LTC4367 datasheet(20/30 Pages) AD

20 / 30 page

LTC7000A

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Where 3.5V < Rising VCC UVLO < 10.5V.

RISING VCC UVLO

FALLING VCC UVLO

VCCUV RESISTOR TO GROUND (k)

90 120 150 180 210 240

7000A F11

Figure 11. VCCUV Resistor Selection

MOSFET Selection

The most important parameters in high voltage applica-

tions for MOSFET selection are the breakdown voltage

BVDSS, on-resistance RDS(ON) and the safe operating area,

SOA.

The MOSFET, when off, will see the full input range of the

input power supply plus any additional ringing than can

occur when driving inductive loads.

External conduction losses are minimized when using low

RDS(ON) MOSFETs. Since many high voltage MOSFETs

have higher threshold voltages (typical VTH ≥ 5V) and

RDS(ON) is directly related to the (VGS–VTH) of the

MOSFET, the LTC7000A/LTC7000A-1 maximum gate drive

of greater than 10V makes it an ideal solution to minimize

external conduction losses associated with external high

voltage MOSFETs.

SOA is specified in Typical Characteristic curves in power

N-channel MOSFET data sheets. The SOA curves show

the relationship between the voltages and current allowed

in a timed operation of a power MOSFET without causing

damage to the MOSFET. The overcurrent trip point (RSNS

and RISET) of the LTC7000A/LTC7000A-1 and TIMER

capacitor should be chosen to stay within the SOA region

of the MOSFET selected for the application.

Limiting Inrush Current During Turn-On

Driving large capacitive loads such as complex electrical

systems with large bypass capacitors should be powered

using the circuit shown in Figure 12. The pull-up gate

drive to the power MOSFET from TGUP is passed through

an RC delay network, RG and CG, which greatly reduces

the turn-on ramp rate of the MOSFET. Since the MOSFET

source voltage follows the gate voltage, the load is pow-

ered smoothly from ground. This dramatically reduces

the inrush current from the source supply and reduces

the transient ramp rate of the load allowing for slower

activation of sensitive electrical loads. The turn-off of the

MOSFET is not affected by the RC delay network as the

pull-down for the MOSFET gate is directly from the TGDN

pin. Note that the voltage rating on capacitor CG needs

to be the same or higher than the external MOSFET and

CLOAD.

Adding CG to the gate of the external MOSFET can cause

high frequency oscillation. A low power, low ohmic value

resistor (10Ω) should be placed in series with CG to

dampen the oscillations as shown in Figure 12 whenever

CG is used in an application. Alternatively, the low ohmic

value resistor can be placed in series with the gate of the

external MOSFET.

1µF

0.047µF

LOAD

SNS+

SNS–

RG 100k

RFLT

7000A F12

LTC7000A/

LTC7000A-1

RSNS

VIN

CLOAD

100µF

TGUP

TGDN

BST

Figure 12. Powering Large Capacitive Loads

The values for RG and CG to limit the inrush current can

be calculated from the below equation:

IIN_RUSH ≅

0.7 • 12V • CLOAD

RG •CG

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