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IZ1937
White LED Step-Up Converter
(Functional equvalent of LT1937 Linear Technology Corporation)
Description
The IZ1937 is a step-up DC/DC converter specifically designed to drive white LEDs with a
constant current. The device can drive two, three or four LEDs in series from a Li-Ion bat-
tery. Series connection of the LEDs provides identical LED currents resulting in uniform
brightness and eliminating the need for ballast resistors. The IZ1937 operates at 1.2MHz,
allowing the use of tiny external components. The output capacitor can be as small as
0.22uF, saving space and cost versus alternative solutions.
A low 95mV feedback voltage minimizes power loss in the current setting resistor for better
efficiency.
The device can used in mobile phones, PDAs, Handheld Computers, calculators, digital
cameras, MP3-players, GPS-receivers.
Features
Internally determined LED current
High Efficiency: 84% Typical
Drives Up to 4 LEDs from a 3.2V Supply
Drives Up to 6 LEDs from a 5V Supply
36V rugged bipolar switch
Fast 1.2MHz switching frequency
Uses tiny 1mm Tall Inductors
Requires only 0.22uF output capacitor
ICs are available for shipment in chip form on unsawn wafers
Tab 1. Contact Pad Description
Contact pad number
Symbol
Description
01 SW Switch pad (Pin)
02
GND
Common pad (pin)
03 VIN Input supply pad (pin)
04
SHDN
Shutdown pad (pin)
05
GND
Common pad (pin)
06 FB Feedback pad (pin)
1 Ver.1.0/9.02.2008
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IZ1937
VIN
03
Reference
voltage
source
FB
06
A1
R1 -
A3
-
+ R3
+
R2 A2
C1
SHDN
04
A1 – error amplifier;
A2 – addrer;
A3 – PWM – comprator;
A4 – comparator;
A5 –driver;
C1 – capacitor;
R1 – R4 – resistor;
VT1 – transistor
Sawtooth
voltage
generator
Oscilator
SW
01
RS -
-flip-flops
A5
A4
+
VT1
R4
-
02,05
GND
Te device contain, switch on/off circuit; output current overrising protection unit, driver-buffer,
output switch.
Fig.1 Block Diagram
2 Ver.1.0/9.02.2008
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Table 2 –Absolute maximum ratings
Symbol Parameter
UIN
USW
UFB
U SHDN
TJ
Тstg
Input voltage
SW pin Voltage
FB pin Voltage
SHDN pin Voltage
Storage Temperature
Junction Temperature
IZ1937
Absolute
maximum ratings
Min.
Max.
- 10
- 36
- 10
- 10
- 150
-60 150
Unit
V
V
V
V
oC
oC
Table 3 – Recommended operating mode
Symbol Parameter
UIN Input voltage
TJ Storage Temperature
Recommended
operating mode
Min.
Max.
2,5 10
- 125
Unit
V
oC
Table 4 Electric Parameters
Parameter, unit
Symbol
Minimum input voltage, V
Maximum input voltage, V
Feedback Voltage, mV
UIN MIN
UIN MAX
UFB
FB pad Bias Current, nA
Supply Current,
mA
uA
Switching Frequency, MHz
Maximum Duty Cycle, %
Switch Leakage Current, uA
IFB
IQ
fS
DCMAX
ILSW
Value
Min. Max.
2,5 -
2,5
- 10
10
86 104
76 114
10 100
10 150
- 2,5
3,75
- 1,0
2,0
0,8 1,6
0,6 1,8
85 -
80
- 5,0
7,5
Testing mode
U SHDN = 3,0 V
Ambient
temperature, °C
UIN = 3,0 V,
U SHDN = 3,0 V,
ISW = 100 mA
Duty Cycle 66 %
UIN = 3,0 V,
U SHDN = 3,0 V
UIN = 3,0 V,
U SHDN = 3,0 V
UIN = 3,0 V,
U SHDN = 0 V
UIN = 3,0 V,
U SHDN = 3,0 V
UIN = 3,0 V,
U SHDN = 3,0 V
UIN = 3,0 V, U SHDN =3,0V,
USW = 5,0 V
25 ± 10
-40 … +125
3 Ver.1.0/9.02.2008
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IZ1937
Table 4 Electric Parameters (continued)
Parameter, unit
Symbol Value
Min. Max.
SHDN Voltage High, V
UIH 1,5 -
1,5
SHDN Voltage Low , V
UIL - 0,4
0,4
Testing mode
UIN = 3,0 V
Note 1
UIN = 3,0 V
Note 1
Ambient
temperature, °C
25 ± 10
-40 … +125
Notes
1 During UIH & UIL measurements threthhold voltage is tested, which has to be within range
from 0,4 to 1,5 V (see fig. 2).
2 Symbol descriptions :
- USHDN - Shutdown pad voltage (contact pad 04);
- UFB Feedback Voltage pad voltage (contact pad
- USW – Switch pad voltage (contact pad 01);
- ISW – Switch pad current (contact pad 01).
06);
UI, V
IC is
switched on
IC is
switched off
UIL=0,4V
Fig.2 Switching on/off diagramm
UIH=1,5V
USHDN, V
4 Ver.1.0/9.02.2008
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Table 5 –Typical Electric Parameters
Symbol
Parameter, unit
ICL Output transistor current limit
UCESAT
ISHDN
Output transistor saturation
voltage
SHDN pad (pin) bias current
Testing mode
Typ.value
UIN = 3,0 V, U SHDN =
3,0 V
UIN = 3,0 V, U SHDN =
3,0 V, ISW = 250 мА
UIN = 3,0 V, U SHDN =
3,0 V
320
350
65
IZ1937
Unit
mA
mV
uA
OPERATION DESCRIPTION
The IZ1937 consist of following units: voltage switch on/off circuit, sawtooth generator, rectangular
pulse generator PWM –comparator RS-latch, error amplifier, output buffer, output switch, current-
limit circuit for output switch.
The IZ1937 uses a constant frequency, current mode control scheme to provide excellent line and
load regulation.
At the start of each oscillator cycle, signal from oscillator applied to S-pin of the RS-latch, the latch
output high level voltage is set. This voltage through buffer turns on the power switch (transistor
VT1). A voltage proportional to the switch current is added to a stabilizing sawtooth voltage and
the resulting sum is applied to the positive terminal of the PWM comparator A3. When this voltage
exceeds the level at the negative input of A3, the SR latch is reset turning off the power switch
VT1. The level at the negative input of A3 is set by the error amplifier A1, and is simply an
amplified version of the difference between the feedback voltage and the reference voltage of
95mV. In this way, the error amplifier A1 contols the peak current level to keep the output in
regulation. If the error amplifier’s output increases, more current is delivered to the output; if it
decreases, less current is delivered.
5 Ver.1.0/9.02.2008
IZ1937-TSe.doc