Datasheet LT1683 (Analog Devices) - 7

LT1683
PIN FUNCTIONS
should be set close to the external clock frequency. Syn- gate drive will not be enabled until VIN > VGCL + 0.8V. If this chronizing the clock to an external reference is useful for pin is tied to VIN, then undervoltage lockout is disabled. creating more stable positioning of the switcher voltage There is an internal 19V Zener tied from this pin to ground and current harmonics. This pin can be left open or tied to provide a fail-safe for maximum gate voltage. to ground if not used.
CT (Pin 7):
The oscillator capacitor pin is used in conjunc-
Slew Control
tion with RT to set the oscillator frequency. For RT = 16.9k:
CAP A, CAP B (Pins 2, 18):
These pins are the feedback COSC(nf) = 129/fOSC(kHz) nodes for the external voltage slewing capacitors. Normally a small 1pf to 5pf capacitor is connected from this pin to
RT (Pin 8):
The oscillator resistor pin is used to set the the drain of its respective MOSFET. charge and discharge currents of the oscillator capacitor. The nominal value is 16.9k. It is possible to adjust this re- The voltage slew rate is inversely proportional to this sistance ±25% to set oscillator frequency more accurately. capacitance and proportional to the current that the part will sink and source on this pin. That current is inversely
Gate Drive
proportional to RVSL.
GATE A, GATE B (Pins 1, 19):
These pins connect to the
RCSL (Pin 15):
A resistor to ground sets the current slew gates of the external N-channel MOSFETs. GATE A and rate for the external drive MOSFETs during switching. The GATE B turn on with alternate clock cycles. These drivers minimum resistor value is 3.3k and the maximum value are capable of sinking and sourcing at least 300mA. is 68k. The time to slew between on and off states of The GCL pin sets the upper voltage of the gate drive. The the MOSFET current will determine how the di/dt related gate pins will not be activated until V harmonics are reduced. This time is proportional to RCSL IN reaches a minimum voltage as defined by the GCL pin (gate undervoltage and RS (the current sense resistor) and maximum cur- lockout). rent. Longer times produce a greater reduction of higher frequency harmonics. The gate drive outputs have current limit protection to safe guard against accidental shorts.
RVSL (Pin 16):
A resistor to ground sets the voltage slew rate for the drains of the external drive MOSFETs. The If the gate drive voltage is greater than about 1V the minimum resistor value is 3.3k and the maximum value opposite gate drive is inhibited thus preventing cross is 68k. The time to slew between on and off states on the conduction. MOSFET drain voltage will determine how harmonics are
GCL (Pin 3):
This pin sets the maximum gate voltage to reduced from this source. This time is proportional to RVSL, the GATE A and GATE B pins to the MOSFET gate drives. CVA/B and the input voltage. Longer times produce more This pin should be either tied to a Zener, a voltage source rolloff of harmonics. CVA/B is the equivalent capacitance or V from CAP A or B to the drain of the MOSFET. IN. If the pin is tied to a Zener or a voltage source, the
Switch Mode Control
maximum gate drive voltage will be approximately V
CS (Pin 4):
This is the input to the current sense amplifier. GCL – 0.2V. If it is tied to VIN, the maximum gate voltage is approximately V It is used for both current mode control and current slewing IN – 1.6. of the external MOSFETs. Current sense is accomplished Approximately 50µA of current can be sourced from this via a sense resistor (RS) connected from the sources of pin if VGCL < VIN – 0.8V. the external MOSFETs to ground. CS is connected to the This pin also controls undervoltage lockout of the gate top of RS. Current sense is referenced to the GND pin. drives. If the pin is tied to a Zener or voltage source, the 1683fd 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Test Circuits Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts