Datasheet LT3800 (Analog Devices) - 9

LT3800
APPLICATIONS INFORMATION
The onboard VCC regulator will provide gate drive power
Charge Pump Doubler
for start-up under all conditions with total MOSFET gate VOUT charge loads up to 180nC. The regulator can operate the B0520 B0520 LT3800 continuously, provided the VIN voltage and/or VCC MOSFET gate charge currents do not create excessive 1μF 1μF power dissipation in the IC. Safe operating conditions LT3800 for continuous regulator use are shown in Figure 1. In BG Si1555DL applications where these conditions are exceeded, VCC must be derived from an external source after start-up.
Charge Pump Tripler
70 VOUT 1μF 60 B0520 B0520 50 B0520 1μF (V) 40 INV VCC 1μF 30 Si1555DL Si1555DL LT3800 20 SAFE 3800 AI01 OPERATING BG CONDITIONS 10 0 50 100 150 200 TOTAL FET GATE CHARGE (nC) 3800 F01
Inductor Auxiliary Winding Figure 1. VCC Regulator Continuous Operating Conditions
In LT3800 converter applications with output voltages in TG the 9V to 20V range, back-feeding VCC and VBOOST from SW the converter output is trivial, accomplished by connect- LT3800 N • V V ing diodes from the output to these supply pins. Deriving CC OUT these supplies from output voltages greater than 20V will • require additional regulation to reduce the feedback voltage. BG Outputs lower than 9V will require step-up techniques to 3800 AI04 increase the feedback voltage to something greater than the 8V VCC regulated output. Low power boost switchers are sometimes used to provide the step-up function, but a simple charge-pump can perform this function in many instances. 3800fc 9 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS FUNCTIONAL DIAGRAM APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS