HV9910BDB3 Supertex, HV9910BDB3 Datasheet - Page 3

• Manufacturer Part Number: HV9910BDB3
• Description: LED Lighting Development Kits LO VOLT HI BRIGHT LED DRIVER DEMOBOARD
• Manufacturer: Supertex

Frequently Asked Questions
4. If the constant off-time mode allows a wider LED voltage range, why not use that mode exclusively? Why do
5. Why is the efficiency of the demoboard higher with a load of two LEDs compared to a single LED load?
All the above factors favor the higher load voltage and thus the demoboard has a higher efficiency when the load is larger.
Although the constant off-time mode allows the demoboard to operate at a higher output voltage, the LED ripple current
is directly proportional to the output voltage in this mode. This makes it difficult to get a good load regulation of the LED
current in the constant off-time mode with a wide variation in the LED string voltage (in this case it will be a 1:4 variation).
At lower LED voltage values, the ripple will be lower and the LED current would be higher.
By switching between the two modes depending on the load, we can get a better current accuracy without having to
adjust the LD voltage or the sense resistor.
Losses in the HV9910BDB3 occur due mainly due to two factors:
Switching losses are dependent on the switching frequency, input voltage and total parasitic capacitance at the node.
At higher switching frequencies, the switching losses are higher.
Conduction losses are dependent on the duty cycle. Since the voltage drop on the FET is smaller than the voltage drop
on the diode (the on-resistance of the FET is very small), the higher the duty cycle, the smaller is the conduction loss.
Please note that we are ignoring the losses in the inductor, which will be identical in both cases.
Also, efficiency = P
input power. So, if the output power is higher, the fixed switching losses are a smaller fraction of the output power and
thereby the efficiency is higher.
Comparing the operation of the converter in both modes at 12V input for this particular demoboard, the following
are the differences:
we need the constant frequency mode?
a. Conduction losses in the FET and diode
b. Switching losses in the FET
a. Output power is higher with 2 LEDs as the load
b. Switching frequency in the constant off-time mode is 55kHz, whereas it is 100kHz in the constant frequency
c. Duty cycle of operation is about higher in the constant off-time mode by a factor of 2 than in the constant
mode
frequency mode
OUT
/ P
IN
= P
10
8
6
4
2
0
OUT
●
2
1235 Bordeaux Drive, Sunnyvale, CA 94089
Constant Frequency Mode
/ (P
OUT
(cont.)
+ losses) = 1/ (1 + losses/P
Load Regulation
4
Constant Off-Time Mode
Load Voltage (V)
Constant off-time mode
With mode change
3
(@ V
IN
= 12V)
6
OUT
Constant Off-time Mode
●
), where P
Tel: [phone]
OUT
is the output power and P
8
●
www.supertex.com
HV9910BDB3
IN
is the