the SS pin to INTVCC. Putting an external

capacitor on SS enables soft starting the output to pre-

vent 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 0.597V

Figure 5. Model for Loop Response

225k

0.597V

86421 F05

CPL

LT8642-1

CURRENT MODE

POWER STAGE

COUT

OUTPUT

Gm = 8.5S

gm = 1.75mS

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