Thermally enhanced Low V
FB
Step-Down LED Driver
ADT6780
OVERVIEW
The ADT6780 is a current mode step-down converter with integrated high side NMOS power switch. It
operates from a 4.5V to 28V input voltage range and supplies up to 2A of load current. Features include
enable control, under voltage lockout, programmable soft start, current limit ,thermal shutdown and PWM
dimming of LEDs. The ADT6780 uses current mode control to regulate the LED current. The LED current
is measured at FB pin voltage and amplified through the internal error amplifier. The error amplifier output
voltage is used to control the high side NMOS power switch and consequently LED current is regulated.
DETAILED DESCRIPTION
Enable and Soft Start
EN pin of the ADT6780 operates both chip enable and UVLO function. EN pin voltage under 800mV shuts
down all the chip function except for pulling up EN pin. When the EN pin voltage exceeds 1.1V, the internal
regulator will be enabled. A EN pin voltage over 2.7V, the soft start capacitor will begin to charge and
enables all the operations including switching function. When the EN pin is floating, EN voltage is high for
its pull-up function.
The soft start function is adjustable. When the EN pin becomes high, a tens of
㎂
current begins charging the
capacitor which is connected from the SS pin to GND. Smooth control of the output voltage is maintained
during start up. The soft start time is adjusted by changing capacitance of C4 and the typical soft start time is
3msec at 100nF of C4.
Switching Frequency
The ADT6780 switching frequency is fixed and set by an internal oscillator. The practical switching
frequency could range from 777kHz to 1050kHz due to device variation. If the FB voltage is under 80mV,
the switching frequency is changed to 125kHz for reducing abrupt inrush current.
Power Boosting
The ADT6780 uses an internal NMOS power switch to step-down the input voltage to the regulated output
current. Since the NMOS power switch requires a gate voltage greater than the input voltage, a boost
capacitor connected between SW and BST drives the gate. The capacitor is internally charged when SW is
low.
Error Amplifier
The high gain error amplifier extracts the difference between the reference voltage and the feedback voltage.
This extracted difference, called error signal, amplified and fed into the COMP, which is for compensation.
The feedback voltage is regulated to the reference voltage, typical 0.2V for the ADT6780.
Current Sensing
The current sensing output is proportional to the current flowing into the inductor, This output goes to the
comparator to make a proper PWM control signal. This output waveform resembles normally ramp shape.
Current Limit Protection
The output over-current protection (OCP) is implemented using a cycle-by-cycle peak detect control circuit.
The switch current is monitored by measuring the high side NMOS switch current. The measured switch
current is compared against a preset voltage which represents the current limit, between 2.2A and 4A. When
the output current is more than current limit, the high side switch will be turned off and PWM duty is
reduced. The output current is monitored in the same manner at each cycle and finally the power switch
almost turned off not to be damaged under fault conditions.
* This specifications are subject to be changed without notice
Jul. 12. 2012 / Preliminary
7/13
http://www.ad-tech.co.kr
ADTECH [ ADTECH ]
