Datasheet SP6690 (Sipex) - 9

APPLICATION INFORMATION Inductor Selection Capacitor Selection
For SP6690, the internal switch will be turned Ceramic capacitors are recommended for their off only after the inductor current reaches the inherently low ESR, which will help produce typical dc current limit (ILIM=350mA). How- low peak to peak output ripple, and reduce high ever, there is typically propagation delay of frequency spikes. 200nS between the time when the current limit For the typical application, 4.7µF input capaci- is reached and when the switch is actually turned tor and 2.2µF output capacitor are sufficient. off. During this 200nS delay, the peak inductor The input and output ripple could be further current will increase, exceeding the current limit reduced by increasing the value of the input and by a small amount. The peak inductor current output capacitors. Place all the capacitors as can be estimated by: close to the SP6690 as possible for layout. For V use as a voltage source, to reduce the output I IN(MAX) PK = ILIM + • 200nS L ripple, a small feedforward (47pF) across the top feedback resistor can be used to provide The larger the input voltage and the lower the sufficient overdrive for the error comparator, inductor value, the greater the peak current. thus reduce the output ripple. In selecting an inductor, the saturation current Refer to Table 2 for some suggested low ESR specified for the inductor needs to be greater capacitors. than the SP6690 peak current to avoid saturating the inductor, which would result in a loss in efficiency and could damage the inductor. Table 2. Suggested Low ESR Capacitor Choosing an inductor with low DCR decreases power losses and increase efficiency.
MANUF. PART NUMBER CAP SIZE
Refer to Table 1 for some suggested low ESR
/VOLTAGE /TYPE
inductors. MURATA GRM32RR71E 2.2µF 1210 770-436-1300 225KC01B /25V /X5R MURATA GRM31CR61A 4.7µF 1206 Table 1. Suggested Low ESR inductor 770-436-1300 475KA01B /10V /X5R TDK C3225X7R1E 2.2µF 1210
MANUF. PART NUMBER DCR Current
847-803-6100 225M /25V /X7R
(
Ω
) Rating (mA)
TDK C3216X5R1A 4.7µF 1206 847-803-6100 475K /10V /X5R MURATA LQH32CN100K11 0.3 450 770-436-1300 (10µH)
LED Current Program
TDK NLC453232T-100K 0.55 500 847-803-6100 (10µH) In the white LEDs application, the SP6690 is generally programmed as a current source. The
Diode Selection
bias resistor Rb, as shown in the typical applica- tion circuit is used to set the operating current of A schottky diode with a low forward drop and the white LED using the equation: fast switching speed is ideally used here to V achieve high efficiency. In selecting a Schottky R FB b = diode, the current rating of the schottky diode IF should be larger than the peak inductor current. where VFB is the feedback pin voltage (1.22V), Moreover, the reverse breakdown voltage of the IF is the operating current of the White LEDs. schottky diode should be larger than the output In order to achieve accurate LED current, 1% voltage. Date: 08/31/09 SP6690 Micro Power Boost Regulator, Series White LED Driver © Copyright 2005 Sipex Corporation 9