LT8551 datasheet(16/32 Pages) AD

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LT8551

Rev 0

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OPERATION

Primary Controller’s Switch State Detection

This section discusses a system with multiple LT8551

devices. For a single LT8551 system, simply ignore the

slave LT8551 devices in this discussion.

The primary controller’s switch states are detected by

the master LT8551, and this information is used by both

master and slave LT8551 devices.

The primary controller’s top switch state is sensed by

the master LT8551’s TGSR, TGSH, and TGSL pins, as

shown in Figure 5. This floating top gate logic signal is

converted to a ground based logic signal, and outputted

to the TGBUF pin. The master LT8551’s TGBUF is then

connected to the downstream slaves’ TGSH, as shown in

Figure 5. The slave’s TGSR and TGSL are connected to the

REG and GND respectively. If the (TGSR–TGSL) voltage is

less than 2.3V (typical), the TGBUF pin will be forced to

ground for both of the master and slave LT8551. See the

Applications Information section for more information.

Figure 5.

REG

• • •

8551 F05

MASTER

LT8551

TGBUF

BGBUF

TGSR

TGSH

TGSL

BGSH

RT/MS

PRIMARY

CONTROLLER

BST

SLAVE

LT8551

TGBUF

BGBUF

TGSR

TGSH

TGSL

BGSH

RT/MS

SLAVE

LT8551

TGBUF

BGBUF

TGSR

TGSH

TGSL

BGSH

RT/MS

Gate Sensing in a Multiple LT8551 System

A similar method is used for the primary controller’s bot-

tom switch state detection, also shown in Figure 5. Since

the primary BG is a ground based signal, only one pin

(BGSH) is needed for the primary’s BG detection. Since

the master LT8551 passes the primary’s switch states to

the slaves, this avoids routing the primary’s noisy BST,

TG, SW and BG signals around the board.

Primary Controller’s Inductor Current Sensing

This section discusses a system with multiple LT8551

devices. For a single LT8551 system, simply ignore the

slave LT8551 devices in this discussion.

The primary controller’s inductor current is detected by

the master LT8551, and this information is used by both

the master and slave LT8551 devices.

As shown in Figure 6, the primary controller’s inductor

current is detected by the master LT8551’s ISP and ISN

pins. This signal is amplified, and then outputted to the

master LT8551’s IAMPP pin. For stability purposes, a cap

is required from the IAMPP to GND, and total capaci-

tance must be between 100pF and 470pF. For a slave

LT8551, the ISP and ISN pins are not used and should

be grounded, and the IAMPP is an input pin. To pass the

primary’s inductor current information from the master

LT8551 to the slave LT8551 devices, all LT8551 devices’

IAMPP and IAMPN pins are connected together, respec-

tively. Connect the IAMPN pins to the master LT8551’s

local ground. The IAMPP and IAMPN are connected to the

inputs of a unity gain differential sense amplify (A7 in the

Block Diagram). See the Applications Information section

for more information.

Figure 6.

REG

• • •

RSENSE

8551 F06

MASTER

LT8551

IAMPP

IAMPN

ISN

ISP

RT/MS

SLAVE

LT8551

IAMPP

IAMPN

ISN

ISP

RT/MS

SLAVE

LT8551

IAMPP

IAMPN

ISN

ISP

RT/MS

Primary Regulator’s Switch Current

Sensing in Multiple LT8551 System

The signal across the primary inductor current sense

resistor is only tens of mV. By passing the amplified cur-

rent signal from the master to the slaves, routing sensitive

small signals around the board is avoided.

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