SLUSCO8B November   2016  – June 2018

PRODUCTION DATA.

1. Features
2. Applications
3. Description
1.     Device Images
4. Revision History
5. Description (continued)
6. Pin Configuration and Functions
7. Specifications
8. Detailed Description
1. 8.1 Overview
2. 8.2 Functional Block Diagram
3. 8.3 Feature Description
4. 8.4 Device Functional Modes
9. Application and Implementation
1. 9.1 Application Information
2. 9.2 Typical Application
1. 9.2.1 TPS5420x 12-V Input, 1.5-A, 3-Piece IR LED Driver With Analog Dimming
2. 9.2.2 TPS5420x 24-V Input, 1-A, 4-Piece WLED Driver With PWM Dimming
10. 10Power Supply Recommendations
11. 11Layout
12. 12Device and Documentation Support
13. 13Mechanical, Packaging, and Orderable Information

• DDC|6

9.2.1.2.3 Output Capacitor Selection

The output capacitor reduces the high-frequency ripple current through the LED string. Various guidelines disclose how much high-frequency ripple current is acceptable in the LED string. Excessive ripple current in the LED string increases the rms current in the LED string, and therefore the LED temperature increases.

1. Look up the total dynamic resistance of the LED string (RLED) using the LED manufacturer’s data sheet.
2. Calculate the required impedance of the output capacitor (ZOUT), given the acceptable peak-to-peak ripple current through the LED string, ILED(ripple). IL(ripple) is the peak-to-peak inductor ripple current as calculated previously in the Inductor Selection section.
3. Calculate the minimum effective output capacitance required.
4. Increase the output capacitance appropriately due to the derating effect of applied dc voltage.

See Equation 9, Equation 10 and Equation 11.

Equation 9.
Equation 10.
Equation 11.

Once the output capacitor is chosen, Equation 12 can be used to estimate the peak-to-peak ripple current through the LED string.

Equation 12.

An OSRAM IR LED, SFH4715A, is used here. The dynamic resistance of this LED is 0.25 Ω at 1.5-A forward current. In this design, a 10-µF, 35-V X7R ceramic capacitor is chosen, the part number is GRM32ER7YA106KA12L, from muRata. The calculated ripple current of the LED is about 20 mA.