Datasheet LT3476 (Analog Devices) - 9

LT3476
ApplicAtions inForMAtion
Ceramic type capacitors using X7R dielectric are best for capacitor that is 1:1000 the value of the compensation temperature and DC bias stability of the capacitor value. capacitor. In the buck configuration, an additional tech- All ceramic capacitors exhibit loss of capacitance value nique is available. The filter capacitor between the CAP with increasing DC voltage bias, so it may be necessary to node and the LED bottom (see the Typical Application on choose a higher value capacitor or larger case size to get the first page) can be moved to between the LED top and the required capacitance at the operating voltage. Always the LED bottom. This circuit change places the inductor check that the voltage rating of the capacitor is sufficient. ripple current through the sense resistor, which improves Table 3 shows some recommended capacitor vendors. pulse-skipping behavior. There is usually less than 1% impact to the current regulation point.
Table 3. Low-ESR Surface Mount Capacitors VENDOR TYPE SERIES Diode Selection
Taiyo-Yuden Ceramic X5R, X7R The Schottky rectifier conducts current during the interval AVX Ceramic X5R, X7R when the switch is turned off. Select a diode with VR rated Murata Ceramic X5R, X7R for the maximum SW voltage. For boost circuits that may use the output disconnect feature, the diode should be
Compensation Design
rated for at least 40V. It is not necessary that the forward The LT3476 uses an internal transconductance error current rating of the diode equal the switch current limit. amplifier whose V The average current I C output compensates the control loop. F through the diode is a function The external inductor, output capacitor, and compensa- of the switch duty cycle, so select a diode with forward tion resistor and capacitor determine the loop stability. current rating of IF = 1.5A • (1-D). If using the PWM fea- The inductor and output capacitor are chosen based on ture for dimming, it may also be important to consider performance, size and cost. The compensation resistor diode leakage from the output (especially at hot) during and capacitor at VC are selected to optimize control loop the PWM low interval. Table 4 has some recommended stability. The component values shown in the typical ap- component vendors. plications circuits yield stable operation over the given range of input-to-output voltages and load currents. For
Table 4. Schottky Diodes
most buck applications, a small filter capacitor (1µF or
VR IAVE VF AT 1A
less) across the load is desirable. In this case, a 10nF
PART NUMBER (V) (A) (mV)
compensation capacitor at V
On Semiconductor
C is usually quite adequate. A compensation resistor of 5kΩ placed between the V MBRM140 40 1 550 C output and the compensation capacitor minimizes channel-
Diodes Inc.
to-channel interaction by reducing transient recovery time. DFLS140L 40 1 550 The boost configuration will have a larger output capacitor, B140 HB 40 1 530 2.2µF to 10µF.
NXP Semiconductor
The following circuit techniques involving the compensa- PMEG4010EJ 40 1 540 tion pin may be helpful where there is a large variation in programmed LED current, or a large input supply range is
Programming the LED Current
expected. At low duty cycles (TON less than 350ns) and low The LED Current is programmed using an external sense average inductor current (less than 500mA), the LT3476 resistor in series with the load. This method allows flex- may start to skip switching pulses to maintain output ibility in driving the load (i.e., sensing one of several parallel regulation. Pulse-skipping mode is usually less desirable strings) while maintaining good accuracy. The VADJ input because it leads to increased ripple current in the LED. sets the voltage regulation threshold across the external To improve the onset of pulse-skipping behavior, place a sense resistor between 10mV and 120mV. A 1.05V refer- capacitor between the SW node and the compensation ence output (REF) is provided to drive the VADJ pins either 3476fb 9 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