AP3125B
Document number: DS36659 Rev. 3 - 2
July 2014
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Operation Description
The AP3125B is specifically designed for off-line AC-DC power
supply used in LCD monitor, notebook adapter and battery charger
applications. It offers a cost effective solution with a versatile
protection function.
Start-up Current and UVLO
The start-up current of AP3125B is optimized to realize ultra low
current (1A typical) so that VCC capacitor can be charged more
quickly. The direct benefit of low start-up current is the availability of
using large start-up resistor, which minimizes the resistor power
loss for high voltage AC input.
An UVLO comparator is included in AP3125B to detect the voltage
on VCC pin. It ensures that AP3125B can draw adequate energy
from hold-up capacitor during power-on. The turn-on threshold is
16V and the turn-off threshold is 7.6V.
Current Sense Comparator and PWM Latch
The AP3125B operates as a current mode controller, the output
switch conduction is initiated by every oscillator cycle and is
terminated when the peak inductor current reaches the threshold
level established by the FB pin. The inductor current signal is
converted to a voltage signal by inserting a reference sense resistor
RS. The inductor current under normal operating conditions is
controlled by the voltage at FB pin. The relation between peak
inductor current (IPK) and VFB is:
Moreover, FOCP with 1.8V threshold is only about 100ns delay,
which can avoid some catastrophic damages such as secondary
rectifier short test. Few drive cycles can alleviate the destruction
range and get better protection.
Leading-edge Blanking
A narrow spike on the leading edge of the current waveform can
usually be observed when the power MOSFET is turned on. A
250ns leading-edge blank is built-in to prevent the false-triggering
caused by the turn-on spike. During this period, the current limit
comparator is disabled and the gate driver can not be switched off.
At the time of turning off the MOSFET, a negative undershoot
(maybe larger than -0.3V) can occur on the SENSE pin. So it is
strongly recommended to add a small RC filter or at least connect a
resistor “R” on this pin to protect the IC (Shown as Figure 1).
FB
SENSE
GATE
GND
BNO 6
1
3
4
AP3125B
VCC
5
2
Large undershoot (more than
-0.3V) may damage the SENSE pin
R
C
Necessary
Figure 1
Built-in Slope Compensation
It is well known that a continuous current mode SMPS may become
unstable when the duty cycle exceeds 50%. The built-in slope
compensation can improve the stability, so there is no need for
design engineer to spend much time on that.
FB Pin and Short Circuit Protection
This pin is normally connected to the opto-coupler and always
paralleled with a capacitor for loop compensation. When the voltage
at this pin is greater than 4.5V and lasts for about 70ms, the IC will
enter the protection mode. For AP3125B, the system will enter
hiccup mode to wait the VCC decreasing to low UVLO level, then the
IC will try to restart until the failure removed. And when this voltage
is less than 1.55V, the IC will stop the drive pulse immediately.
Therefore, this feature can be used for short circuit protection,
which makes the system immune from damage. Normally, output
short makes the VFB value to the maximum because the opto-
coupler is cut off.
VCC Maintain Mode
During light load or step load, VFB will drop and be lower than 1.55V,
thus the PWM drive signal will be stopped, and there is no more
new energy transferred due to no switching. Therefore, the IC
supply voltage may reduce to the shutdown threshold voltage and
system may enter the unexpected restart mode. To avoid this, the
AP3125B hold a so-called VCC maintain mode which can supply
energy to VCC.
When VCC decreases to a setting threshold, the VCC maintain
comparator will output some drive signal to make the system switch
and provide a proper energy to VCC pin. The VCC maintain function
will cooperate with the PWM and burst mode loop which can make
the output voltage variation be within the regulation. This mode is
very useful for reducing startup resistor loss and achieving a better
standby performance with a low value VCC capacitor. The VCC is
not easy to touch the shutdown threshold during the startup process
and step load. This will also simplify the system design. The
minimum VCC voltage is suggested to be designed a little higher
than VCC maintain threshold thus can achieve the best balance
between the standby and step load performance。
System Protection and Pin Fault Protection
The AP3125B provides versatile system and pin fault protections.
The OCP comparator realizes the cycle-by-cycle current limiting
(OCP). In universal input line voltage, the IC realizes the constant
over load protection (OLP). VCC over voltage protection can be
applied as the primary OVP or opto-coupler broken protection. The
AP3125B also has pin fault connection protection including floating
and short connection. The floating pin protection includes the
SENSE, FB, etc. The short pin protection includes the CTRL pin
short protection. When these pins are floated or CTRL pin is
shorted to ground, PWM switching will be disabled, thus protecting
the power system.