LM27951

C2

VIN

C2+

C2-

D1

CIN

3.3 µF

1 µF

COUT

VOUT

3.3 µF

D4

PWM

D4

C1

C1+

C1-

1 µF

Capacitors: 1 µF - TDK C1608X7R1C105K

3.3 µF - TDK C1608X5R1A335K

VIN = 3 V to 5.5 V

IDX = 30 mA max

ISET

GND

EN

D2

D3

D3

D2

D1

RSET

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

RSET

12.5 k

Ω

8.2.2 Detailed Design Procedure

8.2.2.1 Setting LED Currents

The current through the four LEDs connected to D1-4 can be set to a desired level simply by connecting an

appropriately sized resistor (RSET) between the ISET pin of the LM27951 and GND. The LED currents are

proportional to the current that flows out of the ISET pin and are a factor of 200 times greater than the ISET current.

The feedback loop of an internal amplifier sets the voltage of the ISET pin to 1.25 V (typical). The previous

statements are simplified in Equation 4 and Equation 5:

IDx = 200 × (VSET / RSET)

(4)

RSET = 200 × (1.25 V / IDx)

(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).