BD8119FM-M_14 Datasheet, PDF (16/31 Pages) Rohm – White Backlight LED Driver For Medium to Large LCD Panels

BD8119FM-M
(1) Computation of the Input Peak Current and IL_MAX
① Calculation of the maximum output voltage (VOUT_MAX)
To calculate the VOUT_MAX, it is necessary to take into account the VF variation and the number of LED connected
in series.
V_{OUT _ MAX}  V_F  V_F  N 1.0V
Where:
ΔV_F is the VF Variation
N is the Number of LED connection in series
② Calculation of the output current IOUT
I_OUT  I_LED 1.05  M
Where:
M is the Number of LED connections in parallel
③ Calculation of the input peak current IL_MAX
I_{L _ MAX}  I_{L _ AVG}  1 2 I_L
    I_{L _ AVG}  V_{I N}  V_OUT  I_OUT /  V_IN
I _L

V_IN
L

1
f_OSC
 ^VOUT
V_IN  V_OUT
Where:
η is the efficiency
f_OSC is the switching frequency
(a) The worst case scenario for VIN is when it is at the minimum, and thus the minimum value should be applied in the
equation.
(b) An L value of 10µF to 47µF is recommended. The current-mode type of DC/DC conversion is adopted for
BD8119FM-M, which is optimized with the use of the recommended L value in the design stage. This
recommendation is based upon the efficiency as well as the stability. L values outside this recommended range may
cause irregular switching waveform and hence deterioration of stable operation.
(c) η (efficiency) is approximately 80%
VIN
IL
RCS
CS
M1
D2
L
VOUT
M2
D1
COUT
External Application Circuit
(2) The Setting of Over-Current Protection
Choose RCS with the use of the equation VOCP_MIN (=0.54V) / RCS > IL_MAX
When investigating the margin, it is worth noting that the L value may vary by approximately ±30%.
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TSZ22111・15・001
16/27
TSZ02201-0T3T0C600030-1-2
28.Aug.2014 Rev.001