L6726A
Protections
7
7.1
Protections
Overcurrent protection
The overcurrent feature protects the converter from a shorted output or overload, by sensing
the output current information across the low side MOSFET drain-source on-resistance,
R
dsON
. This method reduces cost and enhances converter efficiency by avoiding the use of
expensive and space-consuming sense resistors.
The low side R
dsON
current sense is implemented by comparing the voltage at the PHASE
node when LS MOSFET is turned on with the programmed OCP threshold voltage,
internally held. If the monitored voltage drop (GND to PHASE) exceeds this threshold, an
Overcurrent event is detected. If two overcurrent events are detected in two consecutive
switching cycles, the protection will be triggered and the device will turn off both LS and HS
MOSFETs in a latched condition.
To recover from over current protection triggered, VCC power supply must be cycled.
7.1.1
Overcurrent threshold setting
L6726A allows to easily program an overcurrent threshold ranging from 50 mV to 550 mV,
simply by adding a resistor (R
OCSET
) between LGATE and GND.
During a short time following VCC rising over UVLO threshold, an internal 10µA current
(I
OCSET
) is sourced from LGATE pin, determining a voltage drop across R
OCSET
. This
voltage drop will be sampled and internally held by the device as overcurrent Threshold. The
OC setting procedure overall time length ranges from 5.5 ms to 6.5 ms, proportionally to the
threshold being set.
Connecting a R
OCSET
resistor between LGATE and GND, the programmed threshold will be:
I
OCth
I
OCSET
⋅
R
OCSET
= -------------------------------------------
-
R
dsON
R
OCSET
values range from 5 kΩ to 55 kΩ.
If the voltage drop across R
OCSET
is too low, the system will be very sensitive to start-up
inrush current and noise. This can result in undesired OCP triggering. In this case, consider
increasing R
OCSET
value.
In case R
OCSET
is not connected, the device switches the OCP threshold to a 400 mV
default value: an internal safety clamp on LGATE is triggered as soon as LGATE voltage
reaches 700 mV (typ), enabling the 400 mV default threshold and suddenly ending OC
setting phase.
See
for OC threshold setting procedure timings picture and oscilloscope sample
waveforms.
7.2
Feedback disconnection protection
In order to provide load protection even if FB pin is not connected, a 100 nA bias current is
always sourced from this pin. If FB pin is not connected, bias current will permanently pull
up FB: this forces COMP pin low, avoiding output voltage rising to dangerous levels.
Doc ID 12754 Rev 4
13/35
STMICROELECTRONICS [ STMICROELECTRONICS ]
