TE
CH
tm
T6309B
TM Technology Inc. reserves the right P. 1
Publication Date: SEP. 2005
to change products or specifications without notice.
Revision: A
Micropower Step-up DC/DC Converter
FEATURES
Matched LED Current with Overvoltage
Protection
Drives LEDs in series
Up to 85% Efficiency
Low 400mV Feedback Reference,
Optional 1.19V Feedback Reference
Internal Current Limit 250mA
Under Voltage Lock Out Circuits
0.1uA Low Shutdown Supply Current
6-pin SOT-23 Package
ESD Human Body Mode Over 5KV
APPLICATIONS
Cellular Phones
Portable Communication Devices
Handheld Electronics
PDAs
LED/Display Back Light Driver
PART NUMBER EXAMPLES
GRNERAL DESCRIPTION
The T6309B is a step-up DC/DC converter
that drives LED with a constant current. The
device can driver up to 3 series LEDs from a
single-cell Li-Ion battery. Series connection of
the LEDs provides constant LED current that
results in uniform brightness.
The T6309B features a minimum off-time
current-limited PFM control scheme. The
LED current is set with an external sense
resistor (Rs) and is regulated by the feedback
pin (FB) that regulates the voltage across the
sense resistor to 400mV (typ). The low
400mV feedback reference voltage reduces
power loss and improves efficiency for LED
driver applications. Optional 1.19V feedback
reference is available for normal output
voltage applications like LCD bias. Output
overvoltage protection (OVP) is integrated to
prevent damage in case of output open circuit
condition (e.g. faulty LED). The OVP pin can
monitor the output voltage and turn off the
converter in the event of overvoltage
condition. The T6309B is available in SOT-
23-6 packages.
Part no.
Feedback Referemce
Voltage
T6309B-AX
400mV
T6309B-BX
1.19V
TE
CH
tm
T6309B
TM Technology Inc. reserves the right P. 2
Publication Date: SEP. 2005
to change products or specifications without notice.
Revision: A
PIN ARRANGEMENT
SOT23-6 (Top view)
GND
SW
FB
1
2
3
4
6
5
V
IN
OVP
CE
PIN DESCRIPTION
SYMBOL Pin
No.
DESCRIPTION
SW 1
Switch pin. Connect inductor/diode here. Minimize trace area at this
pin to reduce EMI.
GND
2
Ground pin. Connect to local ground plane.
FB 3
Feedback pin. Reference voltage is 400mV (T6309B-A). Optional
reference voltage is 1.19V (T6309B-B). Connect cathode of lowest
LED and current sense resistor here.
CE 4
Chip enable pin. Tie CE pin to 1.5V or higher to enable device;
0.4V or less to shutdown the device. Do not float this pin.
OVP 5
Overvoltage protection. Connect this pin to the output capacitor of
the converter.
V
IN
6
Power supply Input pin.
BLOCK DIAGRAM
F B
C E
V
I N
1 . 1 9 V
S W
b u f f e r
d r i v e r
O V P
G N D
+
-
c o n t r o l
l o g i c
+
-
c u r r e n t
l i m i t
v o l t a g e
r e f e r e n c e
u n d e r
v o l t a g e
l o c k o u t
m i n i m u m
o f f - t i m e
o n e - s h o t
e r r o r
a m p l i f i e r
o v e r v o l t a g e
p r o t e c t i o n
TE
CH
tm
T6309B
TM Technology Inc. reserves the right P. 3
Publication Date: SEP. 2005
to change products or specifications without notice.
Revision: A
Absolute Maximum Ratings
Supply Voltage ................................................................... -0.3V to 5V
SW Pin Switch Voltage ......................................................... -0.3V to 20V
OVP Pin Overvoltage Protection ............................................... -0.3V to 20V
CE Pin Voltage ................................................................... -0.3V to (VDD+0.3V)
SW Pin Switch Current ......................................................... 250mA
Continuous power dissipation, PD @ T
A
= 25C SOT-23-6 ............. 0.35W
Package Thermal Resistance SOT-23-6, JA ................................ 150C /W
Operating Juction Temperature ................................................ 150C
Operating temperature range ................................................... -40C to +85C
Storage temperature range, TSTG ............................................. -55C to +150C
Lead temperature (soldering, 10sec) .......................................... 260C
ESD Human Body Mode ......................................................... 5KV
Electrical Characteristics
(
V
IN
=+3V, TA = 25C,
unless otherwise noted )
Symbol Description
Conditions
Min.
Typ.
Max
Unit
V
IN
Input
Voltage
2.5
5
V
T6309B-A
340 400 460 mV
V
REF
Reference
Voltage
T6309B-B
1.15 1.19 1.23 V
T
OFF
Switch Minimum Off Time
300 400 550
ns
V
OVP
Voervoltage
Threshold
14 V
I
DD2
Switch Off Current (V
OUT
)
FB=0.5V
60 uA
I
OFF
Shutdown Current (V
IN
) CE=0V
0.1 1 uA
V
IHCE
CE Input Voltage Hight
1.5
V
V
ILCE
CE Input Voltage Low
0.4
V
I
ICE
CE Input Bias Current
0.1
uA
I
IFB
FB Input Bias Current
0.1
uA
UVLO Under Voltage Lockout
1.55 1.75 1.95
V
R
DS(on)
Switch ON Resistance
0.6
ohm
I
LIM
Current
Limit
250 mA
TE
CH
tm
T6309B
TM Technology Inc. reserves the right P. 4
Publication Date: SEP. 2005
to change products or specifications without notice.
Revision: A
Typical Operating Characteristics
(VIN=+3.3V,Load=3 LEDs, L=10uH, C
IN
=4.7uF, C
OUT
=4.7uF, T A=+25 C, unless otherwise noted.)
Vin vs. temperature
1.8
2
2.2
2.4
2.6
2.8
3
20
30
40
50
60
70
Temperature
Vin
note: Efficiency=(VoutIout) / (VinIin)
Vin=5.2V
(2.0V/Div)
Vout
(200mV/Div)
V
SW
(5.0V/Div)
Stability for driving 3 LEDs
(Time=2.0us/Div)
Vin=3.6V
(2.0V/Div)
Vout
(200mV/Div)
V
SW
(5.0V/Div)
Stability for driving 3 LEDs
(Time=2.0us/Div)
VFB Temperature curve
0.38
0.39
0.4
0.41
0.42
0.43
-30 -20 -10 0
10 20 30 40 50 60 70
Temperature (C)
VF
B
(
V)
V in= 3.0V
Load 3LE D
Efficiency (2WLED)
60
65
70
75
80
85
90
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Vin(V)
E
ffi
c
i
e
n
c
y
(
%
)
Io=20mA
Efficiency (3WLED)
60
65
70
75
80
85
90
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Vin(V)
E
ffi
c
i
e
n
c
y
(
%
)
Io=20mA
TE
CH
tm
T6309B
TM Technology Inc. reserves the right P. 5
Publication Date: SEP. 2005
to change products or specifications without notice.
Revision: A
Functional Description
The T6309B is like a standard current-limit
PFM step-up DC-DC converter, but regulates
the voltage across the sense resistor Rs
instead of the output voltage. This results in a
constant LED current regardless the input
voltage and number of LEDs connected. With
integrated overvoltage protection (OVP), the
T6309B can be used as a current source with
overvoltage protection to drive LEDs. This
allows up to 3 LEDS to be connected in series
to the output. The low 400mV feedback
reference voltage reduces power loss and
improves efficiency.
Step-Up Converter
The step-up DC-DC converter operation
can be understood by referring to the block
diagram in Figure 1. PFM comparator
monitors the output voltage via the sense
resistor. When the feedback voltage is higher
than the reference voltage, the MOSFET
switch is turned off. As the feedback voltage
is lower than reference voltage and the
MOSFET switch has been off for at least a
period of minimum off-time decided by the
minimum off-time one-shot, the MOSFET
switch is then turned on one-shot, or until the
current limit signal is asserted.
During the internal MOSFET switch is turned
on, current ramps up in the inductor and store
energy in a magnetic field. When the
MOSFET is turned off, the voltage across the
inductor reverses and forces current through
the diode to the output filter capacitor and
load, so the energy in the inductor is
transferred to output filter capacitor and the
load. As the energy stored in the inductor is
depleted, the current ramps down and the
output diode turns off. The output filter
capacitor stores the charge while the inductor
current is higher than the output current, then
sustains the output voltage until the next
switching cycle.
Current Limit
The T6309B utilizes cycle-by-cycle current
limiting by means of protecting the output
MOSFET switch from overstress and
preventing the small value inductor from
saturation. Current limiting is implemented by
monitoring the output MOSFET current
build-up during conduction, and upon sensing
an over-current conduction immediately
turning off the switch for the duration of the
oscillator cycle. The current through the
output MOSFET is monitored and compared
against a reference signal. When the threshold
is reached, a signal is sent to the PFM
controller block to terminate the power switch
conduction. The current limit threshold is
typically set at 250mA.
Overvoltage Protection
Since T6309B is configured as current
source, the output voltage rises as the output
impedance increases or output is open-circuit
(e.g. fault LED). The output voltage may
exceed the 14V maximum voltage rating of
the internal main switch. An overvoltage
protection circuit is integrated to prevent the
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