0.053

0.054

0.055

0.056

0.057

0.058

0.059

0.06

0.061

0.062

VIN (V)

10uH 75% Eff

10uH 80% Eff

22uH 75% Eff

C021

LM3699

SNVS821A – JANUARY 2014 – REVISED MARCH 2014

www.ti.com

Figure 8. Maximum Output Power Vs Inductance And Efficiency

8.2.2.2.2

Output Voltage Limited

If a output voltage limited situation occurs, when the boost output voltage hits the LM3699 OVP threshold, the

NFET turns off and stays off until the output voltage falls below the hysteresis level (typically 1 V below the OVP

threshold). This results in the boost converter regulating the output voltage to the OVP threshold, causing the

current sinks to go into dropout. The LM3699 OVP setting supports LED strings up to 6 series LEDs (Vƒmax = 3.5

V).

8.2.2.3 Boost Inductor Selection

The boost converter operates using either a 10-µH or 22-µH inductor. The inductor selected must have a

saturation current greater than the peak operating current.

8.2.2.4 Output Capacitor Selection

The LM3699 inductive boost converter requires a 1.0-µF X5R or X7R 50V (0805 size) ceramic capacitor to filter

the output voltage. Pay careful attention to the capacitor tolerance and DC bias response. Smaller body-size 1.0-

µF ceramic capacitors or 25-V, 1.0-µF ceramic capacitors can be used, but for proper operation the degradation

in capacitance due to tolerance, DC bias, and temperature should stay above 0.4 µF. This might require placing

two devices in parallel in order to maintain the required output capacitance over the device operating range and

series LED configuration.

8.2.2.5 Schottky Diode Selection

The Schottky diode must have a reverse breakdown voltage greater than the LM3699’s maximum output voltage.

Additionally, the diode must have an average current rating high enough to handle the LM3699’s maximum

output current, and at the same time the diode peak current rating must be high enough to handle the peak

inductor current. Schottky diodes are required due to their lower forward voltage drop (0.3 V to 0.5 V) and their

fast recovery time.

8.2.2.6 Input Capacitor Selection

The LM3699 inductive boost converter requires a 2.2-µF X5R or X7R ceramic capacitor to filter the input voltage.

The input capacitor filters the inductor current ripple and the internal MOSFET driver currents during turnon of the

internal power switch.

12

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