LTC1435AI datasheet(11/20 Pages) LINER

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LTC1435A

APPLICATIONS INFORMATION

control power to be derived from the output during normal

operation (4.8V < VOUT < 9V) and from the internal regu-

lator when the output is out of regulation (start-up, short

circuit). Do not apply greater than 10V to the EXTVCC pin

and ensure that EXTVCC < VIN.

Significant efficiency gains can be realized by powering

INTVCC fromtheoutput,sincetheVINcurrentresultingfrom

the driver and control currents will be scaled by a factor of

Duty Cycle/Efficiency. For 5V regulators this supply means

connecting the EXTVCC pin directly to VOUT. However, for

3.3V and other lower voltage regulators, additional circuitry

is required to derive INTVCC power from the output.

The following list summarizes the four possible connections

for EXTVCC:

1. EXTVCC left open (or grounded). This will cause INTVCC

to be powered from the internal 5V regulator resulting

in an efficiency penalty of up to 10% at high input volt-

ages.

2. EXTVCC connected directly to VOUT. This is the normal

connection for a 5V regulator and provides the highest

efficiency.

3. EXTVCC connected to an output-derived boost network.

For 3.3V and other low voltage regulators, efficiency

gains can still be realized by connecting EXTVCC to an

output-derived voltage which has been boosted to

greater than 4.8V. This can be done with either the in-

ductive boost winding as shown in Figure 4a or the

capacitive charge pump shown in Figure 4b. The charge

pump has the advantage of simple magnetics.

4. EXTVCC connected to an external supply. If an external

supply is available in the 5V to 10V range (EXTVCC≤VIN),

it may be used to power EXTVCC providing it is compat-

ible with the MOSFET gate drive requirements. When

driving standard threshold MOSFETs, the external sup-

ply must always be present during operation to prevent

MOSFET failure due to insufficient gate drive.

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.

Capacitor CB in the Functional Diagram is charged through

diode DB from INTVCC when the SW pin is low. When the

Figure 4a. Secondary Output Loop and EXTVCC Connection

EXTVCC

VIN

PGND

LTC1435A

N-CH

CIN

VIN

COUT

RSENSE

VOUT

µF

1435A F04b

VN2222LL

BAT85

0.22

µF

Figure 4b. Capacitive Charge Pump for EXTVCC

topside MOSFET is to be turned on, the driver places the

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

hances 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. In most applications 0.1

µF is ad-

equate. The reverse breakdown on DB must be greater than

VIN(MAX).

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

OUT









≥

119

EXTVCC

SFB

SGND

VIN

PGND

LTC1435A

N-CH

CIN

VIN

1N4148

µF

COUT

VSEC

1:N

RSENSE

VOUT

OPTIONAL

EXT VCC

CONNECTION

≤ VSEC ≤ 9V

1435A F04a

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