ADT7350
DETAILED DESCRIPTION
Architecture and Operation
The ADT7350 is a current mode step-down converter uses a PWM control and integrated Power
Switch. An internal error amplifier establishes an integrated error voltage which is compared
with slope compensated ramp signal in order to make internal Power Switch control signal. The
ADT7350 has not only voltage feedback loop but also current feed-forward loop. The peak
current detector limit the over current in cycle by cycle basis and gives the information of load
current to the PWM controller.
At the starting of a cycle, Power Switch is off , the input and output is isolated. The fixed
frequency generator start to operate. After oscillator working , every cycle of beginning, the
Power Switch is turned on. The current flows into inductor thus input is shorted to output via to
inductor.
The Power Switch is turned off when ;
1) The ramp signal which is the sum of the current sensing amplifier output and artificial
compensated signal to prevent from sub-harmonic oscillation crosses the output of
integrator .
2) The inductor current reaches the maximum allowed level (1.8A)
3) The protection signals are trigged.
When the Power Switch is off , the inductor current can not change abruptly. The external
Schottky diode conducts the inductor current.
Enable(EN)
The ADT7350 features a low-power shut down mode to decrease total current consumption. A
logic Low at EN shuts down the controller and internal Power Switch. The typical value of VIH
is 1.2V , VIL is 0.4.
Soft-Start
The internal soft-start circuitry make a reference voltage of error amplifier gradually ramps up to
reduce input surge current during start-up
.
Bootstrap
In order to generate enough large control voltage of Power Switch. The purpose of this block is to
decrease Power Switch turn on resistance.
Protection (OVP/OTP/OCP)
The ADT7350 monitors the output voltage of controller and limit the maximum allowed
feedback voltage in order not to abruptly change output voltage. The output feed-back voltage
over 0.95V, this function is trigged. The ADT7350 monitors the temperature variation using
PTAT(Proportional to Absolute Temperature) current. When the die temperature exceeds typical
+140
°C
, the Over temperature function is trigged. The maximum allowed inductor current is
well controlled to prevent from falling into over-load or shorted output conditions. The
maximum allowed load current is typically 1.8A.
* This specifications are subject to be changed without notice
Aug 21. 2012 / Rev. 0.2
7/10
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