600kHz 3A Step-Down Switching Regulator
I
OMAX
= 3 -
V
O
• (1 - D)
2 • L • f
s
TJ4519
Where:
f
S
= switching frequency,
V
O
= output voltage
D = duty ratio, V
O
/V
I
V
I
= input Voltage.
Figure 2 shows the theoretical maximum load current for the specific cases. In a real application, however, the
allowed maximum load current also depends on the layout and the air cooling condition. Therefore, the maximum
load current may need to be degraded according to the thermal situation of the application.
Maximum Load Current vs. Input Voltage
L=10uH
V
O
= 2.5V
V
O
= 3.3V
V
O
= 5.0V
V
I
(V)
Figure 2. Theoretical maximum load current curves
Inductor Selection
The factors for selecting the inductor include its cost, efficiency, size and EMI. For a typical TJ4519 application,
the inductor selection is mainly based on its value, saturation current and DC resistance. Increasing the inductor
value will decrease the ripple level of the output voltage while the output transient response will be degraded. Low
value inductors offer small size and fast transient responses while they allow large ripple currents, poor
efficiencies and require more output capacitance for low output ripple. The inductor should be able to handle the
peak current without saturating and its copper resistance in the winding should be as low as possible to minimize
its resistive power loss. A good trade-off among its size, loss and cost is to set the inductor ripple current to be
within 15% to 30% of the maximum output current.
The inductor value can be determined according to its operating point under its continuous mode and the
switching frequency as follows:
L=
V
O
• (V
I
- V
O
)
V
I
• f
s
•
δ
• I
OMAX
Where :
fs = switching frequency,
δ
= ratio of the peak to peak inductor current to the output load current
Vo = output voltage.
Jul. 2010 – Preliminary
-6-
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
