LT4351CMS datasheet(10/20 Pages) LINER

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LT4351

sn4351 4351fs

APPLICATIO S I FOR ATIO

In that case the resistor values are set by:

VUV

HYST

UVHYST

FAULT

()

–•

–

•

•–

•

Hysteresis helps prevent erratic behavior due to the noise

on VIN. Two of the most common noise sources are: VIN

dipping when the MOSFETs first turn on and draw down the

voltage on the VINcapacitors,andtheboostregulatorswitch

turning on and drawing current from the VIN capacitors.

Use low ESR capacitors for VIN and OUT filtering.

Note that because the UV pin uses current hysteresis,

placing a capacitor on UV to ground to filter noise will

reduce the effective hysteresis. Filtering can be achieved

by splitting the R2 resistor as shown in Figure 4.

To defeat undervoltage fault detection, the UV pin should

be tied higher than 0.33V. UV can be tied to VIN provided

VIN < 9V. Overvoltage fault detection can be defeated by

grounding the OV pin. Do not exceed VIN.

Boost Regulator

The boost regulator will start working as soon as VIN is

greater than 0.85V. The regulator will supply all the current

for the gate drive amplifier. While the amplifier itself

requires only about 3mA, larger current pulses are re-

quired when charging the MOSFET gate. The reservoir

capacitor on VDD will provide this current (Figure 6).

OVERVOLTAGE FILTERED FAULT

INPUT

REFERRED

VUV = 0.33V

VUV = 0.3V

VUV < 0.3V

VOV > 0.3V

VOV = 0.3V

4351 F05

OVFAULT

UVFAULT + UVHYST

UVFAULT

UNDERVOLTAGE HYSTERESIS

OVERVOLTAGE FAULT:

GATE LOW

UNDERVOLTAGE FAULT:

GATE LOW

GATE CONTROLLED

BY VIN – VOUT

Figure 5. Graphical Representation of the UV and OV Functions

Figure 6

QSW

GND

VDD

CDD

4351 F06

VIN

LT4351

External Shutdown

To externally turn off the MOSFETs, such as to disable the

supply, use an open-collector transistor pulling down on

the UV pin. Note this will not turn off the boost regulator

which will continue to operate.

The regulator performance is relatively insensitive to the

inductor value. The inductor value does control the fre-

quency of operation. A 4.7

µH inductor is recommended

for VIN voltages less than 10V and 10µH for VIN voltages

greater than 10V. Several inductors that work well with the

LT4351 are listed in Table 1. Many different sizes and

shapes are available. Consult each manufacturer for more

detailed information and for their entire selection of related

parts. The switching frequency for the boost regulator is

around 1MHz so ferrite core inductors should be used to

obtain the best efficiency. The inductor must handle a peak

current of 0.7A minimum and have a DC resistance of

0.5

Ω or less. Shielded inductors are recommended to

reduce the noise due to inductive switching.

Table 1. Recommended Inductors

MAX

IND

DCR

PART NUMBER

(

µH)

Ω)

VENDOR

DS1608C-472

4.7

Coilcraft

DS1608C-103

160

(847) 639-6400

www.coilcraft.com

CDRH4D18-4R7

4.7

125

Sumida

CDRH4D28-100

(847) 956-0666

www.sumida.com

LMNP045B4R7N

4.7

Taiyo Yuden

LMNP045B100M

(408) 573-4150

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