SP6690 Datasheet, PDF (9/16 Pages) Sipex Corporation – Micro Power Boost Regulator Series White LED Driver

APPLICATION INFORMAMTION
Inductor Selection
For SP6690, the internal switch will be turned
off only after the inductor current reaches the
typical dc current limit (I_LIM=350mA). How-
ever, there is typically propagation delay of
200nS between the time when the current limit
is reached and when the switch is actually turned
off. During this 200nS delay, the peak inductor
current will increase, exceeding the current limit
by a small amount. The peak inductor current
can be estimated by:
I_PK = I_LIM + ^VIN(MAX) • 200nS
L
The larger the input voltage and the lower the
inductor value, the greater the peak current.
In selecting an inductor, the saturation current
specified for the inductor needs to be greater
than the SP6690 peak current to avoid saturating
the inductor, which would result in a loss in
efficiency and could damage the inductor.
Choosing an inductor with low DCR decreases
power losses and increase efficiency.
Refer to Table 1 for some suggested low ESR
inductors.
Table 1. Suggested Low ESR inductor
MANUF.
PART NUMBER
DCR Current
(Ω) Rating
(mA)
MURATA
LQH32CN100K11 0.3
450
770-436-1300 (10µH)
TDK
NLC453232T-100K 0.55 500
847-803-6100 (10µH)
Diode Selection
A schottky diode with a low forward drop and
fast switching speed is ideally used here to
achieve high efficiency. In selecting a Schottky
diode, the current rating of the schottky diode
should be larger than the peak inductor current.
Moreover, the reverse breakdown voltage of the
schottky diode should be larger than the output
voltage.
Capacitor Selection
Ceramic capacitors are recommended for their
inherently low ESR, which will help produce
low peak to peak output ripple, and reduce high
frequency spikes.
For the typical application, 4.7µF input capaci-
tor and 2.2µF output capacitor are sufficient.
The input and output ripple could be further
reduced by increasing the value of the input and
output capacitors. Place all the capacitors as
close to the SP6690 as possible for layout. For
use as a voltage source, to reduce the output
ripple, a small feedforward (47pF) across the
top feedback resistor can be used to provide
sufficient overdrive for the error comparator,
thus reduce the output ripple.
Refer to Table 2 for some suggested low ESR
capacitors.
Table 2. Suggested Low ESR Capacitor
MANUF.
MURATA
770-436-1300
MURATA
770-436-1300
TDK
847-803-6100
TDK
847-803-6100
PART NUMBER CAP SIZE
/VOLTAGE /TYPE
GRM32RR71E
225KC01B
2.2µF
/25V
1210
/X5R
GRM31CR61A
475KA01B
4.7µF
/10V
1206
/X5R
C3225X7R1E
225M
2.2µF
/25V
1210
/X7R
C3216X5R1A
475K
4.7µF
/10V
1206
/X5R
LED Current Program
In the white LEDs application, the SP6690 is
generally programmed as a current source. The
bias resistor R_b, as shown in the typical applica-
tion circuit is used to set the operating current of
the white LED using the equation:
R_b = ^VFB
I_F
where V_FB is the feedback pin voltage (1.22V),
I_F is the operating current of the White LEDs.
In order to achieve accurate LED current, 1%
Date: 05/25/04
SP6690 Micro Power Boost Regualtor, Series White LED Driver
9
© Copyright 2004 Sipex Corporation