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APPLICATION MANUAL TK11851L
GC3-H032B
Page 1
APPLICATION MANUAL
WHITE LED DRIVER
Step-up DC-DC Converter IC
TK11851L
CONTENTS
1 . DESCRIPTION
2
2 . FEATURES
2
3 . APPLICATIONS
2
4. TYPICAL APPLICATION
2
5. PIN CONFIGURATION
3
6. BLOCK DIAGRAM
3
7. STANDARD APPLICATION CIRCUIT
4
8. ABSOLUTE MAXIMUM RATINGS
5
9. ELECTRICAL CHARACTERISTICS
6
10. TEST CIRCUIT
7
11. TYPICAL CHARACTERISTICS
8
12. PIN DESCRIPTION
11
13. CIRCUIT DESCRIPTION
12
14. WHITE LED APPLICATION
20
15. PACKAGE OUTLINE
23
16. NOTES
24
17. OFFICES
24
MEETING YOUR NEEDS TK11851L
GC3-H032B
Page 2
WHITE LED DRIVER Step-up DC-DC Converter IC
TK11851L
1. DESCRIPTION
The TK11851L is a pulse skipping step-up DC-DC
converter IC designed for White LED driver applications
with the following built in: a very high current switching
transistor (400 mA peak), an inductor current limit detector,
a low voltage reference (V
ref
= 0.515 V), a high speed
oscillator, an error comparator, a zener diode for open-
circuit protection and an ON/OFF control. The IC works
with a very wide operating supply range (2.3 V to 10 V) and
the adjustable output voltage can be set as high as 20 V. The
device is in the ON state when the En pin is pulled to a high
level. Using the very small TK11851L and a few external
components, battery powered systems with LCD displays,
such as digital cameras and cellular phones, can be
effectively backlit with white LEDs. The white LEDs are
connected in series and driven at a constant current, resulting
in uniform brightness and high efficiency. This IC has an
adjustable oscillator, which allows the designer to set the
operating frequency by an external capacitor with the output
controlled by pulse skipping. The reference voltage is a very
low 0.515 V, achieving high efficiency operation with the
constant current output. A small coil can be used because the
inductor current limit detector circuit limits the peak current
(caused by an input voltage increase or the inrush current at
start-up) that flows through the coil. The ON /OFF control is
built-in and the circuit current can be decreased when the En
pin is low (shutdown mode). The white LEDs can be
dimmed by applying a PWM signal to the En (ON / OFF
control) pin. With this method, the white LED brightness is
still controlled by constant current, resulting in constant
chromaticity.
The built-in zener diode can be used for open-circuit
protection in case the output load is disconnected, such as
the string of LEDs opened. The internal zener diode reduces
the external component count.
The TK11851L can also be used in other applications as a
step-up DC-DC converter.
The TK11851L is available in the SOT23L-8 surface mount
package.
2. FEATURES
Very High Speed Adjustable OSC (800kHz Max)
Very Wide Operating Voltage Range (2.3 V to 10 V)
Low Loss Detector (Low Reference Voltage:
V
ref
= 0.515 V)
Operates with Toko D31FU, D31FB Miniature Coil
(Current Limited by Inductor Current Sensor)
Internal Switching Transistor
Open-Circuit Protection (OVP)
Active High On / Off Control
Available Intensity Control Using En Pin with
PWM Signal
Low component Count
Very Small SOT23L-8 Surface Mount Package
3. APPLICATIONS
LED Backlighting and Frontlighting
LED Flashlights
Step-up DC-DC Converters
4. TYPICAL APPLICATION
APPLICATION CIRCUIT (3 LEDs in Series)
L: TOKO Part Number D31FU: 888FU-220M
D52LC: A914BYW-220M
Efficiency vs. Supply Voltage
Efficiency (%) = 100
V
ou
t
I
LED
/(V
in
I
in
)
GND
Is
Ext
Vfb
Gnd
Cx
Vin
2
3
4
6
En
Vovp
Vin
ON/OFF
7
8
5
1
L 22
µH
Cx
68pF
RLED
36 VLED
Cout
2.2
µF
ILED
Cin
4.7
µF
TK11851L
D
SB07-03C
SANYO
50
60
70
80
90
100
2
3
4
5
6
Supply Voltage(V)
Efficiency (%)
D52LC
D31FU
Vout TK11851L
GC3-H032B
Page 3
5. PIN CONFIGURATION
Pin No.
SYMBOL
Function
1
Vin
Power Supply Voltage Input.
2
Is
Inductor current limit.
3
Ext
External Inductor.
4
Vovp
Open-Circuit Protection
5
Vfb
Feedback Voltage.
6
Gnd
Ground.
7
Cx
Timing Capacitor.
8
En
Enable (ON/OFF) Input.

6. BLOCK DIAGRAM
Is
Ext
Vfb
Gnd
Cx
Vin
2
3
6
En
Vovp
Error
Comp
Vref
Is Comp
0.515V
Band Gap Reference
CLK
Q
S
R
7
8
1
Rsc
TK11851L
5
4
OSC
CLK
Cx
0.86V
0.3V
18V
0.1V
0.3 CLK
2.3k 300k 200k 200k 1k Is
Ext
Vfb
Gnd
Cx
Vin
2
3
4
6
En
Vovp
7
8
5
1 TK11851L
GC3-H032B
Page 4
7. STANDARD APPLICATION CIRCUIT
Standard Application circuit for white LED
Standard application circuit for LCD Bias

Cin: Input Capacitor
Cout: Output Capacitor
L: Inductor
D: Schottky Diode
R
LED
: LED Current Setting Resistor
C
X
: OSC Frequency Setting Capacitor
Cin: Input Capacitor
Cout: Output Capacitor
L: Inductor
D: Schottky Diode
R1, R2: Output Voltage Setting Resistor
Cfb: Compensation Capacitor
C
X
: OSC Frequency Setting Capacitor
Is
Ext
Vfb
Gnd
Cx
Vin
2
3
6
En
Vovp
Error
Comp
Vref
Is Comp
0.515V
Band Gap Reference
CLK
Q
S
R
Vin
GND
ON/OFF
7
8
1
Rsc
L
Cx
R
LED
V
LED
Cout
I
LED
Cin
TK11851L
D
5
4
OSC
CLK
Cx
0.86V
0.3V
0.1V
0.3 CLK
Is
Ext
Vfb
Gnd
Cx
Vin
2
3
6
En
Vovp
Error
Comp
Vref
Is Comp
0.515V
Band Gap Reference
CLK
Q
S
R
Vin
GND
ON/OFF
7
8
1
Rsc
L
Cx
Cin
TK11851L
D
5
4
OSC
CLK
Cx
0.86V
0.3V
1k 0.1V
0.3 I
LOAD
Vout
Cout
Cfb
R2
R1
CLK TK11851L
GC3-H032B
Page 5
8. ABSOLUTE MAXIMUM RATINGS
T
a
=25
°
C
PARAMETER
SYMBOL
RATINGS
UNITS
TEST CONDITIONS
Supply Voltage
V
in
20
V
Switch Voltage
V
sw
20
V
Vovp Pin Voltage
V
ovp
-0.3 ~ V
OVP
V
Vfb Pin Voltage
V
fb
-0.3 ~ V
in
V
En Pin Voltage
V
En
-0.3 ~ 20
V
Switch Peak Current
I
sw, peak
400
mA
Power Dissipation
P
D
400
mW
*
Storage Temperature Range
T
stg
-55 ~ +150
°
C
Operating Temperature Range
T
OP
-30 ~ +85
°
C
Maximum Frequency
f
MAX
~800
kHz
Operating Voltage Range
V
OP
2.3 ~ 10
V
P
D
must be decreased at the rate of 3.2mW/°C for operation above 25°C. TK11851L
GC3-H032B
Page 6
9. ELECTRICAL CHARACTERISTICS
V
in
=3V, Is=Open, Ext=Open, V
ovp
=Open, V
fb
=GND, C
X
=300pF, V
En
=V
in
, T
a
=25
°
C unless otherwise specified
Value
Parameter
Symbol
MIN
TYP
MAX
Units
Conditions
Oscillator Section (C
X
Pin)
C
X
Charging Current
I
chg
16
22
30
µA
V
Cx
=0.1V, V
fb
=1V
C
X
Discharging Current
I
dis
35
49
65
µA
V
Cx
=1.0V, V
fb
=1V
Charge and Discharge Current
Ratio
I
dis
/I
chg
2.2
C
X
Threshold Voltage +
V
Cx, High
810
860
910
mV
V
Cx
at I
chg
=0uA, V
fb
=1V
C
X
Threshold Voltage -
V
Cx, Low
260
300
340
mV
V
Cx
at I
dis
=0uA, V
fb
=1V
C
X
Oscillation Voltage
V
OSC
520
560
600
mV
(V
Cx, High
)-(V
Cx, Low
)
Current Limiting Comparator Section (Is Pin)
Inductor Current Limit
I
LIMIT
220
260
300
mA
Note 1
Pin 1-2 total Resistance
R
scext
380
440
500
m Measured Between pin 1
and pin 2
Internal effective Resistance for
Inductor Current Limit Sense
R
sc
300
m Note 2, Note 3
Output Switch Section (Ext Pin)
Output Transistor Saturation
Voltage
V
sw, Sat
0.2
0.4
V
V
CX
=0V, I
sw
=200mA
Output Transistor Off Leak
Current
I
sw, Off
0.01
2
µA
V
fb
=1V, V
sw
=20V
Feedback Comparator Section (Vfb Pin)
Reference Voltage
V
ref
0.495
0.515
0.535
V
Vfb Pin Output Current
I
fbIn
-2
-0.3
µA
V
fb
=0.4V
Shutdown Section (En Pin)
En Input Voltage +
V
En, High
1.2
20
V
On mode
En Input Voltage -
V
En, Low
0.3
V
Shutdown mode
En Pin Input Bias Current
I
EnIn
25
40
µA
V
En
=3V
Open-Circuit Protection Section (Vovp Pin)
Open-Circuit Voltage
V
OVP
17
18.5
20
V
I
Z
=0.5mA
Vin Section (Vin Pin)
Low Voltage Stop
V
in, Low
1.7
2.0
2.3
V
High Voltage Output Stop
V
in, High
11.5
13.5
14.5
V
Shutdown Supply Current
I
in, Off
0.01
1
µA
V
En
=0.3V
Quiescent Supply Current
I
in, On
1
1.5
2
mA
V
fb
=1V
Note 1 I
LIMIT
: The value of current source I
L
(DC measurement) when the switching operation is stopped by the Is
comparator.
Note 2 R
sc
: Guaranteed by the design equation:
sc
in
LIMIT
R
V
300
3
.
2
V
1
.
0
I =
Note 3 Do not connect the output circuit to ground directly, the R
sc
might be damaged.
(Excessive current passes through R
sc
, Schottky Diode and Coil) TK11851L
GC3-H032B
Page 7
10. TEST CIRCUIT
TEST CIRCUIT
Is
Ext
Vfb
Gnd
Cx
Vin
2
3
4
5
6
7
8
En
V
ovp
1
A
I
V
10
µF
A
I
fb
V
fb
A
V
Cx
I
chg
I
dis
I
sw
V
sw
V
I
Z
V
V
ovp
A
I
En
V
En
V
I
L
V
Is
300pF
in
in
TEST CIRCUIT FOR EFFICIENCY MEASUREMANT. (REFERENCE ONLY)
Efficiency: 78%(typ)
A
Cx
300pF
Iin
V
in
C
in
47
µ
V
V
Out
C
out
47
µ
1
Vin
8
En
7
Cx
6
Gnd
5
Vfb
4
Vovp
3
Ext
2
Is
2200p
L
100
µ
D
470k
20k
I
Load
15mA
3.5V
TK11851L
L: 100
µH D63LCB (A921CY-101M) TOKO
D: SB07-03C SANYO TK11851L
GC3-H032B
Page 8
11. TYPICAL CHARACTERISTICS
OSC Period time vs. C
X
Capacitance
V
in
=V
En
=3V, V
fb
=0V
OSC Frequency vs. Temperature
V
in
=V
En
, V
fb
=0V, C
X
=100pF
0
2
4
6
8
10
12
14
0
50
100
150
200
250
300
Cx Capacitance (pF)
OSC Period (mS)
150
170
190
210
230
250
270
290
-30
-10
10
30
50
70
90
Temperature (C)
OSC Frequency (kHz)
Maximum Duty vs. OSC Frequency
V
in
=V
En
=3V, V
fb
=0V
Switch Current vs. Switch Saturation Voltage
V
in
=V
En
=3V, V
fb
=0V, V
Cx
=0V
66
68
70
72
74
76
78
80
82
10
100
1000
OSC Frequency (kHz)
Maximum Duty (%)
0
100
200
300
400
500
600
0
100
200
300
400
500
Switch Saturation Voltage (mV)
Switching Current (mA)
Inductor Current Limit vs. Supply Voltage