LTC4367 datasheet(21/30 Pages) AD

21 / 30 page

LTC7000/LTC7000-1

Rev. E

For more information www.analog.com

APPLICATIONS INFORMATION

UVLO to 8.0V. The value of RVCCUV is calculated and

rounded to the nearest standard value as follows:

RVCCUV =

8.0V

70µA

= 113kΩ

The value of the current sense resistor, RSNS is calculated

next. The LTC7000-1 has a fixed current sense thresh-

old, ΔVTH, of 30mV typical and 22mV minimum. To pro-

vide a minimum 3A load current, the minimum specified

ΔVTH = 22mV should be used for the RSNS calculation

below:

RSNS =

22mV

= 7.3mΩ

The closest standard value is 7mΩ. The power dissipation

of RSNS is 63mW so choose a power rating of greater than

0.25W to provide adequate margin.

The next item to check is to make sure the insertion loss

specification is satisfied. The insertion loss is given by:

PLOSS = ILOAD

2 • R

DS(ON)(MAX) + RSNS

(

)

= 3A

2 • 0.043Ω + 0.007Ω

(

) = 0.45W

Which meets the design specification of less than 0.5W.

The fast output slew rate specification of 1V/µs into a 1µF

load can be met by placing a resistor, RG, in series with

the TGUP pin to the MOSFET gate, as well as connecting

TGDN and a capacitor, CG, to ground on the MOSFET

gate. The values of RG and TG can be calculated from the

following expression:

RG • CG ≅

0.7 • 12V

1V / µs

= 8.4µs

CG needs to have a voltage rating as high as the BVDSS

of the MOSFET. A good choice for CG is the AVX

06032C471KAT2A which has a value of 470pF and a volt-

age rating of 200V. RG is then calculated to be 17.8kΩ.

The bootstrap capacitor CB can be calculated from the

gate charge as specified in the MOSFET data sheet and

CG as follows:

CB >

+ 10 • CG =

30nC

+ 10 • 470pF

≅ 0.33nF. 100nF will be used.

To meet the short-circuit specification, the TIMER pin

should be connected to VCC to enable immediate turn-off

(approximately 70ns) of the MOSFET in the case of an

overcurrent condition. If an overcurrent condition turns

off the MOSFET, it will not turn back on until the INP pin

has cycled low then back high.

The complete circuit is shown in Figure 15.

Figure 15. Design Example

0.1µF

470pF

200V

1µF

SNS+

SNS–

VIN

VCC

TIMER

FAULT

INP

VCCUV

17.8k

7000 F15

LTC7000-1

0.007

IRF7815TRPBF

LOAD

8V TO 135V

3A CONTINUOUS MODE

VIN

8V TO 135V

113k

GND

100

TGUP

TGDN

BST

Turn–On Transient

VIN = 135V

50µs/DIV

VINP

5V/DIV

VLOAD

50V/DIV

IDMOSFET

1A/DIV

7000 F15b

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