tm
TE
CH
T8308
PWM Control 800mA Step-Down Converter
FEATURES
GENERAL DESCRIPTION
• Wide Input Voltage Range: 7V to 30V
• LED Output Current Up to 800mA
• Soft-start
• Single pin on/off and brightness control
using DC voltage or PWM
• High efficiency (up to 97%)
• Up to 1MHz switching frequency
• Typical 5% output current accuracy
• SOT-89 and SOP-8 Lead-free Package
The T8308 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 800mA. Depending upon
supply voltage and external components, this
can provide up to 24 watts of output power.
The T8308 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 T8308 is available
in SOT-89 and SOP-8 Lead-free package.
Applications
• LED/Display Back Light Driver
• Lightings
• Portable Communication Devices
• Handheld Electronics
TM Technology, Inc. reserves the right
to change products or specifications without notice.
PART NUMBER EXAMPLES
P. 1
PART NO.
PACKAGE
T8308-AX
SOT-89
T8308-AD
SOP-8
Publication Date: Oct. 2014
Revision:C
tm
TE
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T8308
PIN ARRANGEMENT(Top view)
PIN DESCRIPTION
Symbol
SOT-89-5
SOP-8
LX
1
3
GND
2
5.6
EN
3
1
ISENSE
4
8
VIN
5
7
Description
Drain of NDMOS switch
Ground
Enable control signal, H: Active, L: Power Down
Connect resistor Rs from this pin to VIN to define nominal average output
current IOUTnom=0.1/Rs
Input voltage (7V to 30V).
Decouple to ground with 47uF or higher X7R ceramic capacitor close to
device.
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 2
Publication Date: Oct. 2014
Revision:C
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T8308
ABSOLUTE MAXIMUM RATINGS
Parameter
Symbol
Value
Unit
VIN
-0.3 to +40
V
ISENSE pin Voltage
-5 to +0.3
V
EN pin Voltage
-0.3 to +6
V
TA
-40 to 125
℃
TLEAD
300
℃
TS
-65 to +150
℃
SOT-89
0.5
W
Voltage on input pin relative to GND
Operating junction Temperature Rang
Maximum Soldering Temperature (at lead, 10sec)
Storage Temperature Rang
Power Dissipation, PD @ TA=25℃
Package Thermal Resistance, θJA
SOP-8
0.8
SOT-89
200
SOP-8
178
℃/W
Electrical Characteristics
(TA=-40 to 85℃ unless otherwise noted. Typical values are at TA=25℃, VDD=24V)
Symbol
Description
Conditions
Min.
Typ.
Max.
Unit
7
-
30
V
VIN
Input voltage
VIRU
Internal regulator start-up threshold
VIN rising
5.65
V
VIRD
Internal regulator shutdown
VIN falling
5.55
V
IQ Off
Quiescent supply current with output off DIM pin grounded
170
uA
IQ On
Quiescent supply current with output
switching
1.8
5
mA
ISENSE
Mean current sense threshold voltage
Measured on ISENSE pin with
(Defines LED current setting accuracy) respect to VIN, VADJ = 1.25V
100
105
mV
ISENTH
Sense threshold hysteresis
ISENSE
ISENSE pin input current
VIH EN
DIM Input Voltage High
VIL EN
DIM Input Voltage Low
0.5
V
ILXM
Continuous LX switch current
0.8
A
RLX
LX Switch ‘On’ resistance
ILXI
LX switch leakage current
TTP
Thermal Shutdown Protect
fLX
TONmin
Operating frequency
Minimum switch ‘ON’ time
DIM pin floating
fsw= 250kHz
95
VSENSE = VIN -0.1
±15
%
10
uA
2
@ILX=0.8A
V
Ω
0.6
5
DIM pin floating,
L=33uH(0.093ohm)
IOUT=0.8A @VLED=3.6V
Driving 1LED
LX switch ‘ON’
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 3
160
uA
℃
280
KHz
240
nS
Publication Date: Oct. 2014
Revision:C
tm
TOFFmin
TPWminR
fLXmax
DLX
TE
CH
Minimum switch ‘OFF’ time
Recommended minimum switch ‘ON’
time
Recommended maximum operating
frequency
Recommended duty cycle range of
output switch at fLXmax
T8308
LX switch ‘OFF’
200
nS
LX switch 'ON' or ‘OFF’
800
nS
1
0.2
MHz
0.9
Notes :
**Parameters are not tested at production. Parameters are guaranteed by design, characterization and process
control.Block Diagram
Block Diagram
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 4
Publication Date: Oct. 2014
Revision:C
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T8308
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 EN pin of the device is unconnected and the
voltage on this pin (VEN) 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 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.
Switching thresholds
Define an average VSENSE switching threshold of 100mV (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 = 100mV/RS
Nominal ripple current is ±15mV/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: Oct. 2014
Revision:C
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T8308
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 = 0.1/RS [for RS ≥ 0.066 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)
0.125
0.2
0.285
Nominal average
800
500
350
output current (mA)
The above values assume that the EN pin is floating.
Note: that RS = 0.125 ohm is the minimum allowed value of sense resistor under these conditions to
maintain switch current below the specified maximum value.
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.
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 6
Publication Date: Oct. 2014
Revision:C
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T8308
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.
Inductor selection
Recommended inductor values for the T8308 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)) , note: tonmin > 240ns
LX switch off time : toff = L∆I / (VLED + VD + Iavg (RS+rL)) , note: toffmin > 200ns
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) {=0.3 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: Oct. 2014
Revision:C
tm
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T8308
TYPICAL APPLICATION CIRCUITS
* note : When outputs the big current, the noise are big, this and the system environment and PCB layout have the
influential, may defer to the actual need to increase the capacitor filtration noise.
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 8
Publication Date: Oct. 2014
Revision:C
tm
TE
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T8308
PACKAGE DIMENSIONS
SOT-89
D
POLISHED(2X)
E
1
B1
3
2
H
L
C
B
10'(2X)
A
e
e1
Symbol
A
B
B1
C
D
E
H
e
e1
L
Dimension in mm
Min.
Max.
1.40
1.60
0.44
0.56
0.36
0.48
0.35
0.44
4.40
4.60
2.29
2.60
3.94
4.25
1.50 BSC
3.00 BSC
0.89
1.2
Dimension in inch
Min.
Max.
0.055
0.063
0.017
0.022
0.014
0.019
0.013
0.017
0.173
0.181
0.090
0.102
0.155
0.167
0.059 BSC
0.118 BSC
0.035
0.047
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 9
Publication Date: Oct. 2014
Revision:C
tm
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T8308
PACKAGE DIMENSIONS
8-LEAD SOP
B
B1
K
T h e rm a l P a d *
J
A1
C1
A
C
C2
F
D
Symbol
A
A1
B
B1
C
C1
C2
D
E
F
J
K
H
Min.
5.70
3.75
1.35
0
0.31
0.30
0.10
Dimension in mm
Typ.
6.00
3.95
1.27
1.55
0.41
0.50
0.15
2.23 REF
2.97 REF
0~8°
E
H
Max.
6.30
4.10
5.13
1.80
1.75
0.15
0.51
0.70
0.25
Min.
0.224
0.148
0.052
0.000
0.012
0.012
0.004
Dimension in inch
Typ.
0.236
0.156
0.050
0.061
0.016
0.020
0.006
0.088 REF
0.117 REF
0~8°
Max.
0.248
0.164
0.202
0.071
0.069
0.006
0.020
0.028
0.010
*Note :
The thermal pad on the IC’s bottom has to be mounted on the copper foil.
To eliminate the noise influence, the thermal pad is suggested to be connected to GND on PCB.
In addition, desired thermal conductivity will be improved, if a heat-conducting copper foil on
PCB is soldered with thermal pad. The thermal pad enhances the power dissipation. As a result, a
large amount of current can be sunk safely in one package.
TM Technology, Inc. reserves the right
to change products or specifications without notice.
P. 10
Publication Date: Oct. 2014
Revision:C