LTC3351 datasheet(39/56 Pages) LINER

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LTC3351

3351f

For more information www.linear.com/LTC3351

APPLICATIONS INFORMATION

Higher shunt current may be accommodated with an ex-

ternal NMOS and resistor across each capacitor as shown

in Figure 13. The shunt current will be VSHUNT divided by

Rx (assuming the RDS(ON) of MNx is small compared to

Rx). The logic level NMOS must be selected so that its

VGS at the desired shunt current is less than one half the

VSHUNTvoltage.Whenusingthiscircuittheshuntresistors

should be increased to 10Ω to minimize the drop across

the LTC3351’s internal shunt FET and thus maximize the

VGS for MNx. Using 10Ω resistors will also reduce the

power dissipation allowing a smaller package size com-

pared to the typical application’s 2.7Ω. When using 10Ω

resistors the internal shunt FET will turn on the external

NMOS, however the internal balancer’s current will not

create enough voltage across the 10Ω resistors to turn

on the external NMOS. Additionally, increasing the 10Ω

resistors to 500Ω will allow the internal balancers to turn

on the external FETs, thus also increasing balance current.

Input Short Protection

TheLTC3351’sinputcurrentsenseamplifier,ISNSP_HSto

ISNSM and ISNSP_CHG to can have at most 0.3V between

them. This is not typically an issue since the LTC3351

regulates input current to 48mV (hot swap current limit)

or 32mV (charger input current limit) using these pins.

The LTC3351 hot swap does not regulate reverse input

current. If the input voltage is rapidly reduced (the input

directly shorted to ground for instance) large currents

will flow from the VOUT capacitance to the VIN short.

Unless external protection is used, these large currents

can quickly produce many volts of drop across the input

sense resistor(s), overstressing the input current sense

pins. If the input is shorted after it has fallen below the

UV threshold, this is not an issue since the hot swap FETs

have already been turned off. However, if the input supply

can be shorted to ground while the LTC3351 is connected,

the input protection circuit in Figure 14 is recommended.

Also if the system load is so low that the LTC3351 circuit

can be disconnected from the input and then shorted to

ground before the input falls below UV, then the same

circuit is also recommended.

Figure 14.

LTC3351

ISNSP_X

ISNSM

VOUT

VIN

3351 F14

ISNSP_X

Input Short Protection Circuit

Since there are several possible configurations for the

input current sense resistors, not all possible protection

circuits are shown. To construct a circuit for another con-

figuration of input current sense resistors, there should

be one Schottky diode for each current sense ampli-

fier (ISNSP_HS-ISNSM and ISNSP_CHG-ISNSM). There

should be 1Ω between the current sense resistor and the

current sense pins, and the diode should be connected

such that it can conduct current in the direction from VOUT

to VIN, this means the anodes will be connected to ISNSM.

In this circuit, the Schottky diode is a simple clamp to limit

the voltage across the pins. The 1Ω resistors are needed

to limit the current in each Schottky diode. Clearly, if only

one sense resistor is used, only one protection diode is

needed.

Supercapacitor Settling

The ESR-only supercapacitor model using a capacitor in

series with an equivalent series resistance (ESR) is over-

simplified.Realsupercapacitorshaveanadditionalsettling

time due to their internal physics

To explain the implications of this settling, assume a super

capacitor of infinite capacitance such that any voltage

change due to charging or discharging can be ignored. In

thesimpleESR-onlymodel,apulseofcurrentwouldsimply

result in a step in voltage at its rising edge and a step back

to its initial voltage at its falling edge. A real supercapaci-

tor’s response to the same pulse of current’s rising edge

will be an instantaneous step in voltage due to the high

frequency ESR, followed by a settling to a higher voltage

due to the “DC” ESR. On the falling edge there will be an

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