Datasheet LT1676 (Analog Devices) - 5

LT1676
U U U PIN FUNCTIONS SHDN (Pin 1):
When pulled below the shutdown mode short as possible to minimize electromagnetic radiation threshold, nominally 0.30V, this pin turns off the regula- and voltage spikes. tor and reduces VIN input current to a few tens of micro-
GND (Pin 4):
This is the device ground pin. The internal amperes (shutdown mode). reference and feedback amplifier are referred to it. Keep When this pin is held above the shutdown mode thresh- the ground path connection to the FB divider and the VC old, but below the lockout threshold, the part will be compensation capacitor free of large ground currents. operational with the exception that output switching
V
action will be inhibited (lockout mode). A user-adjustable
IN (Pin 5):
This is the high voltage supply pin for the output switch. It also supplies power to the internal control undervoltage lockout can be implemented by driving this circuitry during start-up conditions or if the V pin from an external resistor divider to V CC pin is left IN. This action is open. A high quality bypass capacitor that meets the input logically “ANDed” with the internal UVLO, set at nominally ripple current requirements is needed here. (See Applica- 6.7V, such that minimum VIN can be increased above tions Information.) 6.7V, but not decreased (see Applications Information).
SYNC (Pin 6):
Pin used to synchronize internal oscillator If unused, this pin should be left open. However, the high to the external frequency reference. It is directly logic impedance nature of this pin renders it susceptible to compatible and can be driven with any signal between coupling from the high speed VSW node, so a small 10% and 90% duty cycle. The sync function is internally capacitor to ground, typically 100pF or so is recom- disabled if the FB pin voltage is low enough to cause mended when the pin is left “open.” oscillator slowdown. If unused, this pin should be grounded.
VCC (Pin 2):
This pin is used to power the internal control
FB (Pin 7):
This is the inverting input to the feedback circuitry off of the switching supply output. Proper use of amplifier. The noninverting input of this amplifier is inter- this pin enhances overall power supply efficiency. During nally tied to the 1.24V reference. This pin also slows down start-up conditions, internal control circuitry is powered the frequency of the internal oscillator when its voltage is directly from VIN. If the output capacitor is located more abnormally low, e.g., 2/3 of normal or less. This feature than an inch from the VCC pin, a separate 0.1µF bypass helps maintain proper short-circuit protection. capacitor to ground may be required right at the pin.
V V C (Pin 8):
This is the control voltage pin which is the
SW (Pin 3):
This is the emitter node of the output switch output of the feedback amplifier and the input of the and has large currents flowing through it. This node current comparator. Frequency compensation of the over- moves at a high dV/dt rate, especially when in “boost” all loop is effected by placing a capacitor, (or in most cases mode. Keep the traces to the switching components as a series RC combination) between this node and ground.
W U W TIMING DIAGRAMS High dV/dt Mode Low dV/dt Mode
VIN VIN VSW VSW 0 0 SWDR SWDR SWON SWON BOOST BOOST SWOFF SWOFF 1676 TD01 1676 TD02 5