Zetex - AN48, Getting more out of the ZXLD1350

AN48
Getting more out of the ZXLD1350 - high output current
Ray Liu, Systems Engineer, Zetex Semiconductors
Introduction
The ZXLD1350 is a continuous mode inductive step-down converter, designed for driving single
or multiple series connected LEDs efficiently from a voltage source higher than the LED voltage.
The device operates from an input supply between 7V and 30V and provides an externally
adjustable output current of up to 350mA. In order to obtain higher output current to drive LEDs
with higher power, a high current externally connected output stage is required.
700mA driver for multiple 3W LEDs in series
This driver is designed to drive up to six 3W LEDs in series which could deliver total output power
of 15W with an overall efficiency of around 90%.
RS2
LED1
LEDN
L1
RS1
D1
R3
Q3
V6
6
VIN
+
4
ISENSE
U1
C1
R1
LX GND ADJ
1
2
3
R2
D2
C2
Q1
Q2
0
Figure 1
Issue 3 - May 2007
© Zetex Semiconductors plc 2007
Schematic of 700mA driver
1
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AN48
Part list
Table 1
Part ref.
Part no.
Remark
U1
ZXLD1350
Q1
FCX619
Q2
FMMT619
Q3
FMMT619
D1
ZLLS1000
D
25.6V Zener diode
L1
68␮H 1A
RS1
150m⍀
RS2
2.2⍀
R1
2.2K⍀
R2
470⍀
R3
15K⍀
C1
3.3␮F 50V
X5/7R or other low ESR cap
C2
0.1␮F
Optional
Circuit description
The output driver consists of two NPN transistors (Q1 and Q2). Transistor Q2 acts as a small signal
inverter which inverts the original LX switch signal. The collector of Q2 is connected to the base
of transistor Q1 which acts as the power output switch.
Transistor Q3 and Zener diode D2 form a simple regulator to supply a constant voltage to the
driver stage. The voltage at emitter of Q3 is around 5V. This helps to provide a stable driving
current to both Q1 and Q2. The driving currents are around 2mA and 9mA respectively.
Total propagation delay is less than 200ns against the LX pin. Both the rise time and the fall time
of the output switch are less than 70ns when input supply voltage is 30V.
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AN48
Typical performance graphs
5 LEDs in series with total VF = 17.9V
100
800
Output Current (mA)
Efficiency (%)
90
80
70
60
700
600
500
400
50
21
22
23
24
25
26
27
28
29
21
30
22
23
24
25
26
27
28
29
30
Input Voltage (V)
Input Voltage (V)
Efficiency vs Input Voltage (5 LEDs)
Output Current vs Input Voltage (5 LEDs)
4 LEDs in series with total VF = 14.9V
100
800
Output Current (mA)
Efficiency (%)
90
80
70
60
700
600
500
400
50
18
20
22
24
26
28
18
30
20
22
24
26
28
30
Input Voltage (V)
Input Voltage (V)
Efficiency vs Input Voltage (4 LEDs)
Output Current vs Input Voltage (4 LEDs)
3 LEDs in series with total VF = 11.1V
100
800
Output Current (mA)
Efficiency (%)
90
80
70
60
700
600
500
400
50
15
20
25
15
30
Efficiency vs Input Voltage (3 LEDs)
Issue 3 - May 2007
© Zetex Semiconductors plc 2007
20
25
30
Input Voltage (V)
Input Voltage (V)
Output Current vs Input Voltage (3 LEDs)
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AN48
Typical performance graphs (cont.)
2 LEDs in series with total VF = 7.7V
100
800
Output Current (mA)
Efficiency (%)
90
80
70
60
700
600
500
400
50
12
15
18
21
24
27
12
30
15
18
21
24
27
30
Input Voltage (V)
Input Voltage (V)
Efficiency vs Input Voltage (2 LEDs)
Output Current vs Input Voltage (2 LEDs)
1 LED in series with VF = 3.8V
100
800
Efficiency (%)
Output Current (mA)
90
80
70
60
700
600
500
400
50
10
15
20
25
10
30
Efficiency vs Input Voltage (1 LED)
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15
20
25
30
Input Voltage (V)
Input Voltage (V)
Output Current vs Input Voltage (1 LED)
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© Zetex Semiconductors plc 2007
AN48
A driver for supply voltage up to 16V
This driver is a simplified version to the 700mA driver described above. The driver is designed to
drive up to 3 Luxeon® K2 LEDs in series which could deliver a total output power of 10W with a
maximum input supply voltage of 16V.
LED1
LEDN
RS1
L1
D1
V6
+
6
4
VIN
ISENSE
R1
R2
U1
C1
Q1
LX GND ADJ
1
2
3
C2
Q2
0
Figure 2
Schematic of 1A driver
Part List
Table 2
Part ref.
Part no.
U1
ZXLD1350
Q1
ZXTN25020DFH
Q2
ZXTN25020DFH
D1
ZLLS2000
L1
47␮H 1.5A
RS
100m⍀
R1
4.7K⍀
R2
1.5K⍀
C1
4.7␮F 25V
X5/7R or other low ESR cap
C2
0.1␮F
Optional
Issue 3 - May 2007
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Remark
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AN48
Circuit description
This circuit is similar to the 700mA driver described above. The output driver consists of two NPN
transistors (Q1 and Q2). Transistor Q2 acts as a small signal inverter which inverts the original LX
switch signal. The collector of Q2 is connected to the base of transistor Q1 which act as the power
output switch.
Unlike the 700mA driver, the driving current to both Q1 and Q2 varies with the input supply
voltage. Hence, the maximum input supply voltage is limited to 16V. The driving current to Q1 is
between 5mA and 10mA with input supply voltage between 8V and 16V. Lowering the maximum
supply voltage to 16V enables us to use a lower voltage BJT with better switching performance.
Total propagation delay is less than 200ns against the LX pin. Both the rise time and the fall time
of the output switch are less than 60ns when input supply voltage is 16V.
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Issue 3 - May 2007
© Zetex Semiconductors plc 2007
AN48
Typical performance graphs
2 LEDs in series with total VF = 7.1V
1100
Output Current (mA)
100
Efficiency (%)
90
80
70
60
50
900
700
500
10
11
12
13
14
15
16
10
11
12
13
14
15
16
Input Voltage (V)
Output Current vs Input Voltage (2 LEDs)
Input Voltage (V)
Efficiency vs Input Voltage (2 LEDs)
1 LED in series with total VF = 3.5V
1100
Output Current (mA)
100
Efficiency (%)
90
80
70
60
50
900
700
500
8
10
12
14
16
8
Input Voltage (V)
Efficiency vs Input Voltage (1 LED)
Issue 3 - May 2007
© Zetex Semiconductors plc 2007
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12
14
16
Input Voltage (V)
Output Current vs Input Voltage (1 LED)
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AN48
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