LTC1775CS datasheet(15/24 Pages) LINER

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LTC1775

reduce the signal swing at the gate by a diode drop. Thus,

the LTC1775 requires an increased EXTVCC voltage of

about 6V (such as provided by the Figure 6 circuit) when

using this driver.

Output Voltage Programming

The LTC1775 has a pin selectable output voltage deter-

mined by the VPROG pin as follows:

VPROG

VOUT

3.3V

INTVCC

Open

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 8a. When using an external resistive divider, the

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

feedback resistors as shown in Figure 8b. The output

voltage is set by the divider as:

OUT =+









119

Figure 8a. Fixed 3.3V or 5V VOUT

Figure 8b. Adjustable VOUT

VPROG INTVCC

CONNECT FOR

VOUT = 5V

CONNECT FOR

VOUT = 3.3V

LTC1775

VOSENSE

1775 F08a

SGND

COUT

VOUT

VPROG

OPEN

LTC1775

VOSENSE

1775 F08b

SGND

COUT

VOUT

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 dia-

gram) 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 approxi-

mately VIN + INTVCC. During dropout operation, CB sup-

plies the top driver for as long as ten cycles between re-

freshes. Thus, the boost capacitance needs to store about

100 times the gate charge required by the top MOSFET. In

many applications 0.1

µF to 0.47µF 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.

External Gate Drive Buffer

The LTC1775 drivers are adequate for driving up to about

30nC into MOSFET switches. When using large single, or

multiple, MOSFET switches, external buffers may be re-

quired to provide additional gate drive capability. Special

purpose gate driver circuits such as the LTC1693 are ideal

in such cases. Alternately, the external buffer circuit shown

in Figure 7 can be used. Note that the bipolar devices

FMMT619

GATE

OF M1

BOOST

FMMT720

FMMT619

GATE

OF M2

1775 F07

INTVCC

PGND

FMMT720

Figure 7. Optional External Gate Driver

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