MP3910 –PEAK CURRENT MODE BOOST CONTROLLER

MP3910 Rev. 1.11

www.MonolithicPower.com

14

11/6/2014

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2014 MPS. All Rights Reserved.

signal is higher than the COMP voltage, the

comparator output is low, turning off the power

MOSFET.

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.

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.

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.

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.

Short Circuit Protection

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.

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.

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.

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

blanking with 500ns typical value. For some

oscillation condition, one external filter is

necessary to work together with the 500ns LEB

time.

For un-isolated solution, the DC output voltage

is applied to FB pin and it can easily detect the

OVP condition.