tm
TE
CH
T8305
PWM Control
18V Step-Down Controller
Data Sheet
TM Technology Inc.
SD6352-1
tm
TE
CH
T8305
PWM Control 18V Step-Down Controller
Features
General Description
• Wide Input Voltage Range: 7V to 18V
• LED Output Current Up to 3A
• With External Power MOSFET
• Soft-start
• Single pin on/off and brightness control
• High efficiency (up to 95%)
• Up to 1MHz switching frequency
• Typical 5% output current accuracy
• SOT-23 Lead-free Package
The T8305 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 18V and provides
an externally adjustable output current of up to
3A. Depending upon supply voltage and
external components, The T8305 includes the
output switch and a high-side output current
sensing circuit, which uses an external resistor
to set the nominal average output current. The
T8305 is available in SOT23 Lead-free package.
Applications
• CCTV IR LED Driver
• Lightings
• Portable Communication Devices
• Handheld Electronics
TM Technology, Inc. reserves the right
to change products or specifications without notice.
Part Number Examples
PART NO.
T8305AX
P. 1
PACKAGE
SOT23
Publication Date: Mar. 2013
Revision:A
tm
TE
CH
T8305
Pin Arrangement(Top view)
T8305AX
SOT23-6
Pin Description
SYMBOL
SOP-8
ISENSE
1
EN
2
VIN
3
VCC
GD
GND
4
5
6
DESCRIPTION
Connect resistor RS from this pin to VIN to define nominal average
output current IOUTnom = 0.2/RS
Enable control signal, H: Ative, L : Power Down
Input voltage . Decouple to ground with 10uF or higher X7R
ceramic capacitor close to device
12V LDO output
External Power MOSFET Gate Driver
Ground
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 2
Publication Date: Mar. 2013
Revision: A
tm
TE
CH
T8305
Absolute Maximum Ratings
Parameter
Symbol
Value
Unit
VIN
-0.3 to +24
V
ISENSE pin Voltage (Measured with respect to Vin)
-5 to +0.3
V
EN pin Voltage
-0.3 to +6
V
GD pin Voltage
-0.3 to +12
V
TA
-40 to +125
°C
Maximum Soldering Temperature (at leads, 10 sec)
TLEAD
300
°C
Storage Temperature Rang
-65 to +150
°C
Power debauchery, PD @ TA = 25°C
TS
SOT23
0.35
W
Package Thermal Resistance, θJA
SOT23
250
°C/W
Voltage on intput pin relative to GND
Operating Temperature Rang
Electrical Characteristics (TA = -40 to 85°C unless otherwise noted. Typical values are at TA =25°C, VIN =12V)
Symbol
Description
VIN
Input Voltage
Conditions
Min. Typ. Max
7
18
Unit
V
VIRU
Internal regulator start-up threshold
VIN rising
7
V
VIRD
Internal regulator shutdown
VIN falling
6.5
V
IQOFF
Quiescent supply current
with output off
EN pin grounded
170
300
uA
IQON
Quiescent supply current
with output switching
f = 80kHz
1.8
3.0
mA
200
210
mV
VSENSE
Mean current sense threshold Voltage
Measured on ISENSE pin with respect to
(Defines LED current setting accuracy)
190
VSENTH Sense threshold hysteresis
ISENSE
ISENSE pin input current
VIHEN
EN Input Voltage Hight
VILEN
EN Input Voltage Low
±20
VSENSE =VIN – 0.2
20
%
30
2
uA
V
0.5
V
Tr
Rise Time of GD
@Cload = 1600pF
100
ns
Tf
Fall Time of GD
@Cload = 1600pF
100
ns
Vin ≧ 14V
12
V
Vin ＜ 14V
Vin2V
V
VGDO,H GD Pin out Voltage, at High Stage
VGDO,L GD Pin out Voltage, at Low Stage
TTP
0.2
167
Thermal Shutdown Protect
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 3
V
°C
Publication Date: Mar. 2013
Revision: A
tm
TE
CH
T8305
Electrical Characteristics (continuous)
L = 47uH (0.093 ohm)
IOUT=2A @VLED = 3.6V Driving 1 LED
80
KHz
TONmin Minimum switch ‘ON’ time
GD High State
3*
us
TOFFmin Minimum switch ‘OFF’ time
GD Low State
6*
us
GD High State
12
us
fGD
TONminR
Operating frequency
Recommended minimum switch 'ON'
time
fGDmax
Recommended maximum operating
frequency
DGD
Recommended duty cycle range of
output switch at fGDmax
TPD
Internal comparator propagation delay
1
0.3
MHz
0.7
2
us
Notes :
*Parameters are not tested at production. Parameters are guaranteed by design, characterization and process
control.
Block Diagram
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 4
Publication Date: Mar. 2013
Revision: A
tm
TE
CH
T8305
Functional Description
The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a selfoscillating
continuous-mode buck converter.
Device operation
Operation can be best understood by assuming that the Vref of the device is unconnected and the
voltage on pin appears directly at the (+) input of the comparator.
When input voltage VIN is first applied, the initial current in L1 and RS is zero and there is no
output from the current sense circuit. Under this condition, the (-) input to the comparator is at
ground and its output is high. This turns MN on and switches the LX pin low, causing current to
flow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN
and L1 to produce a voltage ramp (VSENSE) across RS. The supply referred voltage VSENSE is
forced across internal resistor R1 by the current sense circuit and produces a proportional current in
internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of the
comparator. When this reaches the threshold voltage, the comparator output switches low and MN
turns off. The comparator output also drives another NMOS switch, which bypasses internal resistor
R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be nominally 20% .
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to VIN. The
current decays at a rate determined by the LED(s) and diode forward voltages to produce a falling
voltage at the input of the comparator. When this voltage returns , the comparator output switches
high again. This cycle of events repeats, with the comparator input ramping between limits of ±
20%.
Switching thresholds
With VREF the ratios of R1, R2 and R3 define an average VSENSE switching threshold of 200mV
(measured on the ISENSE pin with respect to VIN). The average output current IOUTnom is then
defined by this voltage and RS according to:
IOUTnom = 200mV/RS
Output shutdown
The output of the low pass filter drives the shutdown circuit. When the input voltage to this circuit
falls below the threshold, the internal regulator and the output switch are turned off. The voltage
reference remains powered during shutdown to provide the bias current for the shutdown circuit.
Quiescent supply current during shutdown is nominally 35uA and switch leakage is below 5uA.
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 5
Publication Date: Mar. 2013
Revision: A
tm
TE
CH
T8305
Applications Information
Setting nominal average output current with external resistor RS
The nominal average output current in the LED(s) is determined by the value of the external current
sense resistor (RS) connected between VIN and ISENSE and is given by:
IOUTnom = 0.2/RS [for RS ≥ 0.1 ohm]
The table below gives values of nominal average output current for several preferred values of
current setting resistor (RS) in the typical application circuit :
RS (ohm)
0.1
0.132
0.285
0.57
Nominal average
2000
1500
700
350
output current (mA)
.
Note that RS = 0.1ohm is the minimum allowed value of sense resistor under these conditions to
maintain switch current below the specified maximum value. It is possible to use different values of
RS.
Shutdown mode
Taking the EN pin to a voltage below 0.5V for more than approximately 100us, will turn off the
output and supply current will fall to a low standby level of 35uA nominal.
Capacitor selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in
series with the supply source impedance and lowers overall efficiency. This capacitor has to supply
the relatively high peak current to the coil and smooth the current ripple on the input supply. A
minimum value of 47uF is acceptable if the input source is close to the device, but higher values
will improve performance at lower input voltages, especially when the source impedance is high. In
order to avoid high frequency noise influence and improve circuit stability, it is recommended to
shunt a value of 0.22uF Capacitor. The input capacitor should be placed as close as possible to the
IC. For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better
dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this
application and should not be used.
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 6
Publication Date: Mar. 2013
Revision: A
tm
TE
CH
T8305
Inductor selection
Recommended inductor values for the T8305 are in the range 33uH to 100uH. Higher values of
inductance are recommended at higher supply voltages in order to minimize errors due to switching
delays, which result in increased ripple and lower efficiency. Higher values of inductance also result
in a smaller change in output current over the supply voltage range. The inductor should be mounted
as close to the device as possible with low resistance connections to the LX and VIN pins. The
chosen coil should have a saturation current higher than the peak output current and a continuous
current rating above the required mean output current.
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times
within the specified limits over the supply voltage and load current range.
LX switch on time : ton = L∆I / (VIN – VLED - Iavg (RS+rL+RLX)) ,
LX switch off time : toff = L∆I / (VLED + VD + Iavg (RS+rL)) ,
Where:
“L” is the coil inductance (H)
“∆I” is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}
“rL” is the coil resistance (ohm)
“RS” is the current sense resistance
“Iavg” is the required LED current (A)
“VIN” is the supply voltage (V)
“VLED” is the total LED forward voltage (V)
“RLX” is the switch resistance (ohm) {=0.2 ohm nominal}
“VD” is the diode forward voltage at the required load current (V)
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 7
Publication Date: Mar. 2013
Revision: A
tm
TE
CH
T8305
Typical Application Circuits
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 8
Publication Date: Mar. 2013
Revision: A
tm
TE
CH
T8305
Package Dimensions
SOT-23-6
B
B 1
B 2
A 1
C 1
A
C
C 2
F
D
I
Symbol
A
A1
B
B1
B2
C
C1
C2
D
E
F
G
H
I
Min.
2.60
1.40
2.70
0.95
0.90
0
0.30
0.08
Dimension in mm
Typ.
2.80
1.575
2.85
1.90(BSC)
0.95(BSC)
1.20
1.10
0.075
0.40
0.45
0.15
0.60(REF)
H
Max.
3.00
1.60
3.00
Min.
0.102
0.055
0.106
1.45
1.30
0.150
0.037
0.035
0
0.60
0.22
0.012
0.003
G
Dimension in inch
Typ.
0.110
0.062
0.112
0.075(BSC)
0.037(BSC)
0.047
0.043
0.003
0.015
0.018
0.006
Max.
0.118
0.063
0.118
0.057
0.051
0.06
0.023
0.009
0~8°
5~15°
5~15°
TM Technology, Inc. reserves the right
to change products or specifications without notice.
E
P. 9
Publication Date: Mar. 2013
Revision: A