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LM2758

SNVS551E – APRIL 2008 – REVISED MAY 2016

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over temperature (Y5V: 22%, –82% over –30°C to +85°C range; Z5U: 22%, –56% over 10°C to 85°C range).

Under some conditions, a nominal 1 μF Y5V or Z5U capacitor could have a capacitance of only 0.1 μF. Such

detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance

requirements of the LM2758. The voltage rating of the output capacitor must be 6.3 V or more. For example, a

6.3-V, 0603, 2.2-μF output capacitor (TDK C1608X5R0J225) is acceptable for use with the LM2758, as long as

the capacitance on the output does not fall below a minimum of 1 μF in the intended application. All other

capacitors must have a voltage rating at or above the maximum input voltage of the application and a minimum

capacitance of 1 μF.

Table 3. Suggested Capacitors And Suppliers

MANUFACTURER

PART NUMBER

TYPE

MANUFACTURER

VOLTAGE RATING

CASE SIZE

INCH (mm)

2.2 µF for CIN and COUT

C1608X5R0J225

Ceramic X5R

TDK

6.3 V

0603 (1608)

JMK107BJ225

Ceramic X5R

Taiyo-Yuden

6.3 V

0603 (1608)

1 µF for C1 and C2

C1608X5R0J105

Ceramic X5R

TDK

6.3 V

0603 (1608)

JMK107BJ105M

Ceramic X5R

Taiyo-Yuden

6.3 V

0603 (1608)

8.2.2.2 Power Efficiency

Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (PLED) to the power

drawn at the input of the part (PIN). With a 1×/1.5× charge pump, the input current is equal to the charge pump

gain times the output current (total LED current). The efficiency of the LM2758 can be predicted as follows:

PLED = VLED × ILED

(3)

PIN = VIN × IIN

(4)

PIN = VIN × (Gain × ILED + IQ)

(5)

E = (PLED ÷ PIN)

(6)

For a simple approximation, the current consumed by internal circuitry (IQ) can be neglected, and the resulting

efficiency will become:

E = VLED ÷ (VIN × Gain)

(7)

Neglecting IQ results in a slightly higher efficiency prediction, but this impact will be negligible due to the value of

IQ being very low compared to the typical torch and flash current levels (100 mA to 500 mA). It is also worth

noting that efficiency as defined here is in part dependent on LED voltage. Variation in LED voltage does not

affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced

analysis, it is recommended that power consumed by the circuit (VIN × IIN) be evaluated rather than power

efficiency.