MP3910 –PEAK CURRENT MODE BOOST CONTROLLER
signal is higher than the COMP voltage, the
comparator output is low, turning off the power
MOSFET.
When the output is shorted to the ground, the
part works in OCP mode and current is limited
cycle-by-cycle, the part may run into OLP
protection.
If the voltage on the ISENSE pin exceeds the
current-limit threshold voltage with typical value
of 185mV, MP3910 will turn off the GATE
output for that cycle, until the internal oscillator
starts the next cycle, and sense current again.
MP3910 limits the current of MOSFET cycle-by-
cycle.
But if the peak current cannot be limited by
185mV ISENSE voltage in every cycle due to
leading edge blanking (LEB) time, the current
may run out of control and transformer may run
into saturation. If the monitored ISENSE voltage
reaches 0.35V, the part will turn off the GATE
out and run into hiccup mode by discharge SS
capacitor with 1.66uA current. It will also re-
start up if SS voltage is discharged to 0.2V.
Over Load Protection (OLP)
The peak current is limited cycle-by-cycle, if the
load continues increasing after triggering OCP
protection, the output voltage will decrease and
the peak current will trigger OCP every cycle.
MP3910 set the over load detection by continue
monitoring the ISENSE pin voltage.
In case the short circuit is removed, the output
voltage will recover only after the next new
restart cycle.
For boost converter, it has no method to limit
current from the input to the output in the
condition of output short circuit. If protection
from this type condition is desired, it is
necessary to add some secondary protection
circuit.
Once the SS voltage is charged to 3.65V after
start up, the OLP protection is enabled. If an
OCP signal is detected, the soft-start charging
current is disabled and one over current
discharge source is enabled, the SS voltage
drops with the rate of 17.8uA current. At the
same time, one 50us one-shot timer is activated
and it remains active for 50µs after the OCP
condition stops. The 17.8uA discharge source
cannot be turn off until the one-shot timer
becomes inactive. If the OCP disappears before
at least 50us prior than the SS capacitor
discharging to 3.27V, MP3910 will run back to
normal work condition and the SS capacitor will
be re-charged to 3.65V with 54uA rate. If the
SS capacitor is discharged to 3.27V, MP3910
will register it as over load condition and turn off
the gate output until next re-start cycle. At the
same time, 17.8uA discharge current is
disabled and the 1.66uA over load discharge
source is enabled. After the SS voltage is
discharged to 0.2V, MP3910 will re-start up with
new soft-start cycle. This is hiccup mode
protection.
Over Voltage Protection
For isolated-flyback application, the positive
plateau of auxiliary winding voltage is
proportional to the output voltage, MP3910
features the over voltage protection by using
the auxiliary winding voltage instead of directly
monitoring the output voltage. The auxiliary
voltage can be monitored by FB pin through
resistor divider, once the voltage is higher than
OVP reference voltage, MP3910 turns off the
GATE output and discharge SS voltage with
1.66uA current until SS voltage is lower than
0.2V, then the part will initial one new re-start
cycle.
To avoid the mis-trigger due to the oscillation of
the leakage inductance and the parasitic
capacitance, the OVP sampling has a TOVPS
blanking with 500ns typical value. For some
oscillation condition, one external filter is
necessary to work together with the 500ns LEB
time.
The OLP detection function is disabled after the
SS voltage is discharged to be lower than
3.27V and it will be re-enabled after SS voltage
is re-charged to 3.65V. So the OLP only occurs
after the soft-start is completed.
For un-isolated solution, the DC output voltage
is applied to FB pin and it can easily detect the
OVP condition.
Short Circuit Protection
MP3910 Rev. 1.11
11/6/2014
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