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
Example
5. Output Capacitor Selection (COUT
)
5. Output Capacitor Selection (COUT
)
A. COUT = 100µF to 470µF standard aluminium
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
electrolytic.
between 100µF and 470µF is recommended.
B. Capacitor voltage rating = 20V.
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.
Procedure (Adjustable Output Version: TC2574-ADJ)
Procedure
Example
Given Parameters:
OUT = 24V
Given Parameters:
VOUT = Regulated Output Voltage
VIN (max) = Maximum DC Input Voltage
LOAD (max) = Maximum Load Current
V
VIN (max) = 40V
ILOAD (max) = 0.4A
I
1. Programming Output Voltage
1. Programming Output Voltage (selecting R1 and R2)
To select the right programming resistor R1 and R2
value (see Figure 2) use the following formula:
Select R1 and R2
R2
V
OUT = 1.23
(
1.0 +
)
Select R1 = 1.0kΩ
R1
R2
R1
VOUT = VREF
(
1.0 +
)
where VREF = 1.23V
VOUT
VREF
10V
R2 = R1
(
– 1.0)
= 1.0k
(
– 1.0
)
1.23V
Resistor R1 can be between 1.0kΩ and 5.0kΩ. (For best
temperature coefficient and stability with time, use 1% metal
R1 = 18.51kΩ, choose a 18.7kΩ metal film resistor.
film resitors).
VOUT
R2 = R1
(
– 1.0)
VREF
2. Input Capacitor Selection (CIN
)
2. Input Capacitor Selection (CIN
)
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 +VIN 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.
A 22µF aluminium electrolytic capacitor located near the
input and ground pin provides sufficient bypassing.
TC2574-1 1/6/00
8
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