LT8228 datasheet(16/69 Pages) AD

16 / 69 page

LT8228

Rev. A

For more information www.analog.com

PIN FUNCTIONS

SNS1P, SNS1N (Pins 1, 2): Positive and Negative Input

Terminals of the V1 Bidirectional Current Sense Amplifier

(CSA1 in the Block Diagram section). The pins allow cur-

rent monitoring and regulation of the V1 input current in

buck mode and V1 output current in boost mode. Current

sense polarity is positive for current flowing out of V1 into

V2. Place input gain resistors RIN1 between the current

sense resistor RSNS1 and these pins. Typical bias current

into these pins is 90µA for common mode voltage above

2.5V. As common mode voltage decreases below 2.5V,

bias current decreases and reverses direction. Refer to

the curve of IB1 over VCM1 in the Typical Performance

Characteristics section.

CSA1 is connected in a negative feedback loop to make

SNS1N and SNS1P pin voltages equal. The voltage across

the current sense resistor and the input gain resistors

generates a difference in current flowing into the SNS1N

and SNS1P pins, ISNS1N and ISNS1P. The current flowing

through RSNS1, ISNS1, includes the V1 current, the input

bias current of CSA1’s negative feedback terminal and the

differential current given by Equation 1.

ISNS1N – ISNS1P=

ISNS1•RSNS1

RIN1

(1)

In buck mode, this current difference is generated out of

the ISET1P and IMON1 pins. In boost mode, it is gener-

ated out of the ISET1N, ISHARE and IMON1 pins. Limit

the difference between SNS1N and SNS1P pin currents

to ±100µA by choosing the values of RSNS1 and RIN1

appropriately. Refer to the RSNS1 and RIN1 Selection in

Applications Information section for more details.

UV1 (Pin 3): Undervoltage Detection Input for V1. It is

a high impedance pin with the undervoltage detection

threshold set at 1.2V typically. The undervoltage level is

set using a resistor divider connected between V1 node

and ground. If V1 needs reverse voltage protection, con-

nect the resistor divider in series with a diode whose

anode is connected to V1. The status of the UV1 pin is

reported at the REPORT pin in buck mode.

If the DRXN pin is externally set high for buck mode oper-

ation and the UV1 pin voltage falls below its threshold

voltage, the FAULT and SS pins pull low and the LT8228

stops switching. If the DRXN pin is high but not externally

controlled, and the UV1 pin voltage falls below the thresh-

old voltage, the regulation mode changes from buck to

boost and the DRXN pin is internally driven low. See the

Operation section for more information. Tie the pin to

INTVCC if not used.

FB1 (Pin 4): V1D Feedback Voltage and Overvoltage

Detection Input. This pin is one of the boost mode error

amplifier’s (EA1 in the Block Diagram section) inverting

terminals. It is a high impedance pin and senses the V1D

voltage through an external resistor divider network. The

pin is regulated to the typical internal reference voltage

of 1.21V in boost mode.

V1D overvoltage detection threshold is set at 1.3V typi-

cally. The status of V1D overvoltage is reported at the

REPORT pin in boost mode. If the DRXN pin is externally

set low for boost mode operation and the FB1 pin voltage

rises above its overvoltage threshold voltage, the FAULT

pin pulls low. If the DRXN pin is low but not externally

controlled, and the FB1 pin voltage rises above the over-

voltage threshold voltage for a duration of 1024 switching

clock cycle, the regulation mode changes from boost to

buck and the DRXN pin is pulled high by the external pull-

up resistor. Tie the pin to ground if not used.

IMON1 (Pin 5): V1 Current Monitor Output. The current

out of this pin is equal to the absolute voltage across

the current sense resistor RSNS1 divided by the value of

the input sense resistor RIN1. This current represents

V1 input current in buck mode and V1 output current in

boost mode. Connecting a resistor RMON1, from IMON1

to ground generates a voltage VMON1 for monitoring by an

external ADC. The maximum dynamic range for IMON1 is

2.5V. To set RMON1, first determine the maximum monitor

voltage VMON1MAX based on ADC input dynamic range.

Next, calculate the value of RMON1 with Equation 2.

RMON1 =

RIN1

ISNS1MAX • RSNS1

• VMON1MAX

(2)

where ISNS1MAX is the maximum of the programmed V1

output current limit IV1N(LIM) in boost mode or the pro-

grammed V1 input current limit IV1P(LIM) in buck mode.

A filtering capacitor can be added to read the average

current at the ADC input. Refer to the RMON1 Selection for

V1 Current Monitoring in Applications Information section

LT8228 Datasheet(PDF) 16 Page - Analog Devices

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