LTC4367 datasheet(18/30 Pages) AD

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LTC7000A

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

from VIN to ground. A simple resistive divider can be

used as shown in Figure 9 to meet specific VIN voltage

requirements. When RUN is less than 1.11V or OVLO is

greater than 1.21V, TGDN will be pulled to TS and the

external MOSFET will be turned off. The approximate

delay time for the OVLO pin to turn on or turn off the

external MOSFET is 2.5µs. The approximate delay time

for the RUN pin falling lower than 1.11V to turn off the

external MOSFET is 3.5µs.

SUPPLY

VIN

7000A F08

LTC7000A

RUN

LTC7000A

RUN

Figure 8. RUN Pin Interface to Logic

7000A F09

VIN

LTC7000A

OVLO

RUN

Figure 9. Adjustable UV and OV Lockout

The current that flows through the R3 – R4 – R5 divider

will directly add to the shutdown, sleep and active current

of the LTC7000A, and care should be taken to minimize

the impact of this current on the overall current used by

the application circuit. Resistor values in the megaohm

range may be required to keep the impact of the quiescent

shutdown and sleep currents low. To pick resistor values,

the sum total of R3 + R4 + R5 (RTOTAL) should be chosen

first based on the allowable DC current that can be drawn

from VIN. The individual values of R3, R4 and R5 can then

be calculated from the following equations:

= RTOTAL •

1.21V

Rising VIN OVLO Threshold

= RTOTAL •

1.21V

Rising VIN UVLO Threshold

– R5

= RTOTAL – R5 – R4

For applications that do not need a precise external OVLO

the OVLO pin is required to be tied directly to ground.

The RUN pin in this type of application can be used as an

external UVLO using the above equations with R5 = 0Ω.

Similarly, for applications that do not require a precise

UVLO, the RUN pin can be tied to VIN. In this configura-

tion, the UVLO threshold is limited by the internal VIN

UVLO thresholds as shown in the Electrical Characteristics

table. The resistor values for the OVLO can be computed

using the above equations with R3 = 0Ω.

Be aware that the OVLO pin cannot be allowed to exceed

its absolute maximum rating of 6V. To keep the voltage

on the OVLO pin from exceeding 6V, the following

relationship should be satisfied:

VIN(MAX) •

R3+R4+R5

⎛

⎝

⎜

⎞

⎠

⎟ < 6V

If the VIN(MAX) relationship for the OVLO pin cannot be

satisfied, an external 5V Zener diode should also be placed

from OVLO to ground in addition to any lockout setting

resistors.

Bootstrapped Supply (BST-TS)

An external bootstrapped capacitor, CB, connected

between BST and TS supplies the gate drive voltage for

the MOSFET driver. The LTC7000A/LTC7000A-1 keeps

the BST-TS supply charged with an internal charge pump,

allowing for duty cycles up to 100%. When the high side

external MOSFET is to be turned on, the driver places

the CB voltage across the gate-source of the MOSFET.

This enhances the high side MOSFET and turns it on. The

source of the MOSFET, TS, rises to VIN and the BST pin

follows. With the high side MOSFET on, the BST voltage

is above the input supply; VBST = VIN + 12V. The boost

capacitor, CB, supplies the charge to turn on the external

MOSFET and needs to have at least 10 times the charge to

turn on the external MOSFET fully. The charge to turn on

the external MOSFET is referred to gate charge, QG, and

is typically specified in the external MOSFET data sheet.

Gate charge can range from 5nC to hundreds of nCs and is

influenced by the gate drive level and the type of external

MOSFET used. For most applications, a capacitor value

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