Datasheet LTC3765 (Analog Devices) - 9

LTC3765
OPERATION
The LTC3765 is a forward converter start-up controller and
Gate Drive Encoding
gate driver for use in a single-switch forward converter The LTC3766 secondary-side forward controller sends a with active clamp reset. When connected through a pulse pulse-encoded signal through a small pulse transformer transformer to the LTC3766 secondary-side synchronous and series DC restore capacitor to the IN+ and IN– pins of forward controller, it forms a highly efficient forward con- the LTC3765. After a brief start-up sequence to establish a verter with secondary-side regulation, galvanic isolation communication lock between the two parts, the LTC3765 between input and output, and synchronous rectification. extracts clock and duty cycle information from the signal The LTC3765 and LTC3766 bias voltages are generated and uses it to control the PG and AG gate driver outputs. from a proprietary self-starting architecture which elimi- nates the need for an additional bias supply. Figure 2 shows how the LTC3766 drives the pulse trans- former in a complementary fashion, with a duty cycle of
Linear Regulator
79%. At the appropriate time during the positive cycle, The LTC3765 features an external series pass linear the LTC3766 applies a short (150ns) zero voltage pulse regulator controller that eliminates the long start-up time across the pulse transformer, indicating the end of the associated with a conventional trickle charger. The NDRV PG “on” time. pin regulates the gate of an external NMOS transistor to DUTY CYCLE = 15% DUTY CYCLE = 0% ramp up the VCC supply with a well controlled 35µs ramp 150ns 150ns time to the 8.5V regulation point. For low input supply voltage applications where the threshold of the external NMOS transistor limits the VCC voltage, an internal charge V + – IN – VIN pump boosts NDRV to a voltage higher than VIN so that the external NMOS can be fully enhanced.
Self-Starting Start-up
1 CLK PER 1 CLK PER 3765 F02 When power is first applied and when the RUN pin and
Figure 2. Gate Drive Multiplexing Scheme
VCC have satisfied their respective start-up requirements, the LTC3765 begins open-loop operation using its own
Gate Drivers and Delay Adjustment
internal oscillator. Power is supplied to the secondary The active clamp gate driver (AG) and the primary switch by switching the gate drivers with a gradually increas- gate driver (PG) are “in-phase,” with a programmable ing duty cycle from 0% to 70% as controlled by the rate overlap time set by the DELAY pin. In an active clamp of rise of the voltage on the SSFLT pin. A peak charge forward converter topology, the delay time between the circuit powered from an auxiliary winding off of the main active clamp PMOS turn-off and the primary switch NMOS transformer allows the LTC3766 to begin operation even turn-on is critical for optimizing efficiency. When the active for small duty cycles. When the LTC3766 has adequate clamp is on, the drain of the primary NMOS, or primary voltage to satisfy its start-up requirements, it provides switch node (SWP), is driven to a voltage of approximately duty cycle information through the pulse transformer as VIN/(1 – Duty Cycle) by the main transformer. When the shown in Figure 2. The LTC3765 detects this signal and active clamp turns off, the current in the magnetizing transfers control of the gate drivers to the LTC3766. The inductance of the transformer ramps this voltage linearly LTC3765 turns off the linear regulator and, through an down to VIN. Transitional power loss in the primary switch on-chip rectifier, also extracts power from this signal. is minimized by turning it on when this voltage is at a minimum. 3765fb For more information www.linear.com/LTC3765 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Timing Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts