Technical Note
15/22
BD60A00NUX, BD60A60NUX
www.rohm.com
2011.12 - Rev.A
© 2011 ROHM Co., Ltd. All rights reserved.
●
The coil selection
The DC/DC is designed by more than 4.7µH. When L value sets to a lower value, it is possibility that the specific
sub-harmonic oscillation of current mode DC / DC will be happened.
Please do not let L value to 3.3µH or below.
And, L value increases, the phase margin of DC / DC becomes to zero. Please enlarge the output capacitor value when you
increase L value.
●
Output capacitor selection
Output Capacitor smoothly keeps output voltage and supplies LED current.
If LED current is set more than 20mA with Vcoil=2.3V
～
5.5V, please make output capacitor more than 2.2µF. Otherwise
phase margin of DC/DC decreases and might oscillate.
Output Voltage consists of Charge (FET ON) and Discharge (LED current). So Output voltage has Output ripple Voltage
every FET switching.
Output ripple voltage is calculated as following.
Output ripple Voltage
- Switching cycle = T
- Total LED current = I
LED
- Switching ON duty = D
- Output ripple Voltage = V
ripple
- Output Capacitor = C
OUT
- Output Capacitor (real value) = C
real
- Decreasing ratio of Capacitor = C
error
C
real
= C
OUT
× C
error
(Capacitor value is decreased by Bias, so)
C
real
= I
LED
× (1-D) × T / V
ripple
C
OUT
= I
LED
× (1-D) × T / V
ripple
/ C
error
(Example 1)
In case of, V
IN
=5.0V, fsw = 0.6MHz, V
OUT
=32.5V, I
LED
=20mA, C
OUT
= 2.2µF, C
error
= 50%
T = 1 / 0.6MHz
D = 1 – V
IN
/ V
OUT
= 1 – 5/32.5
V
ripple
= I
LED
× (1-D) × T / (C
OUT
×C
error
) = 20mA × (5/32.5) ×(1 / 0.6MHz) / (2.2µF×0.5)
=
4.7mV
●
LED selection
Please select LED VF that input voltage is smaller than output voltage (VOUT).
0V 35V
50V
Output voltage
Capa [µF
]
C
out
C
real