600kHz 3A Step-Down Switching Regulator
The peak to peak inductor current is:
TJ4519
I
p-p
=
δ
• I
OMAX
After the required inductor value is selected, the proper selection of the core material is based on the peak
inductor of the core material is based on the peak inductor current and efficiency specifications. The core must be
able to handle the peak inductor current I
PEAK
without saturation and produce low core loss during the high
frequency operation.
I
p-p
I
PEAK
= I
OMAX
+
2
The power loss for the inductor includes its core loss and copper loss. If possible, the winding resistance should
be minimized to reduce inductor’s copper loss. The core must be able to handle the peak inductor current I
PEAK
without saturation and produce low core loss during the high frequency operation. The core can be found in the
manufacturer’s datasheet. The inductor’s copper loss can be estimated as follows:
P
COPPER
= I
2LRMS
• R
WINDING
Where:
I
LRMS
is the RMS current in the inductor. This current can be calculated as follows:
I
LRMS
= I
OMAX
• 1 +
1
•
δ
2
12
Output Capacitor Selection
Basically, there are two major factors to consider in selecting the type and quantity of the output capacitors.
The first one is the required ESR (Equivalent Series Resistance) which should be low enough to reduce the
output voltage deviation during load changes. The second one is the required capacitance, which should be high
enough to hold up the output voltage. Before the TJ4519 regulates the inductor current to a new value during a
load transient, the output capacitor delivers all the additional current needed by the load. The ESR and ESL of the
output capacitor, the loop parasitic inductance between the output capacitor and the load combined with inductor
ripple current are all major contributors to the output voltage ripple. Surface mount ceramic capacitors are
recommended.
Input Capacitor Selection
The input capacitor selection is based on its ripple current level, required capacitance and voltage rating. This
capacitor must be able to provide the ripple current drawn by the converter. For the continuous conduction mode,
the RMS value of the input capacitor current I
CIN(RMS)
can be calculated from:
I
CIN(RMS )
= I
OMAX
•
V
O
• ( V
I
- V
O
)
V
2I
This current gives the capacitor’s power loss through its R
CIN(ESR)
as follows:
P
CIN
= I
2CIN(RMS )
• R
CIN(ESR )
The input ripple voltage mainly depends on the input capacitor’s ESR and its capacitance for a given load, input
voltage and output voltage. Assuming that the input current of the converter is constant, the required input
capacitances for a given voltage ripple can be calculated by:
Jul. 2010 – Preliminary
-7-
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
