LTC1439CG datasheet(16/32 Pages) LINER

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LTC1438/LTC1439

APPLICATIONS INFORMATION

compatible with the MOSFET gate drive requirements.

When driving standard threshold MOSFETs, the exter-

nal supply must be always present during operation to

prevent MOSFET failure due to insufficient gate drive.

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CBconnected to the BOOST

1 and BOOST 2 pins supply the gate drive voltages for the

topside MOSFETs. Capacitor CB in the Functional Dia-

gram is charged through diode DB from INTVCC when the

SW1(SW2) pin is low. When one of the topside MOSFETs

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 SW1(SW2) rises to VIN and the BOOST

1(BOOST 2) pin follows. With the topside MOSFET on,

the boost voltage is above the input supply: VBOOST = VIN

+ VINTVCC. The value of the boost capacitor CB needs to

be 100 times that of the total input capacitance of the

topside MOSFET(s). The reverse breakdown on DB must

be greater than VIN(MAX).

Output Voltage Programming

The LTC1438/LTC1439 have pin selectable output voltage

programming. Controller 1 on the LTC1438-ADJ is a

dedicated adjustable controller. The output voltage is

selected by the VPROG1(VPROG2) pin as follows on all of the

other parts:

VPROG1,2 = 0V

VOUT1,2 = 3.3V

VPROG1,2 = INTVCC

VOUT1,2 = 5V

VPROG2 = Open (DC)

VOUT2 = Adjustable

Except for the LTC1438-ADJ, the top of an internal resis-

tive divider is connected to SENSE – 1 pin in Controller 1.

For fixed output voltage applications the SENSE – 1 pin is

connected to the output voltage as shown in Figure 5a.

When using an external resistive divider for an adjustable

regulator, the VPROG2 pin is left open (VPROG1 is internally

left open on the LTC1438-ADJ) and the VOSENSE2 pin is

connected to the feedback resistors as shown in Figure 5b.

The adjustable controller will force the externally attenu-

ated output voltage to 1.19V.

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

voltages.

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

inductive 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

Figure 4a. Secondary Output Loop and EXTVCC Connection

VIN

VSEC

VOUT

COUT

1438 F04a

µF

RSENSE

•

CIN

TGL1

N-CH

OPTIONAL EXTVCC

CONNECTION

≤ VSEC ≤ 9V

N-CH

1N4148

LTC1438

LTC1439*

1:1

TGS1*

SW1

BG1

PGND

SGND

SFB1

EXTVCC

*TGS1 ONLY AVAILABLE ON THE LTC1439

VIN

VOUT

COUT

1438 F04b

µF

0.22

µF

RSENSE

CIN

TGL1

N-CH

VN2222LL

LTC1438

LTC1439*

BAT85

TGS1*

SW1

BG1

PGND

EXTVCC

*TGS1 ONLY AVAILABLE ON THE LTC1439

Figure 4b. Capacitive Charge Pump for EXTVCC

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