|
BD93942F Datasheet, PDF (16/30 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type) | |||
|
◁ |
BD93942F
Datasheet
OCP Settings/DCDC Componentsâ Current Capacity Selection Method
One of the function of CS pin - when its pin voltage>0.45 it stops the DCDC. Thus, RCS resistor value
need to be checked after the peak current flow through the inductor is calculated. Furthermore, DCDC external
componentsâ current capacity needs to be greater than peak current flowing through this inductor.
ï¼Inductor peak current Ipeak calculation methodï¼
Firstly, ripple voltage which occurs at the CS pin is decided depending on the DCDC application conditions.
The conditions when made as below;
Output voltage=VOUT[V]
LED total current=IOUT[A]
DCDC input voltage=VIN[V]
DCDC efficiency =η[%]
Total required average input current IIN:
I IN
ï½ VOUT [V ]ï´ IOUT [ A]
VIN [V ]ï´ï¨[%]
[ A]
Inductor ripple current IL[A] which occurs at inductor L[H] during
DCDC drive operation with switching frequency=fsw[Hz] is as follows
ÎIL ï½ (VOUT [V ] ï VIN [V ]) ï´ VIN [V ] [ A]
L[H ] ï´ VOUT [V ] ï´ f SW [Hz ]
Therefore, ILâs peak current Ipeak can be calculated using below equation
Figure 17. DC/DC convertor application circuit
Ipeak
ï½
I IN [ A] ï«
ïIL[ A]
2
[ A]ï(1)
(Resistor RCS connected to CS pin selection method)
This Ipeak flows in RCS and generates voltage. (refer to time chart
diagram on the right). This voltage value, VCSpeak can be calculated as below
VCS peak ï½ Rcs ï´ Ipeak [V ]
This VCSpeak when reach 0.45V, will stop the DCDC output.
Thus when selecting RCS value, below condition needs to be met.
Rcs[ï] ï´ Ipeak [ A] ï¼ 0.45[V ]
(DCDC Componentsâ Current Capacity Selection Method)
When OCP reach detection voltage CS=0.45V, Iocp current
I OCP
ï½
0.45[V ]
Rcs[ï]
[ A]ï(2)
Figure 18. Inductor current waveform
Ipeak current (1)ãIOCP current (2)ãand componentsâ MAX current capacity needs to satisfy the following
I peak ï¼ I OCP ï¼ Rated current of components
Above condition needs to be satisfied when selecting DCDC application parts eg. FET, inductor, diode etc.
Furthermore, continuous mode is recommended for normal DCDC applications. Inductorâs ripple current MIN limit value,
lmin becoming
Im
in
ï½
I IN
[ A]
ï
ïIL[ A] [
2
A]
ï¾
0
Is a condition to be met. If this is not met, it is called discontinuous mode.
www.rohm.com
© 2015 ROHM Co., Ltd. All rights reserved.
TSZ22111ã»15ã»001
16/26
TSZ02201-0F5F0C100050-1-2
12.Aug.2015 Rev.002
|
▷ |