Datasheet LT3476 (Analog Devices) - 8

LT3476
ApplicAtions inForMAtion
switch, the state of the PWM pin is sampled only during
Input Capacitor Selection
a narrow time slot at the beginning of each switch period. For proper operation, it is necessary to place a bypass
Inductor Selection
capacitor to GND close to the VIN pin of the LT3476. A 1µF, or greater, capacitor with low ESR should be used. The inductors used with the LT3476 should have a satura- A ceramic capacitor is usually the best choice. tion current rating of 2.5A or greater. For best loop stability results, the inductor value selected should provide a ripple In the buck configuration, the capacitor at the input to the current of 350mA or more. For buck (step-down) or boost power converter has large pulsed currents due to the cur- (step-up) configurations, and using a 21kΩ resistor on rent returned through the Schottky diode when the switch R is off. For best reliability, this capacitor should have low T (TSW ~ 1µs), inductor values from 4.7µH to 10µH are recommended for most applications. In the buck mode, ESR and ESL and meet the ripple current requirement, the inductor value can be estimated using the formula: ( ) D IRMS = ISW • (1− D) • D L(µH) = BUCK • TSW(µS) • (VCAP − VLED), ∆I where D is the switch duty cycle. A 2.2µF ceramic type V capacitor placed close to the Schottky and the ground D LED BUCK = plane is usually sufficient for each channel. VCAP V
Output Capacitor Selection
LED is the voltage across the LED string and VCAP is the input voltage to the converter. In the boost mode, the The selection of output filter capacitor depends on the load inductor value can be estimated using the formula: and the converter configuration, i.e., step-up or step-down. D For LED applications, the equivalent resistance of the LED L(µH) = BOOST • TSW(µS) • VIN , is typically low, and the output filter capacitor should be ∆I sized to attenuate the current ripple from the inductor to V D CAP − VIN 35mA or less. The following equation is useful to estimate BOOST = VCAP the required capacitor value: VIN is the input voltage and VCAP is the voltage across T C SW the LED string. Table 2 below provides some suggested FILT = 2 • RLED components and vendors. A typical filter capacitor value for RLED = 5Ω and TSW =
Table 2. Inductors
1µs is 0.47µF. For loop stability, consider the output pole
VALUE IRMS DCR HEIGHT
is at the frequency where closed loop gain should be
PART NUMBER (µH) (A) (Ω) (mm)
unity, so the dominant pole for loop compensation will
Sumida
be established by the capacitor at the VC input. CDRH6D38-100 10 2.0 0.028 4.0 For the LED boost applications, to achieve the same LED CDRH5D28-5R3 5.3 1.90 0.028 3.0 ripple current the required filter capacitor value is about CDRH73-100 10 1.68 0.072 3.4 five times larger than the value calculated above due to
Toko
the pulsed nature of the source current. A 2.2µF ceramic D63CB 10 1.49 0.042 3.5 type capacitor placed close to the Schottky and the ground D63CB 4.7 2.08 0.026 3.5 plane of the IC is usually sufficient for each channel.
Cooper-ET
As the output capacitor is subject to high ripple current, SD25-4R7 4.7 1.80 0.047 2.5 ceramic capacitors are recommended due to their low ESR and ESL at high frequency. 3476fb 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts