Table 1. Design Parameters

8.2.2.1 Setting LED Currents

The current through the four LEDs connected to D_1-4 can be set to a desired level simply by connecting an appropriately sized resistor (R_SET) between the I_SET pin of the LM27951 and GND. The LED currents are proportional to the current that flows out of the I_SET pin and are a factor of 200 times greater than the I_SET current. The feedback loop of an internal amplifier sets the voltage of the I_SET pin to 1.25 V (typical). The previous statements are simplified in Equation 4 and Equation 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 mΩ 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).

Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM27951. 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 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 LM27951.

The voltage rating of the output capacitor must be 10 V or more. All other capacitors must have a voltage rating at or above the maximum input voltage of the application.

8.2.2.3 Parallel Dx Outputs for Increased Current Drive

Outputs D_1-4 may be connected together to drive a one or two LEDs at higher currents. In a one LED configuration, all four parallel current sources of equal value are connected together to drive a single LED. The LED current programmed must be chosen such that the current provided from each of the outputs is programmed to 25% of the total desired LED current. For example, if 60 mA is the desired drive current for the single LED, R_SET must be selected so that the current out of each current source is 15 mA. Similarly, if two LEDs are to be driven by pairing up the D_1-4 outputs (that is, D_1-2, D_3-4), R_SET must be selected so that the current out of each current source output is 50% of the desired LED current.

Connecting the outputs in parallel does not affect the internal operation of the LM27951 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 apply to this parallel output configuration, just as they do to the standard 4-LED application circuit.

8.2.2.4 Power Efficiency

Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (P_LED) to the power drawn at the input of the part (P_IN). With a 1.5×/1× 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 LM27951 can be predicted as follows:

Neglecting I_Q results in a slightly higher efficiency prediction, but this impact is no more than a few 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 that power consumed by the circuit (V_IN × I_IN) be evaluated rather than power efficiency.

8.2.2.5 Power Dissipation

The power dissipation (P_DISSIPATION) and junction temperature (T_J) can be approximated with Equation 10 and Equation 11. P_IN is the power generated by the 1.5×/1× charge pump, P_LED is the power consumed by the LEDs, T_Ais the ambient temperature, and R_θJA is the junction-to-ambient thermal resistance for the 14-pin WSON package. V_IN is the input voltage to the LM27951, V_LED is the nominal LED forward voltage, and I_LED is the programmed LED current.

The junction temperature rating takes precedence over the ambient temperature rating. The LM27951 may be operated outside the ambient temperature rating, so long as the junction temperature of the device does not exceed the maximum operating rating of 115°C. The maximum ambient temperature rating must be derated in applications where high power dissipation and/or poor thermal resistance causes the junction temperature to exceed 115°C.