LTC2977 datasheet(31/138 Pages) AD

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WINDOW COMPARATOR PROGRAMMING

In normal operation, VOUTn voltage should be always

close to half of the VINSn voltage. A floating window

comparator monitors the voltage on the VOUTn pin and

compares it with VINSn/2. The window hysteresis volt-

age can be programmed and is equal to the voltage at

the HYS_PRGMSn pin. There is a precision 10μA cur-

rent flowing out of HYS_PRGMSn pin. A single resistor

from HYS_PRGMSn pin to GND sets the HYS_PRGMSn

pin voltage, which equals the resistor value multiplied by

10μA current (e.g. the voltage is 1V with a 100k resis-

tor from the HYS_PRGMSn pin to GND). With a 100k

resistor on the HYS_PRGMSn pin, the VINSn/2 voltage

has to be within (VOUTn ± 1V) window during startup and

normal operation, otherwise a fault is triggered and the

LTM4664A divider stages stop switching.

The window hysteresis voltage can be linearly pro-

grammed from 0.3V to 2.4V with different resistor values

on HYS_PRGMNSn pin. If the HYS_PRGMSn pin is tied

to INTVCC, a default 0.8V hysteresis window is applied

internally. The hysteresis window voltage has to be pro-

grammed large enough to tolerate the VOUTn pin voltage

ripple and voltage drop at maximum load conditions. See

Figure 5. Small internal RC filters can be used on these two

pins to reject noise higher than the switching frequency.

Usually front end circuit breakers control the rate change

and slew rate on the main input power which will eliminate

this problem. As long as the input supply moves much

slower than the operating frequency of the 4:1 divider,

then the regulator will be able to balance without a need

to pre-balance.

OVERCURRENT PROTECTION

The LTM4664A 4:1 divider provides overcurrent protection

through a sensing resistor placed on the high voltage side.

A precision rail to rail comparator monitors the differential

voltage between INSNSSn+ pin and INSNSSn– pin which

are Kelvin connected to a sensing resistor. Whenever the

INSNSSn+ pin voltage is 50mV higher than INSNSSn– pin

voltage, overcurrent fault is triggered and the FAULTSn pin

is pulled down to ground. At the same time the divider

stage stops switching and starts retry mode based on

the timer pin setup. The overcurrent fault will be cleared

when the TIMERSn pin voltage reaches 4V and the voltage

across the sensing resistor is less than 50mV.

The current through the sensing resistor is a pulse cur-

rent during charging/discharging of the flying capacitors,

which may result a voltage higher than the 50mV thresh-

old at heavy loads. To prevent the inrush current from

falsely triggering the overcurrent protection, an RC filter

is required at the INSNSSn+ pin and INSNSSn– The RC

filter timer constant has to be larger than a switching

period. Typically a 100Ω and 0.1μF filter is good for most

of applications, and this filter is already included inside the

LTM4664A. The current limit can be selected by choos-

ing different sense resistor values. For example, 10mΩ

sensing resistor sets current limit at 50mV/10mΩ = 5A

ideally. Due to the switching ripple, the actual current limit

is always lower than the ideal case. In real circuits, the

current limit is around 4.2A with the 0.1μF/100Ω filter and

200kHz switching frequency. If overcurrent protection is

not used, short INSNSSn+ pin and INSNSSn– pin together

and connect them to the drain of the top as shown in

Figure 1 stage 2. Stage 2 current limit is not usually nec-

essary since stage 1 already has it implemented. Also

the dual 25A/30A regulators have overcurrent protection

discussed later in the data sheet.

LTM4664A

Rev. 0

For more information www.analog.com

4:1 DIVIDER APPLICATION INFORMATION

Figure 5. Relationship Between HYS_PRGM Pin Voltage

and VOUTn_SENSE Window Comparator Voltage

VHYS_PRGM (V)

2.5

1.5

0.5

4664A F05

LTC2977 Datasheet(PDF) 31 Page - Analog Devices

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