LTC4367 datasheet(21/30 Pages) AD

21 / 30 page

LTC7000A

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

For the values shown in Figure 12 the inrush current will be:

IIN_RUSH ≅

0.7 • 12V • 100µF

100kΩ • 0.047µF

≅ 180mA

Correspondingly, the ramp rate at the load for the circuit

in Figure 12 is approximately:

∆ VLOAD

∆T

≅

0.7 • 12V

RG •CG

≅ 2V/ ms

When CG is added to the circuit in Figure 12, the value

of the bootstrap capacitor, CB, must be increased to be

able to supply the charge to both to MOSFET gate and

capacitor CG. The relationship for CB that needs to be

maintained when CG is used is given by:

CB >

MOSFET QG

+ 10 •CG

Optional Schottky Diode Usage on TS

When turning off a power MOSFET that is connected to

an inductive load (inductor, long wire or complex load),

the TS pin can be pulled below ground until the current in

the inductive load has completely discharged. The TS pin

is tolerant of voltages down to –6V, however, an optional

Schottky diode with a voltage rating at least as high as the

load voltage should be connected between TS and ground

to prevent discharging the load through the TS pin of the

LTC7000A/LTC7000A-1. See Figure 13.

SNS+

SNS–

TGUP

TGDN

7000A F13

LTC7000A/

LTC7000A-1

RSNS

M1A

VIN

LOAD

RFLT

Figure 13. Optional Schottky Diode Usage

Reverse Current Protection

To protect the load from discharging back into VIN when

the external MOSFET is off and the VIN voltage drops

below the load voltage, two external N-channel MOSFETs

should be used and must be configured in a back-to-back

arrangement as shown in Figure 14. Dual N-channel

packages such as the Vishay/Siliconix Si7956DP are a

good choice for space saving designs.

SNS+

SNS–

TGUP

TGDN

INP

7000A F14

RSNS

M1A

M1B

VIN

LOAD

LTC7000A/

LTC7000A-1

RFLT

Figure 14. Protecting Load from Voltage Drops on VIN

Design Example

As a design example, consider a fast power supply switch

with the following specifications: VIN = VLOAD = 8V to

135V, ILOAD = 3A, Insertion Loss < 0.5W at room temp

with maximum load, output rise time with a 1µF load

is 1V/µs (1A inrush current) and a shorted load should

immediately turn off the MOSFET.

The first item to select is the N-channel MOSFET. The

IRF7815PBF is selected because it has sufficient breakdown

voltage (BVDSS_MIN = 150V), sufficient continuous current

rating for a 3A load (ID_MAX = 4.1A) and the on-resistance

is low enough (RDS(ON)_MAX = 43mΩ) to be able to meet

the power loss specification.

Examining the MOSFET data sheet, the VGS vs RDS(ON)

typical performance curve shows a sharp increase in

RDS(ON) as the MOSFET VGS gets below 8.0V. Since the

default VCC UVLO is 7.0V, a resistor (RVCCUV) should be

placed between VCCUV and ground to increase the VCC

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