LTC1625IGN datasheet(14/24 Pages) LINER

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LTC1625

Note that RDS(ON) also varies with the gate drive level. If

gate drives other than the 5.2V INTVCC are used, this must

be accounted for when selecting the MOSFET RDS(ON).

Particular care should be taken with applications where

EXTVCC is connected to the output. When the output

voltage is between 4.7V and 5.2V, INTVCC will be con-

nected to the output and the gate drive is reduced. The

resulting increase in RDS(ON) will also lower the current

limit. Even applications with VOUT > 5.2V will traverse this

region during start-up and must take into account the

reduced current limit.

Topside MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor (CB in the functional

diagram) connected to the BOOST pin supplies the gate

drive voltage for the topside MOSFET. This capacitor is

charged through diode DB from INTVCC when the SW node

is low. Note that the voltage across CB is about a diode

drop below INTVCC. When the top MOSFET turns on, the

switch node voltage rises to VIN and the BOOST pin rises

to approximately VIN + INTVCC. During dropout operation,

CB supplies the top driver for as long as ten cycles between

refreshes. Thus, the boost capacitance needs to store

about 100 times the gate charge required by the top

MOSFET. In many applications 0.22

µF is adequate.

When adjusting the gate drive level , the final arbiter is the

total input current for the regulator. If you make a change

and the input current decreases, then you improved the

efficiency. If there is no change in input current, then there

is no change in efficiency.

Output Voltage Programming

The LTC1625 has a pin selectable output voltage deter-

mined by the VPROG pin as follows:

VPROG = 0V

VOUT = 3.3V

VPROG = INTVCC

VOUT = 5V

VPROG = Open

VOUT = Adjustable

Remote sensing of the output voltage is provided by the

VOSENSE pin. For fixed 3.3V and 5V output applications an

internal resistive divider is used and the VOSENSE pin is

connected directly to the output voltage as shown in

Figure 6a. When using an external resistive divider, the

APPLICATIONS INFORMATION

VPROG pin is left open and the VOSENSE pin is connected to

feedback resistors as shown in Figure 6b. The output

voltage is set by the divider as:

OUT =+









119

VPROG

VOUT = 5V: INTVCC

VOUT = 3.3V:

GND

LTC1625

VOSENSE

1625 F06a

SGND

COUT

VOUT

Figure 6a. Fixed 3.3V or 5V VOUT

VPROG

OPEN

LTC1625

VOSENSE

1625 F06b

SGND

COUT

Figure 6b. Adjustable VOUT

Run/Soft Start Function

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

start function and a means to shut down the LTC1625. Soft

start reduces surge currents from VIN by gradually in-

creasing the controller’s current limit ITH(MAX). This pin

can also be used for power supply sequencing.

Pulling the RUN/SS pin below 1.4V puts the LTC1625 into

a low quiescent current shutdown (IQ< 30µA). This pin can

be driven directly from logic as shown in Figure 7. Releas-

ing the RUN/SS pin allows an internal 3

µA current source

to charge up the external capacitor CSS. If RUN/SS has

been pulled all the way to ground there is a delay before

starting of approximately:

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