LTC1437AIG datasheet(16/28 Pages) LINER

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LTC1436A

LTC1436-PLL-A/LTC1437A

APPLICATIONS INFORMATION

Topside MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor CB connected to the Boost

pin supplies the gate drive voltage for the topside

MOSFET(s). Capacitor CB in the functional diagram is

charged through diode DB from INTVCC when the SW pin

is low. When one of the topside MOSFET(s) is to be turned

on, the driver places the CB voltage across the gate source

of the desired MOSFET. This enhances the MOSFET and

turns on the topside switch. The switch node voltage SW

rises to VIN and the Boost pin rises to VIN + INTVCC. The

value of the boost capacitor CB needs to be 100 times

greater than the total input capacitance of the topside

MOSFET(s). In most applications 0.1

µF is adequate. The

reverse breakdown on DB must be greater than VIN(MAX).

Output Voltage Programming

The output voltage is pin selectable for all members of the

LTC1436A/LTC1437A family. The output voltage is

selected by the VPROG pin as follows:

VPROG = 0V

VOUT = 3.3V

VPROG = INTVCC

VOUT = 5V

VPROG = Open (DC)

VOUT = Adjustable

The LTC1436A/LTC1437A family also has remote output

voltage sense capability. The top of an internal resistive

divider is connected to VOSENSE. For fixed 3.3V and 5V

output voltage applications the VOSENSE pin is connected

to the output voltage as shown in Figure 5a. When using

an external resistive divider, the VPROG pin is left open (DC)

and the VOSENSE pin is connected to the feedback resistors

as shown in Figure 5b.

Figure 4b. Capacitive Charge Pump for EXT VCC

Figure 5b. LTC1436A/LTC1437A Adjustable Applications

Figure 5a. LTC1436A/LTC1437A Fixed Output Applications

Power-On Reset Function (POR)

The power-on reset function monitors the output voltage

and turns on an open drain device when it is out of

regulation. An external pull-up resistor is required on the

POR pin.

When power is first applied or when coming out of

shutdown, the POR output is pulled to ground. When the

output voltage rises above a level which is 5% below the

final regulated output value, an internal counter starts.

After counting 216 (65536) clock cycles, the POR pull-

down device turns off.

The POR output will go low whenever the output voltage

drops below 7.5% of its regulated value for longer than

approximately 30

µs, signaling an out-of-regulation condi-

tion. In shutdown, the POR output is pulled low even if the

regulator’s output is held up by an external source.

Run/Soft Start Function

The RUN/SS pin is a dual purpose pin that provides the

soft start function and a means to shut down the

LTC1436A/LTC1437A. Soft start reduces surge currents

from VIN by gradually increasing the internal current limit.

Power supply sequencing can also be accomplished

using this pin.

EXTVCC

VIN

TGL

TGS

PGND

LTC1436A

LTC1437A

N-CH

CIN

VIN

0.22

µF

BAT85

COUT

BAT85

µF

RSENSE

VN2222LL

1436 F04b

VPROG

SGND

LTC1436A

LTC1437A

1436 F05a

COUT

VOUT

GND: VOUT = 3.3V

INTVCC: VOUT = 5V

VOSENSE

OPEN (DC)

1436 F05b

100pF

1.19V

≤ VOUT ≤ 9V

VPROG

SGND

LTC1436A

LTC1437A

VOSENSE

VOUT = 1.19V 1 +

()

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