ADD5207 Datasheet, PDF (13/16 Pages) Analog Devices – Four-String, White LED Driver

Data Sheet
Using the duty cycle and switching frequency (fSW), users can
determine the on time with the following equation:
t _ON
=
D
f _SW
The input (VIN) and output (VOUT) voltages determine the
switch duty cycle (D) with the following equation:
D = ^{VOUT − VIN}
V_OUT
Choose the output capacitor based on the following equation:
( ) _COUT
≥
I _L × V_OUT − V_IN
f _SW ×V_OUT × ∆V_OUT
Capacitor manufacturers include: Murata Manufacturing Co.,
Ltd., AVX, Sanyo, and Taiyo Yuden Co., Ltd.
Diode Selection
The output diode conducts the inductor current to the output
capacitor and loads while the switch is off. For high efficiency,
minimize the forward voltage drop of the diode. Schottky diodes
are recommended. However, for high voltage, high temperature
applications, where the Schottky diode reverse leakage current
becomes significant and degrades efficiency, use an ultrafast
junction diode. The output diode for a boost regulator must be
chosen depending on the output voltage and the output current.
The diode must be rated for a reverse voltage equal to or greater
than the output voltage used. The average current rating must
be greater than the maximum load current expected, and the peak
current rating must be greater than the peak inductor current.
Using Schottky diodes with lower forward voltage drop decreases
power dissipation and increases efficiency. The diode must be
rated to handle the average output load current. Many diode
manufacturers derate the current capability of the diode as a
function of the duty cycle. Verify that the output diode is rated
to handle the average output load current with the minimum
duty cycle.
The minimum duty cycle of the ADD5207 is:
D _MIN
= ^{VOUT − VIN_MAX}
V_OUT
where VIN_MAX is the maximum input voltage.
For example, DMIN is 0.5 when VOUT is 30 V and VIN_MAX is 15 V.
Schottky diode manufacturers include ON Semiconductor,
Diodes Incorporated, Central Semiconductor Corp., and Sanyo.
ADD5207
Loop Compensation
The external inductor, output capacitor, and the compensation
resistor and capacitor determine the loop stability. The induc-
tor and output capacitor are chosen based on performance, size,
and cost. The compensation resistor (RC) and compensation
capacitor (CC ) at COMP are selected to optimize control loop
stability. For typical LED application of the ADD5207, a 6.8 kΩ
compensation resistor in series with a 2.2 nF compensation
capacitor at COMP is adequate.
VOUT_FB
g_m
HEADROOM CONTROL
R_C
C2
C_C
Figure 15. Compensation Components
A step-up converter produces an undesirable right-half plane
zero in the regulation feedback loop. Capacitor C2 is chosen
to cancel the zero introduced by output capacitance ESR.
Solving for C2,
C2 = ^{ESR ×COUT}
R_C
For low ESR output capacitance, such as with a ceramic
capacitor, C2 is optional.
LAYOUT GUIDELINES
When designing a high frequency, switching, regulated power
supply, layout is very important. Using a good layout can solve
many problems associated with these types of supplies. The
main problems are loss of regulation at high output current
and/or large input-to-output voltage differentials, excessive
noise on the output and switch waveforms, and instability.
Using the following guidelines helps minimize these problems.
Make all power (high current) traces as short, direct, and thick
as possible. It is good practice on a standard printed circuit
board (PCB) to make the traces an absolute minimum of 15 mil
(0.381 mm) per ampere. The inductor, output capacitors, and
output diode should be as close to each other as possible. This
helps reduce EMI radiated by the power traces that carry high
switching currents. Close proximity of the components also
reduces lead inductance and resistance, which in turn reduce noise
spikes, ringing, and resistive losses that produce voltage errors.
Rev. A | Page 13 of 16