SP6691EB Datasheet, PDF (3/6 Pages) Sipex Corporation – Evaluation Board Manual

Table 2. Suggested Low ESR capacitor
MANUFACTURE
MURATA
770-436-1300
MURATA
770-436-1300
TDK
847-803-6100
TDK
847-803-6100
PART NUMBER
GRM32RR71E
225KC01B
GRM31CR61A
475KA01B
C3225X7R1E
225M
C3216X5R1A
475K
CAP
/VOLTAGE
2.2uF
/25V
4.7uF
/10V
2.2uF
/25V
4.7uF
/10V
SIZE
/TYPE
1210
/X5R
1206
/X5R
1210
/X7R
1206
/X5R
7) LED Current Program
In the white LEDs application, the
SP6691EB is generally programmed as a
current source. The bias resistor R_b is used
to set the operating current of the white LED
using the equation:
R_b
=
V_FB
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% precision resistors are
recommended. Table 3 below shows the R_b
selection for different white LED currents.
For example, to set the operating current to
be 20mA, R_b is selected as 60.4 Ohm, as
shown in the schematic.
Table 3. Bias Resistor Selection
I_F (mA)
5
10
12
15
20
R_b (Ω)
243
121
102
80.6
60.4
8) Vout Programming
The SP6691EB can be programmed as
either a voltage source or a current source.
To program the SP6691 as voltage source,
the SP6691 requires 2 feedback resistors R₁
& R₂ to control the output voltage. The
formula for the resistor selection are shown
below.
R₁
=


V_out
1.22
− 1

•
R₂
9) Open Circuit Protection
When any white LED inside the white LED
module fails or the LED module is
disconnected from the circuit, the output and
the feedback control will be open, thus
resulting in a high output voltage, which may
cause the SW pin voltage to exceed it
maximum rating. In this case, a zener diode
can be used at the output to limit the voltage
on the SW pin and protect the part. The
zener voltage should be larger than the
maximum forward voltage of the White LED
module.
10) Brightness Control
Dimming control can be achieved by
applying a PWM control signal to the
EN/PWM pin. The brightness of the white
LEDs is controlled by increasing and
decreasing the duty cycle of The PWM
signal. A 0% duty cycle corresponds to zero
LED current and a 100% duty cycle
corresponds to full load current. While the
operating frequency range of the PWM
control is from 60Hz to 700Hz, the
recommended maximum brightness
frequency range of the PWM signal is from
60Hz to 200Hz. A repetition rate of at least
60Hz is required to prevent flicker. The
magnitude of the PWM signal should be
higher than the minimum SHDN voltage
high.
11) Layout Consideration
Both the input capacitor and the output
capacitor should be placed as close as
possible to the IC. This can reduce the
copper trace resistance which directly
affects the input and output ripples. The
feedback resistor network should be kept
close to the FB pin to minimize copper trace
connections that can inject noise into the
system. The ground connection for the
feedback resistor network should connect
directly to the GND pin or to an analog
ground plane that is tied directly to the GND
pin. The inductor and the schottky diode
should be placed as close as possible to the
switch pin to minimize the noise coupling to
the other circuits, especially the feedback
network.
3