LM27951

D1

3.3 µF

1 µF

3.3 µF

D4

PWM

1 µF

Capacitors: 1 µF - TDK C1608X7R1C105K

3.3 µF - TDK C1608X5R1A335K

GND

EN

D2

D3

10

LM27951

SNVS416C – NOVEMBER 2005 – REVISED FEBRUARY 2016

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8.2 Typical Application

Figure 5. LM27951 Typical Application

8.2.1 Design Requirements

For typical white-LED switched capacitor applications, use the parameters listed in Table 1.

Table 1. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Minimum input voltage

2.8 V

Output current

20 mA

12.5 k

Ω

8.2.2 Detailed Design Procedure

8.2.2.1 Setting LED Currents

statements are simplified in Equation 4 and Equation 5:

(4)

(5)

8.2.2.2 Capacitor Selection

The LM27951 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors

are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance

(ESR) — < 20 m

Ω typical. Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are

not recommended for use with the LM27951 due to their high ESR, compared to ceramic capacitors.

For most applications, it is preferable to use ceramic capacitors with X7R or X5R temperature characteristic with

the LM27951. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over

temperature (X7R: ±15% over –55°C to 125°C; X5R: ±15% over –55°C to +85°C).