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参数资料
| 型号： | TC2576VAT |
| 元件分类： | 稳压器 |
| 英文描述： | 7.5 A SWITCHING REGULATOR, 63 kHz SWITCHING FREQ-MAX, PSFM5 |
| 封装： | TO-220, 5 PIN |
| 文件页数： | 8/25页 |
| 文件大小： | 213K |
| 代理商： | TC2576VAT |
TC2576
16
TC2576-2 12/04/00
3.0A Step-Down Switching Regulator
2001 Microchip Technology Inc.
DS21399A
This circuit configuration is able to deliver approximately
0.7 A to the output when the input voltage is 12V or higher.
At lighter loads the minimum input voltage required drops to
approximately 4.7V, because the buck–boost regulator to-
pology can produce an output voltage that, in its absolute
value, is either greater or less than the input voltage.
Since the switch currents in this buck–boost configura-
tion are higher than in the standard buck converter topology,
the available output current is lower.
This type of buck–boost inverting regulator can also
require a larger amount of startup input current, even for light
loads. This may overload an input power source with a
current limit less than 5.0A.
Such an amount of input start-up current is needed for
at least 2.0msec or more. The actual time depends on the
output voltage and size of the output capacitor.
Because of the relatively high startup currents required
by this inverting regulator topology, the use of a delayed
startup or an undervoltage lockout circuit is recommended.
Using a delayed startup arrangement, the input capaci-
tor can charge up to a higher voltage before the switch–
mode regulator begins to operate.
The high input current needed for startup is now partially
supplied by the input capacitor CIN.
It has been already mentioned above, that in some
situations, the delayed start–up or the undervoltage lockout
features could be very useful. A delayed start–up circuit
applied to a buck–boost converter is shown in Figure 9,
Figure 15 in the “Undervoltage Lockout” section describes
an undervoltage lockout feature for the same converter
topology.
Design Recommendations
The inverting regulator operates in a different manner
than the buck converter and so a different design procedure
has to be used to select the inductor L1 or the output
capacitor COUT.
The output capacitor values must be larger than what is
normally required for buck converter designs. Low input
voltages or high output currents require a large value output
capacitor (in the range of thousands of
F).
The recommended range of inductor values for the
inverting converter design is between 68
H and 220 H. To
select an inductor with an appropriate current rating, the
inductor peak current has to be calculated.
The following formula is used to obtain the peak inductor
current:
IPEAK ≈
ILOAD (VIN – IVOUTI)
+
VIN x tON
VIN
2L1
where tON ≈
IVOUTI
1.0
, and fOSC = 52kHz.
VIN + IVOUTI
x
fOSC
Under normal continuous inductor current operating
conditions, the worst case occurs when VIN is minimal.
With the inverting configuration, the use of the ON/OFF
pin requires some level shifting techniques. This is caused
by the fact, that the ground pin of the converter IC is no
longer at ground. Now, the ON/OFF pin threshold voltage
(1.3V approximately) has to be related to the negative output
voltage level. There are many different possible shutdown
methods, two of them are shown in Figures10 and 11.
Figure 9. Inverting Buck-Boost Regulator with Delayed Startup
Figure 10. Inverting Buck-Boost Regulator Shutdown Circuit
Using an Optocoupler
TC2576
1
3
5
GND
ON/OFF
+VIN
R2
47 k
CIN
100
F
NOTE: This picture does not show the complete circuit.
R1
47 k
R3
470
Shutdown
Input
MOC8101
–VOUT
Off
On
5.0 V
0
+VIN
Figure 11. Inverting Buck-Boost Regulator Shutdown Circuit Using
a PNP Transistor
NOTE: This picture does not show the complete circuit.
R2
5.6 k
Q1
2N3906
TC2576
1
3
5
GND
ON/OFF
R1
12 k
–VOUT
+VIN
Shutdown
Input
Off
On
+V
0
+VIN
CIN
100
F
D1
IN5822
L1
68
H
Output
2
4
Feedback
12 to 25V
Unregulated
DC Input
CIN
100mF
/50 V
5
1
ON/OFF
+VIN
–12V @ 100mA
Regulated
Output
COUT
2200
F
/16V
TC2576
C1
0.1
F
R1
47k
R2
47k
3
GND
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