LT8601 datasheet(13/26 Pages) LINER

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LT8601

8601fa

For more information www.linear.com/LT8601

applicaTions inForMaTion

operaTion

Power-On Reset Timer

The LT8601 includes a power-on reset timer. The power-

on reset timeout period is adjustable using an external

capacitor on the CPOR pin as described in the Applications

Information section. The timer is initiated when the

POREN pin is higher than 1.2V (typical).

The output of the POR timer, the RST pin, is an open-drain

output with a weak internal pull-up of 100kΩ (typical) to

approximately 2V. RST is held low until the expiration of

the POR timer. The RST pin is only valid when the LT8601

is enabled and INTVCC is above 2.7V.

Setting the Output Voltages

The output voltages are set by the resistor dividers on the

outputs as shown in Figure 1. The formula used is:

R1=R2•

VOUTx

VFB

–1

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟

where VOUTx is the output voltage of regulator x and VFB

is the feedback reference voltage. VFB is 1V for the high

voltage regulators (1 and 2) and 800mV for the low voltage

channel. R2 should be 200k or less to avoid noise problems.

To improve the frequency response, a feedforward capaci-

tor Cff may also be used. Typical values are 10pF to 100pF.

Great care should be taken to route the FB node away from

noise sources, such as an inductor or a SW line.

Switching Frequency

The LT8601 uses a constant frequency architecture that can

be programmed from 250kHz to 2.2MHz by tying a resistor

from the RT pin to ground. Table 1 shows the closest 1%

resistor value of RT for common switching frequencies.

Table 1. Switching Frequency vs RT Value

SWITCHING FREQUENCY (MHz)

RT (kΩ)

0.25

255

0.35

178

0.5

124

0.75

80.6

1.0

60.4

1.25

47.5

1.5

39.2

1.75

33.2

2.0

28.7

2.2

26.1

The following equation approximates the values shown

in Table 1:

RT =

61.9

fS –0.009

–1.9

where RT is in kΩ and fS is in MHz.

Selection of the operating frequency is mainly a trade-off

between efficiency and component size. The advantage

of high frequency operation is that smaller inductor and

capacitor values may be used. The advantage of low fre-

quency operation is higher efficiency.

The high switching frequency also decreases the duty

cycle range because of finite minimum on- and off-times

independent of the switching frequency. The minimum

and maximum duty cycles are:

DCMIN = fS • tON(MIN)

DCMAX = 1 – fS • tOFF(MIN)

where fS is the switching frequency, tON(MIN) is the

minimum switch on-time, and tOFF(MIN) is the minimum

switch off-time. These equations illustrate how duty cycle

range increases when switching frequency decreases.

Information about individual channel minimum on and

off times can be found in the Electrical Characteristics

table and Typical Performance curves section.

Figure 1. Feedback Resistor Divider

8601 F01

FBx

SWx

VOUTx

COUT

LT8601

Cff

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