CS5212

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12

Rpullup Value Selection for Boost Circuit Startup

The CS5212 application circuit incorporates a pullup

resistor, R7, into the boost circuitry. This resistor is essential

to achieving startup of the boost circuit. At startup, the

GATE(H) output may be limited to 0.8 V, due to internal Vbe

drops. Until the boost circuitry charges up, the GATE(H)

output cannot provide sufficient Vgs to turn on the MOSFET.

A resistor from GATE(H) to BST allows bypassing of the

GATE(H) driver until the boost circuitry is charged. The time

constant, set by the pullup resistor and the Cin of the top

MOSFET, must be fast enough to turn on the MOSFET

during the switching period. The following equation is used

to determine Rpullup:

Rpullup t

1

(Cin

fSW)

Choosing components according to this equation will

insure that approximately 63% of the boost voltage will be

applied to GATE(H) within one switching period. To start

charge pumping, the control MOSFET must pull up the

switching node above 0.6 V, two Schottky drops, which will

by GATE(H) must be 0.6 V greater than Vth of the top FET.

Both high− and low−side switches must be sublogic level

ensure proper up.

ORDERING INFORMATION

Device

Operating

Temperature Range

Package

CS5212ED14

SOIC−14

55 Units / Rail

CS5212ED14G

SOIC−14

(Pb−Free)

55 Units / Rail

CS5212EDR14

SOIC−14

2500 / Tape & Reel

CS5212EDR14G

SOIC−14

(Pb−Free)

2500 / Tape & Reel

CS5212GD14

SOIC−14

55 Units / Rail

CS5212GD14G

SOIC−14

(Pb−Free)

55 Units / Rail

CS5212GDR14

SOIC−14

2500 / Tape & Reel

CS5212GDR14G

SOIC−14

(Pb−Free)

2500 / Tape & Reel

†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging

Specifications Brochure, BRD8011/D.