tm
TE
CH
T6322A
PWM Control 1.5A Step-Down Converter
FEATURES
GENERAL DESCRIPTION
• Wide Input Voltage Range: 7V to 30V
• LED Output Current Up to 1.5A
• 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-23 and SOP-8 Lead-free Package
The T6322A 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 1.5A. Depending upon supply
voltage and external components, this can
provide up to 42 watts of output power. The
T6322A includes the output switch and a highside output current sensing circuit, which uses
an external resistor to set the nominal average
output current. Output current can be adjusted
above, or below the set value, by applying an
external control signal to the 'ADJ' pin. The
ADJ pin will accept either a DC voltage or a
PWM waveform. Depending upon the control
frequency, this will provide either a continuous
or a gated output current. The PWM filter
components are contained within the chip. The
T6322A is available in SOT-23 and SOP-8
Lead-free package.
Applications
• LED/Display Back Light Driver
• Lightings
• Portable Communication Devices
• Handheld Electronics
PART NUMBER EXAMPLES
PART NO.
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PACKAGE
T6322A-AXG
SOT-23
T6322A-ADG
SOP-8
Publication Date: NOV. 2008
Revision: A
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T6322A
PIN ARRANGEMENT(Top view)
LX
GND
1
ADJ
2
6
5
VIN
EN
3
4
ISENSE
EN
1
8
Is e n s e
ADJ
LX
2
3
7
6
V IN
GND
LX
4
5
GND
SOT-23-6
SOP-8
PIN DESCRIPTION
SYMBOL
SOT-23
SOP-8
LX
GND
1
2
3,4
5,6
ADJ
3
2
ISENSE
4
8
EN
5
1
VIN
6
7
DESCRIPTION
Drain of NDMOS switch
Ground
Multi-function On/Off and brightness control pin:
• Leave floating for normal operation.(VADJ = VREF = 1.25V giving
nominal average output current IOUTnom = 0.1/RS)
• Drive with DC voltage (0V < VADJ < 2.5V) to adjust output current
from 0% to 200% of IOUTnom
• Drive with PWM signal from open-collector or open-drain transistor,
to adjust output current. Adjustment range 0% to 100% of IOUTnom for
f >10kHz and 0% to 100% of IOUTnom for f < 500Hz
Connect resistor RS from this pin to VIN to define nominal average
output current IOUTnom = 0.1/RS
Enable control signal, H: Ative, L : Power Down
Input voltage (7V to 30V). Decouple to ground with 10uF or higher
X7R ceramic capacitor close to device
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Publication Date: NOV. 2008
Revision: A
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T6322A
ABSOLUTE MAXIMUM RATINGS
Parameter
Voltage on intput pin relative to GND
Symbol
VIN
Value
-0.3 to +36
Unit
V
-5 to +0.3
V
-0.3 to +6
V
TA
-40 to +125
°C
TLEAD
300
°C
TS
-65 to +150
°C
SOT-23
0.35
SOP-8
0.8
SOT-23
250
SOP-8
178
ISENSE pin Voltage
ADJ and EN pin Voltage
Operating Temperature Rang
Maximum Soldering Temperature (at leads, 10 sec)
Storage Temperature Rang
Power Dissipation, PD @ TA = 25°C
Package Thermal Resistance, θJA
W
°C/W
Electrical Characteristics
(TA = -40 to 85°C unless otherwise noted. Typical values are at TA =25°C, VDD =12V)
Symbol
Description
VIN
Input Voltage
Conditions
Min. Typ. Max
7
30
Unit
V
VIRU
Internal regulator start-up threshold
VIN rising
5.65
V
VIRD
Internal regulator shutdown
VIN falling
5.55
V
IQOFF
Quiescent supply current
with output off
EN pin grounded
35
uA
IQON
Quiescent supply current
with output switching
ADJ pin floating
f = 250kHz
1.8
5.0
mA
100
105
mV
VSENSE
Mean current sense threshold Voltage Measured on ISENSE pin with respect to
(Defines LED current setting accuracy) VIN, VADJ = 1.25V
95
VSENTH Sense threshold hysteresis
ISENSE
ISENSE pin input current
VSENSE =VIN – 0.1
VREF
Internal reference voltage
Measured on ADJ pin with pin floating
∆VREF/∆T Temperature coefficient of VREF
VADJ
External control voltage range on
ADJ pin for DC brightness control*
0
VIHEN
EN Input Voltage Hight
2
VILEN
EN Input Voltage Low
RADJ
Resistance between ADJ pin and VREF 0< VADJ < 2.5V
ILXM
Continuous LX switch current
RLX
LX Switch ‘On’ resistance
ILXI
LX switch leakage current
TTP
Thermal Shutdown Protect
±15
%
10
uA
1.25
V
50
ppm/K
2.5
V
0.8
@ ILX = 1.5 A
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V
200
0.3
160
V
Kohm
1.5
A
0.45
ohm
5
uA
°C
Publication Date: NOV. 2008
Revision: A
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T6322A
Electrical Characteristics (continuous)
DPWML
Duty cycle range of PWM signal
applied to ADJ pin during low
frequency PWM dimming mode
Brightness control range
Duty cycle range of PWM signal
applied to ADJ pin during high
DPWMH frequency PWM dimming mode
Brightness control range
fLX
PWM frequency<500Hz
PWM amplitude = VREF
Measured on ADJ pin
0.01
PWM frequency>10KHz
PWM amplitude = VREF
Measured on ADJ pin
0.16
100:1
1
5:1
ADJ pin floating, L = 33uH (0.093 ohm)
IOUT=1A @VLED = 3.6V Driving 1 LED
Operating frequency
1
280
KHz
TONmin Minimum switch ‘ON’ time
LX switch ‘ON’
240**
ns
TOFFmin Minimum switch ‘OFF’ time
LX switch ‘OFF’
200**
ns
LX switch ‘ON’
800
ns
Recommended minimum switch 'ON'
TONminR time
fLXmax
Recommended maximum operating
frequency
DLX
Recommended duty cycle range of
output switch at fLXmax
TPD
Internal comparator propagation delay
1
0.3
MHz
0.7
50
ns
Notes :
*100% brightness corresponds to VADJ = VADJ(nom) = VREF. Driving the ADJ pin above VREF will increase
the VSENSE. threshold and output current proportionally.
**Parameters are not tested at production. Parameters are guaranteed by design, characterization and
process control.
Block Diagram
V
CC
D1
RS
LED
L1
LX
V IN
M N
V ref.
C1
4 .7 u F
R1
B u ffe r
+
R4
-
ADJ
+
EN
V o lta g e
re g u la to r
B and
gap
R2
In te rn a l
VD D
O ver
T em p.
D riv e r
R3
GND
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T6322A
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 ADJ pin of the device is unconnected and
the voltage on this pin (VADJ) 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 (VADJ), 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 15% of VADJ.
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 to VADJ, the comparator output
switches high again. This cycle of events repeats, with the comparator input ramping between limits
of VADJ ± 15%.
Switching thresholds
With VADJ = VREF, the ratios of R1, R2 and R3 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
Adjusting output current
The device contains a low pass filter between the ADJ pin and the threshold comparator and an
internal current limiting resistor (200kohn nom) between ADJ and the internal reference voltage.
This allows the ADJ pin to be overdriven with either DC or pulse signals to change the VSENSE
switching threshold and adjust the output current. The filter is third order, comprising three sections,
each with a cut-off frequency of nominally 4kHz. Details of the different modes of adjusting output
current are given in the applications section.
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.
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T6322A
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)
0.066
0.1
0.142
0.285
Nominal average
1500
1000
700
350
output current (mA)
The above values assume that the ADJ pin is floating and at a nominal voltage of VREF (=1.25V).
Note that RS = 0.066ohm 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 if the ADJ pin is driven from an external voltage. (See
next section)
Output current adjustment by external DC control voltage
The ADJ pin can be driven by an external dc voltage (VADJ), as shown, to adjust the output current
to a value above or below the nominal average value defined by RS.
ADJ
+
T6322A
GND
DC
GND
The nominal average output current in this case is given by:
IOUTdc = (VADJ /1.25) x (0.1V / RS), [for 0 < VADJ <2.5V]
Note that 100% brightness setting corresponds to VADJ = VREF. When driving the ADJ pin above
1.25V, RS must be increased in proportion to prevent IOUTdc exceeding 1.5A maximum.
The input impedance of the ADJ pin is 200kohm ±25%.
Output current adjustment by PWM control
Directly driving ADJ input
A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the ADJ pin, as
shown below, to adjust the output current to a value above or below the nominal average value set
by resistor RS:
V ADJ
0V
PW M
ADJ
T6322A
GND
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T6322A
Driving the ADJ input via open collector transistor
The recommended method of driving the ADJ pin and controlling the amplitude of the PWM
waveform is to use a small NPN switching transistor as shown below:
PW M
ADJ
T6322A
GND
This scheme uses the 200k resistor between the ADJ pin and the internal voltage reference as a
pull-up resistor for the external transistor.
Driving the ADJ input from a microcontroller
Another possibility is to drive the device from the open drain output of a microcontroller. The
diagram below shows one method of doing this:
MCU
ADJ
T6322A
GND
If the NMOS transistor within the microcontroller has high Drain / Source capacitance, this
arrangement can inject a negative spike into ADJ input of the T6322A and cause erratic operation
but the addition of a Schottky clamp diode (cathode to ADJ) to ground and inclusion of a series
resistor (10K) will prevent this. See the section on PWM dimming for more details of the various
modes of control using high frequency and low frequency PWM signals.
Shutdown mode
Taking the EN pin to a voltage below 0.8V for more than approximately 100us, will turn off the
output and supply current will fall to a low standby level of 35uA nominal.
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T6322A
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 T6322A 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)
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T6322A
TYPICAL APPLICATION CIRCUITS
V IN 7V to 30V
R S 0.1 ohm
C1
47uF
C2
0.22uF
LED
L1
47uH
D1
VIN
NC
ADJ
T6322A
ISENSE
LX
GND
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T6322A
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
E
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
5~15°
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Max.
0.118
0.063
0.118
0.057
0.051
0.06
0.023
0.009
0~8°
5~15°
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T6322A
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
0.10
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
0.25
0.51
0.70
0.25
Min.
0.224
0.148
0.052
0.001
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
0.004
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.
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Publication Date: NOV. 2008
Revision: A