LTC1649CS datasheet(10/16 Pages) LINER

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LTC1649

tors that work well in LTC1649 applications. A common

way to lower ESR and raise ripple current capability is to

parallel several capacitors. A typical LTC1649 application

might require an input capacitor with a 5A ripple current

capacity and 2% output shift with a 10A output load step,

which requires a 0.005

ΩoutputcapacitorESR.SanyoOS-

CON part number 10SA220M (220

µF/10V) capacitors

feature 2.3A allowable ripple current at 85

°C and 0.035Ω

ESR; three in parallel at the input and seven at the output

will meet the above requirements.

Input Supply Considerations/Charge Pump

The LTC1649 requires four supply voltages to operate:

VIN, VCC, PVCC1 and PVCC2. VIN is the primary high power

input, supplying current to the drain of Q1 and the input to

the internal charge pump at the VIN pin. This supply must

be between 2.7V and 6V for the LTC1649 to operate

properly. An internal charge pump uses the voltage at VIN

to generate a regulated 5V output at CPOUT. This charge

pump requires an external 1

µF capacitor connected be-

tween the C + and C – pins, and an external 10

µF reservoir

capacitor connected from CPOUT to ground. The voltage at

CPOUT must always be greater than or equal to VIN. If VIN

is expected to rise above 5V, an additional Schottky diode

(D5) should be added from VIN to CPOUT.

CPOUT is typically connected to PVCC2 directly, providing

the 5V supply that the G2 driver output uses to drive Q2.

PVCC2 requires a 10µF bypass to ground; this capacitor

can double as the CPOUT reservoir capacitor, allowing a

typical application with CPOUT and PVCC2 connected to-

gether to get away with only a single 10

µF capacitor at this

node, located close to the PVCC2 pin. VCC can also be

powered from CPOUT, but is somewhat sensitive to noise.

PVCC2 happens to be a significant noisemaker, so most

applications require an RC filter from CPOUT/PVCC2 to VCC.

Ω and 10µF are typical filter values that work well in

most applications.

PVCC1 needs to be boosted to a level higher than CPOUT to

provide gate drive to Q1. The LTC1649 initially used a

charge pump from VIN to create CPOUT; the typical appli-

cation uses a second charge pump to generate the PVCC1

supply. This second charge pump consists of a Schottky

diode (DCP) from CPOUT to PVCC1, and a 1µF capacitor

from PVCC1 to the source of Q1. While Q2 is on, the diode

charges the capacitor to CPOUT. When Q1 comes on, its

source rises to VIN, and the cap hauls PVCC1 up to (CPOUT

+ VIN), adequate to fully turn on Q1. When Q1 turns back

off, PVCC1 drops back down to CPOUT; fortunately, we’re

not interested in turning Q1 on at this point, so the lower

voltage doesn’t cause problems. The next time Q1 comes

on, PVCC1 bounces back up to (CPOUT + VIN), keeping Q1

happy. Figure 4 shows a complete power supply circuit for

the LTC1649.

Figure 4. LTC1649 Power Supplies

APPLICATIONS INFORMATION

DRIVE

CIRCUITRY

VCC

CHARGE

PUMP

VIN

LTC1649

CPOUT

PVCC2

µF

PVCC1

VIN

COUT

VOUT

1649 F04

CIN

µF

*OPTIONAL

FOR VIN ≥ 5V

Ω

µF

C+

C–

DCP

D5*

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