0.5A Step-Down Switching Regulator
TC2574
Procedure (Fixed Output Voltage Version) (Continued)
In order to simplify the switching regulator design, a step-by-step design procedure and examples are provided.
Procedure
5. Output Capacitor Selection (C
OUT
)
A.
Since the TC2574 is a forward–mode switching regulator
with voltage mode control, its open loop 2–pole–1–zero
frequency characteristic has the dominant pole–pair deter
mined by the output capacitor and inductor values.
For stable operation and an acceptable ripple voltage,
(approximately 1% of the output voltage) a value
between 100µF and 470µF is recommended.
B.
Due to the fact that the higher voltage electrolytic
capacitors generally have lower ESR (Equivalent Series
Resistance) numbers, the output capacitor’s voltage
rating should be at least 1.5 times greater than the output
voltage. For a 5.0Vregulator, a rating at least 8.0V is a
appropriate, and a 10Vor 16V rating is recommended.
Example
5. Output Capacitor Selection (C
OUT
)
A.
C
OUT
= 100µF to 470µF standard aluminium
electrolytic.
B.
Capacitor voltage rating = 20V.
Procedure (Adjustable Output Version: TC2574-ADJ)
Procedure
Given Parameters:
V
OUT
= Regulated Output Voltage
V
IN
(max) = Maximum DC Input Voltage
I
LOAD
(max) = Maximum Load Current
1. Programming Output Voltage
To select the right programming resistor R1 and R2
value (see Figure 2) use the following formula:
V
OUT
= V
REF
1.0 +
Given Parameters:
V
OUT
= 24V
V
IN
(max) = 40V
I
LOAD
(max) = 0.4A
1. Programming Output Voltage (selecting R1 and R2)
Select R1 and R2
R2
V
OUT
= 1.23 1.0 +
Select R1 = 1.0kΩ
R1
Example
(
)
(
R2
R1
)
where V
REF
= 1.23V
R2 = R1
V
(
V
OUT
– 1.0
REF
10V
)
= 1.0k
(
1.23V – 1.0
)
Resistor R1 can be between 1.0kΩ and 5.0kΩ. (For best
temperature coefficient and stability with time, use 1% metal
film resitors).
V
OUT
R2 = R1
– 1.0
V
REF
R1 = 18.51kΩ, choose a 18.7kΩ metal film resistor.
(
)
2. Input Capacitor Selection (C
IN
)
To prevent large voltage transients from appearing at the
input and for stable operation of the converter, an aluminium
or tantalum electrolytic bypass capacitor is needed between
the input pin +V
IN
and ground pin GND. This capacitor
should be located close to the IC using short leads. This
capacitor should have a low ESR (Equivalent Series
Resistance) value. For additional information see input
capacitor section in the “EXTERNAL COMPONENTS”
section of this data sheet.
2. Input Capacitor Selection (C
IN
)
A 22µF aluminium electrolytic capacitor located near the
input and ground pin provides sufficient bypassing.
TC2574-1 1/6/00
8
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
