Datasheet LT3478, LT3478-1 (Analog Devices) - 7

LT3478/LT3478-1
U U U PI FU CTIO S SW (Pins 1, 2):
Switch Pin. Collector of the internal NPN hysteresis allows programming of undervoltage lockout power switch. Both pins are fused together inside the IC. (UVLO) hysteresis. ⎯S⎯H⎯D⎯N above 1.4V turns the part on Connect the inductor and diode here and minimize the and removes a 10µA sink current from the pin. ⎯S⎯H⎯D⎯N = 0V metal trace area connected to this pin to minimize EMI. reduces VIN current < 3µA. ⎯S⎯H⎯D⎯N can be directly connected to V
V
IN. If left open circuit the part will be turned off.
IN (Pin 3):
Input Supply. Must be locally bypassed with a capacitor to ground.
CTRL1 (Pin 12):
CTRL1 pin voltage is used to program maximum LED current (CTRL2 = V
V
REF). CTRL1 voltage
S (Pin 4):
Inductor Supply. Must be locally bypassed can be set by a resistor divider from V with a capacitor to ground. Can be shorted to V REF or an external IN if only voltage source. Maximum LED current is given by: one supply is available (see L (Pin 5) function).
L (Pin 5):
Inductor Pin. An internal resistor between V (LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps S and L pins monitors inductor current to protect against (LT ) 3478 Max LED Current = inrush current. Exceeding 6A immediately turns off the . 0 1 internal NPN power switch and discharges the soft-start Mi ( n CTR , L 1.0 ) 5 • Amps R pin. Input current monitoring can be disabled by connect- SEN NSE ing the inductor power supply directly to the L pin and (linear for 0.1V < CTRL1< 0.95V ; CTRL2 = VREF) For maxi- leaving the VS pin open (requires local bypass capacitor mum LED current, short CTRL1 and CTRL2 pins to VREF. to GND on L pin; not VS pin).
CTRL2 (Pin 13):
The CTRL2 pin is available for program-
VOUT (Pin 6):
Output voltage of the converter. Connect a ming a decrease in LED current versus temperature capacitor from this pin to ground. Internal circuitry moni- (setting temperature breakpoint and slope). This feature tors VOUT for protection against open LED faults. allows the output LED(s) to be programmed for maximum
LED (Pin 7):
Connect the LED string from this pin to allowable current without damage at higher temperatures. ground. An internal (LT3478-1)/external (LT3478) resistor This maximizes LED usage and increases reliability. A between the VOUT and LED pins senses LED current for CTRL2 voltage with negative temperature coeffi cient is accurate control. created using an external resistor divider from VREF with temperature dependant resistance. If not used, CTRL2
OVPSET (Pin 8):
Programs VOUT overvoltage protection should be tied to V level (OVP) to protect against open LED faults. OVP = REF. (OVPSET • 41)V. OVPSET range is 0.3V to 1V for an OVP
PWM (Pin 14):
Input pin for PWM dimming control. Above range of typically 12.3V to 41V. 1V allows converter switching and below 1V disables switching with V
V
C pin level maintained. With an external
C (Pin 9):
Output of the transconductance error amplifi er MOSFET placed in series with the ground side of the LED and compensation pin for the converter regulation loop. string, a PWM signal driving the PWM pin and MOSFET
VREF (Pin 10):
Bandgap Voltage Reference. This pin can gate provides accurate dimming control. The PWM signal supply up to 100µA. Can be used to program CTRL1, can be driven from 0V to 15V. If unused, the pin should CTRL2, OVPSET pin voltages using resistor dividers to be connected to VREF. ground.
RT (Pin 15):
A resistor to ground programs switching ⎯
S
⎯
H
⎯
D
⎯
N (Pin 11):
The ⎯S⎯H⎯D⎯N pin has an accurate 1.4V frequency between 200kHz and 2.25MHz. threshold and can be used to program an undervoltage
SS (Pin 16):
Soft-Start Pin. Placing a capacitor here pro- lockout (UVLO) threshold for system input supply using a grams soft-start timing to limit inductor inrush current resistor divider from supply to ground. A 10µA pin current during start-up due to the converter. When inductor current 34781f 7