ADT6780 ADTECH [ADTech], ADT6780 Datasheet - Page 9

• Manufacturer Part Number: ADT6780
• Description: Thermally enhanced Low VFB Step-Down LED Driver
• Manufacturer: ADTECH [ADTech]

Jul. 12. 2012 / Preliminary
APPLICATION INFORMATION
Input Capacitor
The input capacitor is used to filter out discontinuous, pulsed input current and to maintain input voltage
stable. Therefore input capacitor should be able to supply the AC current to the step-down converter. Its
input ripple voltage can be estimated by:
where, CIN is input capacitor value.
The voltage rating of input capacitor must be greater than the maximum input voltage plus ripple voltage.
Since the input capacitor absorbs the input switching current, it requires an proper ripple current rating. The
RMS current in the input capacitor can be approximated by:
The worst-case condition occurs at V
approximately half of the I
current rating higher than I
capacitors because of their low ESR and high current rating. When selecting ceramic capacitors, X5R or
X7R type dielectric ceramic capacitors should be used for their better temperature and voltage
characteristics. For most applications, a 10㎌ ceramic capacitor is sufficient.
Output Capacitor
The output capacitor is required to maintain the DC output voltage. In a step-down converter circuit, output
ripple voltage is determined by the inductor value, switching frequency, output capacitor value and ESR.
That is:
where,
C
ESR is the equivalent series resistance of the output capacitor.
Low ESR capacitors are preferred to keep the output voltage ripple low. When low ESR ceramic capacitor is
used as output capacitor, its ESR value can be waived. So, the impedance at the switching frequency is
dominated by the capacitance. Therefore the output voltage ripple is:
On the other hand, in the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the
switching frequency. In this case, the output voltage ripple is:
In a step-down converter, output capacitor current is continuous. Usually, the ripple current rating of the
output capacitor is not concerned because of its low ripple current. For most applications, a 10㎌ ceramic
capacitor is sufficient.
O
is output capacitor value,
Thermally enhanced Low V
* This specifications are subject to be changed without notice
OUT
CIN_RMS
. For reliable operation and best performance, the input capacitors must have
ΔV
I
ΔV
CIN_RMS
at worst operating conditions. Ceramic capacitors are preferred for input
IN
OUT
IN
ΔV
=
=2×V
F
OUT
=
SW
=
ΔV
I
ΔI
(continued)
I
OUT
×
OUT
=
OUT
OUT
L
C
ΔI
×
IN
×
(50% duty condition), and its worst RMS current is
⎛
⎜ ⎜
⎝
=
L
×
ESR
×
ΔI
9/13
V
V
V
⎛
⎜ ⎜
⎝
V
OUT
L
OUT
8
IN
IN
×
+
×
FB
(
F
×
8
×
ESR
SW
Step-Down LED Driver
×
⎛
⎜ ⎜
⎝
⎛
⎜ ⎜
⎝
1
1
1
F
×
−
−
SW
)
1
C
V
V
V
V
×
O
OUT
OUT
IN
IN
C
⎞
⎟ ⎟
⎠
O
⎞
⎟ ⎟
⎠
⎞
⎟ ⎟
⎠
⎞
⎟ ⎟
⎠
http://www.ad-tech.co.kr
ADT6780