LTC4413 datasheet(17/32 Pages) AD

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LTC4421

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

when initially charging the output from 0V, so that

ILOAD,CHG = 0. The application circuit in Figure 5 utilizes

an LTC2965 voltage monitor on the output to disable the

output DC/DC converter until VOUT rises above 9V. Once

VOUT rises above 9V, the DC/DC remains enabled until

VOUT drops below 1V.

N-Channel MOSFET Selection

The LTC4421 drives N-Channel MOSFETs to conduct or

block current from an input supply voltage and an output

load current. The important features of the MOSFETs are:

1. BVDSS, the absolute maximum drain-source

voltage

2. VGS,MAX, the absolute maximum VGS voltage

3. VGS(TH), the threshold voltage

4. RDS(ON), the on-resistance

5. SOA, the safe operating area

The maximum allowable drain-source voltage, BVDSS,

must be higher than all supply voltages, as there are vari-

ous scenarios where the output voltage can be at the high-

est supply voltage when the input is at 0V, and vice versa.

Additionally, it must be higher than the clamping voltage

of Transient Voltage Suppressor (TVS) diodes D1 and D2.

Supplies with high input parasitic inductance may require

additional precautions. See the Input and Output Short

Circuits and Supply Transient Protection section for more

Figure 5. Load Current Hold Off. The LTC2965 Voltage Monitor Disables the DC/DC Output Load Current Until VOUT > 9V

SiR158DP

2.5m

RSENSE1

CCP0

1µF

COUT

220µF

RTH3

619k

RTH2

332k

RTH1

42.2k

CPO

CPOREF

CG1

47nF

VOUT

LTC4421

12V

VIN

INH

INL

PS RS

OUT

GND

LTC2965

DC/DC

ENABLE

LTC4421 F05

REF

GATE1

SOURCE1

SENSE1 OUT1

information. Choose MOSFETs having VGS,MAX = ±20V, to

handle the LTC4421’s 14V maximum gate drive voltage.

When the back-to-back MOSFETs turn on and conduct

current to the output, large drain-source voltages can

occur on the input side MOSFET as the output is charg-

ing. However, the drain-source voltage of the output side

MOSFET is limited to about 1V due to the body diode turn-

ing on, so the output side MOSFET is always in triode. As

a result, the input side MOSFET has much more stringent

SOA requirements. A MOSFET with lower SOA and lower

VGS(TH) can be used on the output side to minimize power

loss in that MOSFET.

The chosen MOSFET must be able to withstand an out-

put short circuit for longer than tTMR,FLT. During output

shorts, the LTC4421 regulates the short-circuit current

using its current limit regulation circuitry and runs the

overcurrent fault timer. When the short persists longer

than the programmed tTMR,FLT time, the LTC4421 turns

off the MOSFETs. The worst-case occurs when the out-

put is resistively shorted and the output voltage, VSHORT,

remains above the foldback comparator falling threshold

voltage, which is 410mV. In this case, the power during

the short circuit is given in Equation 11.

POWER ≈ V

IN • ILIM =

IN • 25mV

(

)

SENSE

(11)

where VIN is in the input voltage and VIN >> VSHORT.

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