PWM Control 1.5A Step-Down Converter
FEATURES
• 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 accurac y • SOT-23 and SOP-8 Lead-free Package Applications
• LED/Display Back Light Driver
• Lightings
• Portable Communication Devices
• Handheld Electronics GENERAL DESCRIPTION
The T6322A is a continuous mode inductive step-down converter, designed for driving single or multiple ries 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 high-side output current nsing circuit, which us an external resistor to t the nominal average output current. Output current can be adjusted above, or below the t 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.
PART NUMBER EXAMPLES
PART NO. PACKAGE
T6322A-AXG SOT-23
T6322A-ADG SOP-8
PIN ARRANGEMENT(Top view)
A D J
L X
V IN G N D I SEN SE
EN
L X L X
E N
G N D
G N D V IN A D J I n
SOT-23-6 SOP-8
PIN DESCRIPTION
SYMBOL
SOT-23 SOP-8
DESCRIPTION
LX 1 3,4 Drain of NDMOS switch GND 2 5,6
Ground
ADJ 3 2
Multi-function On/Off and brightness control pin:
• Leave floating for normal operation.(V ADJ = V REF = 1.25V giving nominal average output current I OUTnom = 0.1/RS)
• Drive with DC voltage (0V < V ADJ < 2.5V) to adjust output current from 0% to 200% of I OUTnom
• 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 I OUTnom for f < 500Hz
I SENSE 4 8 Connect resistor R S from this pin to V IN to define nominal average output current I OUTnom = 0.1/R S
EN 5 1 Enable control signal, H: Ative, L : Power Down
V IN
6
7
Input voltage (7V to 30V). Decouple to ground with 10uF or higher X7R ceramic capacitor clo to device
ABSOLUTE MAXIMUM RATINGS
Parameter Symbol Value Unit Voltage on intput pin relative to GND V IN-0.3 to +36 V
I SENSE pin Voltage -5 to +0.3 V ADJ and EN pin Voltage-0.3 to +6 V Operating Temperature Rang T A-40 to +125 °C Maximum Soldering Temperature (at leads, 10 c) T
LEAD
300 °C Storage Temperature Rang T S-65 to +150 °C
SOT-23 0.35
Power Dissipation, PD @ TA = 25°C
SOP-8 0.8
W
SOT-23 250
Package Thermal Resistance, θJA
SOP-8 178
°C/W Electrical Characteristics
(TA = -40 to 85°C unless otherwi noted. Typical values are at TA =25°C, VDD =12V)
Symbol Description Conditions Min. Typ. Max Unit V IN Input Voltage 7 - 30 V V IRU Internal regulator start-up threshold V IN rising 5.65 V V IRD Internal regulator shutdown V IN falling 5.55 V
I QOFF Quiescent supply current
with output off
EN pin grounded35 uA
I QON Quiescent supply current
with output switching ADJ pin floating
f = 250kHz
1.8 5.0 mA
V SENSE Mean current n threshold Voltage (Defines LED current tting accuracy)Measured on I SENSE pin with respect to
V IN, V ADJ = 1.25V95 100 105 mV
V SENTH Sen threshold hysteresis±15 % I SENSE I SENSE pin input current V SENSE =V IN – 0.110 uA
V REF Internal reference voltage Measured on ADJ pin with pin floating 1.25 V ∆V REF/∆T Temperature coefficient of V REF50 ppm/K
V ADJ External control voltage range on
ADJ pin for DC brightness control*
0 2.5 V V IHEN EN Input Voltage Hight 2 V V ILEN EN Input Voltage Low 0.8 V R ADJ Resistance between ADJ pin and V REF0< V ADJ < 2.5V200 Kohm
I LXM Continuous LX switch current 1.5 A
R LX LX Switch ‘On’ resistance @ I LX = 1.5 A0.3 0.45 ohm
I LXI LX switch leakage current 5 uA
T TP Thermal Shutdown Protect160 °C
Electrical Characteristics (continuous) Duty cycle range of PWM signal
applied to ADJ pin during low frequency PWM dimming mode 0.01 1
D PWML
Brightness control range PWM frequency<500Hz
PWM amplitude = V REF
Measured on ADJ pin
100:1
Duty cycle range of PWM signal
applied to ADJ pin during high frequency PWM dimming mode 0.16 1
D PWMH
Brightness control range PWM frequency>10KHz
PWM amplitude = V REF
Measured on ADJ pin
5:1
f LX Operatin
g frequency ADJ pin floating, L = 33uH (0.093 ohm)
I OUT=1A @V LED = 3.6V Driving 1 LED
280 KHz
T ONmin Minimum switch ‘ON’ time LX switch ‘ON’240**ns T OFFmin Minimum switch ‘OFF’ time LX switch ‘OFF’200**ns
T ONminR Recommended minimum switch 'ON'
time
LX switch ‘ON’800 ns
f LXmax Recommended maximum operating
frequency
1 MHz
D LX Recommended duty cycle range of
output switch at f LXmax
0.3 0.7
T PD Internal comparator propagation delay 50
ns
Notes :
*100% brightness corresponds to V ADJ = V ADJ(nom) = V REF. Driving the ADJ pin above V REF will increa the V SENSE. threshold and output current proportionally.
**Parameters are not tested at production. Parameters are guaranteed by design, characterization and process control.
Block Diagram
V C C
C1
4.7u
Functional Description
The device, in conjunction with the coil (L1) and current n resistor (RS), forms a lfoscillating 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 (V ADJ) appears directly at the (+) input of the comparator.
When input voltage V IN is first applied, the initial current in L1 and RS is zero and there is no output from the current n 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 V IN to ground, via RS, L1 and the LED(s). The current ris at a rate determined by V IN and L1 to produce a voltage ramp (V SENSE) across RS. The supply referred voltage V SENSE is forced across internal resistor R1 by the current n 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 (V ADJ), the comparator output switches low and MN turns off. The comparator output also drives another NMOS switch, which bypass internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is t by R3 to be nominally 15% of V ADJ.
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to V IN. 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 V ADJ ± 15%.
Switching thresholds
With V ADJ = V REF, the ratios of R1, R2 and R3 define an average V SENSE switching threshold of 100mV (measured on the I SENSE pin with respect to V IN). The average output current I OUT nom is then defined by this voltage and R S according to:
I OUTnom = 100mV/R S
Nominal ripple current is ±15mV/R S
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 pul signals to change the VSENSE switching threshold and adjust the output current. The filter is third order, comprising three ctions, each with a cut-off frequency of nominally 4kHz. Details of the different modes of adjusting output current are given in the applications ction.
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.
