LTC1435A Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LTC1435A

High Efficiency Low Noise Synchronous Step-Down Switching Regulator

Linear Technology 

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 from the output, since the VIN current resulting from

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

+

VIN

CIN

VIN

OPTIONAL

EXT VCC

CONNECTION

5V ≤ VSEC ≤ 9V

TG

EXTVCC

R6 LTC1435A

SFB

SW

R5

SGND

BG

PGND

N-CH

1N4148

L1

1:N

VSEC

+

RSENSE

+

1µF

VOUT

COUT

N-CH

1435A F04a

Figure 4a. Secondary Output Loop and EXTVCC Connection

+

VIN

TG

EXTVCC

LTC1435A

SW

BG

PGND

VIN

CIN

+

1µF

BAT85 0.22µF

BAT85

N-CH

N-CH

VN2222LL

BAT85

L1

RSENSE

VOUT

+

COUT

1435A F04b

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 0.1µF 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:

VOUT

=

1.19V



1+

R2

R1

, VOUT

≥

1.19

V

11

Share Link: