AT1799
2A Step-Down Voltage Regulator
General Description
The AT1799 features a PWM step-down converter operating with a fixed switching
frequency of 380 KHz and uses internal power MOSFET to provide maximum
efficiency. The output voltage of the converter can be set as low as 1.215V with
external resistive divider. When trans-conductance signal is higher than saw-tooth
wave, then PWM comparator will produce ON-time signal to set internal flip-flop,
which turns on the switching power MOSFET. The external inductor current ramps up
linearly, storing energy in a magnetic field. Once peak current of flowing through
switching power MOSFET over current-limited threshold or trans-conductance signal
level is lower than saw-tooth wave, the switching power MOSFET will turns off, the
flip-flop resets, and external schottky diode turns on. This forces the current through
the inductor to ramp back down, transferring the energy stored in the magnetic field to
the output capacitor and load. To add higher flexibility to the selection of external
component values, the device uses external loop compensation.
Enable Control
Digital logic of EN provides an electrical ON/OFF control of the power supply. Connecting
this pin to ground or to any voltage less than 1.0V will completely turn OFF the regulator. In
this state, current drain from the input supply is only 20µA, the internal reference, error
amplifier, comparators, and biasing circuitry turn off . When EN control is not required that it
should be left open circuited.
Soft-Start
AT1799 is built-in soft-start function. When power up, after input voltage is above the
under-voltage lockout threshold, then soft-start allows a gradual increase of COMP voltage to
get to a steady state operating point. Therefore, reducing input surge currents. During this
time, the COMP voltage increases and the output duty cycle is allowed to increase from zero
to the value required for regulation. The maximum load current is available after the soft-start
cycle is completed. When EN pin is taken low, the soft-start capacitor is discharge to ground.
Short-Circuit Protection
If feedback voltage of the regulator falls below 0.7V, the oscillator clock switched
operating frequency to 42KHz and duty cycle is limited to 5% to reduce input power
delivered to output. If the short circuitry is not removed, the protection circuitry will
activate sustainability. When short-circuit problem is to eliminate, oscillator clock will
switch back to 380KHz automatically.
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Tel: 886-3-563-0878
8/23/2006 REV:1.1
Fax: 886-3-563-0879
WWW: http://www.aimtron.com.tw
Email: service@aimtron.com.tw
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AIMTRON [ AIMTRON TECHNOLOGY ]
