SP6691EB
Evaluation Board Manual
Ideal for series white LED driver
High output voltage, up to 30V
Low quiescent current: 20uA
Ultra low shutdown current: 10nA
High Efficiency: up to 80%
SOT23-5 Package & SMT
components for small, low profile
Power Supply
DESCRIPTION AND BOARD SCHEMATIC
The
SP6691EB Evaluation Board
is designed to help the user evaluate the performance of the
SP6691EB as a series white LED driver. The evaluation board is a completely assembled and
tested surface mount board which provides easy probe access points to all SP6691EB Inputs
and Outputs so that the user can quickly connect and measure electrical characteristics and
waveforms.
SP6691
Date: 01/26/05
SP6691EB Evaluation Board Manual
Copyright 2005 Sipex Corporation
USING THE EVALUATION BOARD
1) Powering Up the SP6691EB Circuit
The SP6691EB Evaluation Board can be
powered from inputs from a +1.2V to +5.0V.
Connect with short leads directly to the “VIN”
and “GND” posts.
Monitor the Output
Voltage and connect the Load between the
“VOUT” post and the “GND” post.
2) Using the J1 Jumper: Enabling the
SP6691EB Output and using the
Shutdown Mode
The SP6691EB output will be enabled if the
J1 Jumper is in the bottom or pin 1 to 2
position. If J1 is in the pin 2 to 3 or top
position, the Shutdown pin is brought to
GND, which puts the SP6691EB in the low
quiescent Shutdown Mode.
3) Using the Posts
Since the part might get damaged when the
output is open loop, two divider resistors
(R
1
=1M, R
2
=64.9K) are used to provide the
feedback loop and set the output voltage.
For the white LEDs application, these two
resistors (R
1
, R
2
) need to be removed from
the evaluation board first to avoid over-
voltage and then plug the white LED module
between “VOUT” and “FB” posts. The bias
resistor R
b
should also be installed on the
board.
4) Inductor Selection
For SP6691EB, the internal switch will be
turned off only after the inductor current
reaches the typical dc current limit
(I
LIM
=450mA). However, 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:
V
in (max)
I
pk
=
I
LIM
+
⋅
200nS
L
The larger the input voltage and the lower
the inductor value, the greater the peak
current.
2
In selecting an inductor, the saturation
current specified for the inductor needs to be
greater than the SP6691EB 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
MANUFACTURE
PART NUMBER
LQH32CN100K11
(10uH)
NLC453232T-100K
(22uH)
DCR
(Ω)
0.3
0.55
Current
Rating
(mA)
450
500
MURATA
770-436-1300
TDK
847-803-6100
5) 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.
6) 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, 4.7uF input
capacitor and 2.2uF 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 SP6691EB 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
reducing the output ripple.
Refer to Table 2 for some suggested low
ESR capacitors.
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:
V
R
b
=
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)
R
b
(Ω)
5
243
10
121
12
102
15
80.6
20
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 R
1
& R
2
to control the output voltage. The
formula for the resistor selection are shown
below.
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
V
R
1
=
out
−
1
•
R
2
1.22
POWER SUPPLY DATA
For the standard evaluation board (4x20mA series white LEDs application), in which the output
voltage is around 15V and output current is 20mA, the power supply data is provided in Fig 1. to
Fig. 4. The white LEDs used here were from LUMEX (Part Number: SML-LX2832UWC-TR).
90
Average Output Current (mA)
20
85
80
Efficiency (%)
75
70
65
60
55
50
2.7
3
3.3
3.6
3.9
4.2
Input Voltage (V)
16
12
8
4
0
0
20
40
60
80
100
PWM Duty Cycle (%)
Fig. 1 Efficiency vs Input Voltage
Fig. 2 Average Io vs SHDN duty cycle
V
sw
I
L
(0.5A/DIV)
V
out
(AC)
V
out
(AC)
Fig. 3 Typical Switching Waveform
(V
in
=3.3V)
Fig. 4 Output Ripple (V
in
=2.7V)
.
4
EVALUATION BOARD LAYOUT
FIGURE 1: SP6691EB COMPONENT PLACEMENT
FIGURE 2: SP6691EB PC LAYOUT TOP SIDE
FIGURE 3: SP6691EB PC LAYOUT BOTTOM SIDE
5
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