DN78

DN78
ZXSC310 with reverse polarity protection
Ray Liu - Applications Engineer, Zetex Semiconductors
Description
The schematic diagram shown in Figure 1 is a typical example of the ZXSC310 used in a LED
flashlight application. The input voltage can either be one or two alkaline cells. If the battery is put
in the flashlight the wrong way, the reverse polarity can damage the ZXSC310 and switching
transistor, Q1. Implementing a mechanical reverse protection method can be expensive, and not
always reliable. This paper describes methods of electronic reverse protection, without efficiency
loss, for the ZXSC series ICs and related LED flashlight application circuits.
Circuit problems caused by the reverse polarity battery
If a negative voltage appears at the input terminal of Figure 1 then reverse current will flow from
the ground pin of the ZXSC310 to the VCC terminal and back to the battery. This current is high
and will damage the ZXSC310. Some of this reverse current will also flow through the VDRIVE
terminal of the ZXSC310 and into Q1 base-collector completing the circuit to the battery.
The reverse current through base-collector of Q1 turns the transistor on in the reverse direction
and causes high current to flow from ground, through emitter-collector to the battery, resulting
in battery drainage and possible damage to the switching transistor, Q1.
A common method of reverse polarity protection
A common method of reverse protection is to add a Schottky diode in series with the battery
positive. The problem with this method of reverse protection is that there is a loss of efficiency
due to the forward voltage drop of the diode, typically 5% to 10% depending upon input voltage,
reducing the usable battery life. The proposed method of reverse protection for the ZXSC series
IC's gives full protection with no loss of efficiency.
D1
L1
VIN
U1
Q2
VCC
C2
VDRIVE
Stdn
ISENSE
GND
ZXSC310
Figure 1
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
R1
Schematic diagram
1
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DN78
Reverse protection without efficiency loss
By adding current limiting resistor and Schottky diode, the reverse current flow can be eliminated
without a loss of efficiency.
Flashlight circuit with bootstrap
For the bootstrap circuit in Figure 2, the current through the ZXSC310 is blocked by the reversed
biased Schottky diode, D1.
The current from VDRIVE, which turns on Q1 in the reverse direction, is diverted via D2 back to the
battery so that Q1 does not turn on. R2 is a current limiting resistor to control this VDRIVE current.
This value is typically set to 100⍀ to 500⍀ to minimize battery current drain without affecting the
normal operation of the circuit.
D1
L1
VIN
D2
U1
VCC
C1
VDRIVE
Stdn
ISENSE
GND
Q1
R2
C2
R1
ZXSC310
Figure 2
Ref
Part number
Manufacturer
Comments
U1
ZXSC310E5
Zetex
LED driver in SOT23-5
Q1
ZXTN25012EFL
Zetex
Low sat. NPN in SOT23
D1
Value
750mA
BAT750
Zetex
750mA Schottky in SOT23
200mA
BAT54
Zetex
200mA Schottky in SOT23
68␮H
Generic
Generic
ISAT>0.4A, R<0.8⍀
270m⍀
Generic
Generic
0805 size
100⍀
Generic
Generic
0805 size
C1
10␮F/6.3V
Generic
Generic
C2
22␮F/6.3V
Generic
Generic
D2
(1)
L1
R1
R2
(1)
Table 1
Bill of materials
NOTES:
(1) Add for reverse protection
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2
Issue 3 - August 2007
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DN78
Typical operating characteristics
(For typical application circuit where Tamb = 25°C unless otherwise stated)
Without Protection
With Protection
Without Protection
100
50
Output Current (mA)
Efficiency (%)
90
80
70
60
50
3
2.6
2.2
1.8
1.4
40
30
20
10
0
1
3
2.6
Input Voltage (V)
1.8
1.4
1
Input Voltage vs Output Curren
Without Protection
With Protection
Without Protection
100
With Protection
4
80
60
40
Output Voltage (V)
Input Current (mA)
2.2
Input Voltage (V
Input Voltage vs Efficiency
20
0
-20
-40
-60
-80
-100
3
With Protection
60
2
1
0
-1
-2
Input Voltage (V)
1
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
Input Voltage (V)
Figure 3
© Zetex Semiconductors plc 2007
2
0
2.8
-3
Input Voltage vs Input Current
Issue 3 - August 2007
3
Input Voltage vs Output Voltage
Performance graphs
3
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DN78
Other circuit examples using reverse polarity protection
Flashlight circuit without bootstrap
The circuit shown in Figure 4 is for an LED flashlight application without bootstrap. As described
previously, reverse current can flow from the GND terminal to VCC and back to the battery. To
block this current path an extra diode, D2b, is added. It is recommended that a Schottky diode be
used for this application to maximize the start-up input voltage from VCC(MAX) to VCC(MIN) + D2b
VF, 3V to 1V. The Schottky diode, D2a, and resistor, R2, work in the same way as described in the
bootstrap circuit in Figure 2. A dual Schottky diode, BAT54S, is recommended for D2 in order to
achieve low component count.
L1
D1
VIN
U1
VCC
C1
R2
C2
Stdn
GND
ISENSE
ZXSC310
Figure 4
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Q1
VDRIVE
R1
LED flashlight application without bootstrap
4
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
DN78
Other circuit examples using reverse polarity protection
Flashlight circuit without bootstrap
Figure 5 is a step down converter with reverse polarity protection. The main application for this
circuit is a four alkaline cell flashlight driving a high powered LED. Again the protection circuit
operates as described above. A dual Schottky diode, BAT54S, is recommended for D2 in order to
achieve low component count.
C2
L1
D1
VIN
D2a
D2b
R2
VCC
VDRIVE
Q1
Stdn
C1
ISENSE
GND
ZXSC310
R1
Figure 5
Issue 3 - August 2007
© Zetex Semiconductors plc 2007
Step down converter with reverse polarity protection
5
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DN78
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or
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© Zetex Semiconductors plc 2007
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