LT8551 datasheet(20/32 Pages) AD

20 / 32 page

LT8551

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

compare the calculated value of TJ to the manufacturer’s

data sheets to help choose the appropriate PMOS that

will not overheat.

TJ = TA + P • RTH(JA)

(1)

Where:

TJ is the junction temperature of the PMOS

TA is the ambient air temperature

P is the power dissipation of the PMOS.

RTH(JA) is the MOSFET’s thermal resistance from the

junction to the ambient air. Refer to the manufacturer’s

data sheets.

To reduce the power dissipation in the external PMOS,

it’s helpful to power up the chip with a lower voltage aux

power supply (VBIAS) instead of sharing the same power

supply with the power stage, especially in a high input

voltage application. Large enough copper area on the PC

board is needed for the PMOS to alleviate thermal stress.

To ensure the loop stability, it’s also recommended to

choose a PMOS with Qg < 40nC.

Figure 13.

VIN (V)

100

220

13.6

8551 F13

CONSTANT

POWER

SET TO 100 mA

REG LDO Current Limit vs VBIAS Voltage

Operating Frequency Selection

The expander system (primary controller and LT8551

devices) adopts a constant frequency ranging from

100kHz to 1MHz determined by the master LT8551. The

primary controller and slave LT8551 devices are syn-

chronized to the master LT8551 by connecting the mas-

ter LT8551’s CLK1 pin to their SYNC pins as shown in

Figure 1. To minimize noise, it is recommended to add

an RC filter between master CLK1 and each primary’s or

slave’s SYNC pin. This RC filter should be close to SYNC

pins with typical value of R = 10Ω, C = 220pF.

The frequency can be set either by the internal oscillator,

or can be synchronized to an external clock source. A

trade-off between efficiency and component size exists in

selecting the switching frequency. Low frequency opera-

tion increases efficiency by reducing MOSFET switching

losses, but requires more inductance and/or capacitance

to maintain low output ripple voltage. The switching fre-

quency can be set by placing an appropriate resistor from

the RT/MS pin to ground and tying the SYNC pin low or

high. The frequency can also be synchronized to an exter-

nal clock source driven into the SYNC pin. The following

sections provide more details.

Internal Oscillator

The free-running switching frequency of the master

LT8551 can be set using the internal oscillator by tying

a resistor from RT/MS pin to ground while the SYNC pin

is driven low (<0.8V) or high (>1.2V). The oscillator fre-

quency is calculated using the following formula:

fOSC =

25,000

RT +0.15

kHz

(2)

Where fOSC is in kHz and RT is in kΩ. Conversely, RT (in

kΩ) be calculated from the desired frequency (in kHz)

using:

RT =

25,000

fOSC

Ω–0.15kΩ

(3)

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