Data Sheet

LT8638-2

analog.com

Rev.0

21 of 35

500kHz. The slope compensation is set by the RT value, while the minimum slope compensation required to avoid

subharmonic oscillations is established by the inductor size, input voltage, and output voltage. Since the

synchronization frequency does not change the slopes of the inductor current waveform, if the inductor is large

enough to avoid subharmonic oscillations at the frequency set by RT, then the slope compensation is sufficient for

all synchronization frequencies.

FB Resistor Network

The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor

values according to equation 1.

R1 = R2 (

VOUT

0.6V

− 1)

(1)

Reference designators see the BLOCK DIAGRAM. 1% resistors are recommended to maintain output voltage accuracy.

When using large FB resistors, a 4.7pF to 47pF phase-lead capacitor should be connected from VOUT to FB.

Setting the Switching Frequency

The LT8638-2 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 3MHz

by using a resistor tied from the RT pin to ground. Table 4 shows the necessary RT value for a desired switching

frequency.

The RTresistor required for a desired switching frequency can be calculated using Equation 2.

RT =

44.8

fSW

− 5.9

where, RT is in kΩ and fSW is the desired switching frequency in MHz.

Table 4. SW Frequency vs. RT Value

fSW (MHz)

RT (kΩ)

0.2

226

0.3

143

0.4

105

0.5

82.5

0.6

66.5

0.7

56.2

0.8

48.7

1.0

38.3

1.2

31.6

1.4

26.1

1.6

22.1

1.8

19.1

2.0

16.9

2.2

15.4

3.0

10.5