600kHz 3A Step-Down Switching Regulator
I
D- AVG
= I
OMAX
• (I - D)
TJ4519
Thermal Considerations
There are three major power dissipation sources for the TJ4519. The internal switch conduction loss its
switching loss due to the high frequency switching actions and the base drive boost circuit loss. These losses can
be estimated as:
P
total
= I
O 2
• R
on
• D + 10.8 • 10
-3
• I
O
• V
I
+
10
• I • D • ( V
boost
)
1000
O
Where:
I
O
= load current;
Ron = on-equivalent resistance of the switch;
V
BOOST
= input voltage or output based on the boost circuit connection.
The junction temperature of the TJ4519 can be further determined by:
T
J
= T
A
+
θ
JA
• P
total
θ
JA
is the thermal resistance from junction to ambient. Its value is a function of the IC package, the application
layout and the air cooling system.
The freewheeling diode also contributes a significant portion of the total converter loss. This loss should be
minimized to increase the converter efficiency by using Schottky diodes with low forward drop (V
F
).
P
diode
= V
F
• I
O
• (1 - D)
Loop Compensation Design
The TJ4519 has an internal error amplifier and requires a compensation network to connect between the
COMP pin and GND pin as shown in Figure 3. The compensation network includes C4, C5 and R3. R1 and R2
are used to program the output voltage according to:
V
O
= 0.8 • (1+
R
1
)
R
2
Assuming the power stage ESR (equivalent series resistance) zero is an order of magnitude higher than the
closed loop bandwidth, which is typically one tenth of the switching frequency, the power stage control to output
transfer function with the current loop closed (Ridley model) for the TJ4519 will be as follows:
G
VD
(s) =
2.5 • R
L
s
1+
1
R
L
• C
Where:
R
L
= Load,
C = Output capacitor.
The goal of the compensation design is to shape the loop to have a high DC gain, high bandwidth, enough phase
margin, and high attenuation for high frequency noises. Figure 3 gives a typical compensation network which
offers 2 poles and 1 zero to the power stage:
Jul. 2010 – Preliminary
-9-
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
