LM27952

SNVS364B – MAY 2005 – REVISED MAY 2013

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CAPACITOR SELECTION

The LM27952 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

<20m

Ω typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not

recommended for use with the LM27952 due to their high ESR, as compared to ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with

the LM27952. 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).

Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the

LM27952. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -

20%) and vary significantly 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 1µF Y5V or Z5U capacitor could have a

capacitance of only 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet

the minimum capacitance requirements of the LM27952.

The voltage rating of the output capacitor should be 10V or more. All other capacitors should have a voltage

rating at or above the maximum input voltage of the application.

PARALLEL DX OUTPUTS FOR INCREASED CURRENT DRIVE

all four parallel current sinks of equal value drive the single LED. The LED current programmed should be

chosen so that the current through each of the outputs is programmed to 25% of the total desired LED current.

current through each of the current sink inputs is 15mA. Similarly, if two LEDs are to be driven by pairing up the

the desired LED current.

Connecting the outputs in parallel does not affect internal operation of the LM27952 and has no impact on the

Electrical Characteristics and limits previously presented. The available diode output current, maximum diode

voltage, and all other specifications provided in the Electrical Characteristics table apply to this parallel output

configuration, just as they do to the standard 4-LED application circuit.

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 1.5x/1x charge pump, the input current is equal to the charge pump

gain times the output current (total LED current). For a simple approximation, the current consumed by internal

circuitry can be neglected and the efficiency of the LM27952 can be predicted as follows:

(7)

(8)

(9)

(10)

percentage points when several LEDs are driven at full power. 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

THERMAL PROTECTION

Internal thermal protection circuitry disables the LM27952 when the junction temperature exceeds 150°C (typ.).

This feature protects the device from being damaged by high die temperatures that might otherwise result from

excessive power dissipation. The device will recover and operate normally when the junction temperature falls

below 140°C (typ.). It is important that the board layout provide good thermal conduction to keep the junction

temperature within the specified operating ratings.

10

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