EVAL6730 STMicroelectronics, EVAL6730 Datasheet - Page 18

• Manufacturer Part Number: EVAL6730
• Description: EVAL BOARD FOR L6730XX
• Manufacturer: STMicroelectronics
• Type: Motor / Motion Controllers & Driversr

Specifications of EVAL6730

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Power - Output: 66W
• Voltage - Output: 3.3V
• Current - Output: 20A
• Voltage - Input: 4.5 ~ 14V
• Regulator Topology: Buck
• Frequency - Switching: 400kHz
• Board Type: Fully Populated
• Utilized Ic / Part: L6730
• Input Voltage: 8 V to 52 V
• Product: Power Management Modules
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: L6208
• Other names: 497-5501

AN1451 APPLICATION NOTE
2.12 Over Current Protection
To implement an Over Current (i.e. short circuit) Protection, a dedicated Over Current Detection (OCD) circuitry
(see Figure 18 for a simplified schematic) senses the current in each high side. Power DMOS are actually made
up with thousands of individual identical cells, each carrying a fraction of the total current flowing. The current
sensing element, connected in parallel to the Power DMOS, is made only with few such cells, having a 1:N ratio
compared to the power DMOS. The total drain current is split between the output and the sense element ac-
cording to the cell ratio. Sensed current is, then, a small fraction of the output current and will not contribute
significantly to power dissipation.
Figure 18. Over Current Detection simplified circuitry.
This sensed current is compared to an internally generated reference to detect an over current condition. An
internal open drain mosfet turns on when the sum of the currents in the bridges 1A and 2A or 1B and 2B reaches
the threshold (5.6A typical value); the open drain is internally connected to the EN pin. To ensure an over current
protection, connect this pin to an external RC network (see Figure 18).
Figure 19 shows the device operating in overcurrent condition (short to ground). When an over current is de-
tected the internal open drain mosfet pulls the EN pin to GND switching off all 8 power DMOS of the device and
allowing the current to decay. Under a persistent over current condition, like a short to ground or a short between
two output pins, the external RC network on the EN pin (see Figure 18) reduces the r.m.s. value of the output
current by imposing a fixed disable-time after each over current occurrence. The values of R
selected to ensure proper operation of the device under a short circuit condition. When the current flowing
through the high side DMOS reaches the OCD threshold (5.6 A typ.), after an internal propagation delay
(t
(V
decay as it circulates through the freewheeling diodes. Since the DMOS are off , there is no current flowing
through them and no current to sense so the OCD circuit, after a short delay (t
open drain device off, and R
(V
rent can be very high, the external RC network provides a disable time (t
(see Figure 19).
18/43
OCD(ON)
TH(OFF)
TH(ON)
C or LOGIC
+5V
), after the t
) the open drain starts discharging C
) all the Power DMOS turn off after the internal propagation delay (t
R
C
EN
EN
.
.
D(ON)EN
EN
60
delay, the DMOS turn on and the current restarts. Even if the maximum output cur-
R
EN
DS(ON)
TYP.
can charge C
TO GATE
LOGIC
OPEN-DRAIN
INTERNAL
POWER SENSE
COMPARATOR
COMPARATOR
1 cell
OCD
EN
EN
OCD
. When the voltage at EN pin reaches the turn-on threshold
. When the EN pin voltage falls below the turn-off threshold
POWER DMOS
OVER TEMPERATURE
n cells
OUT
I
1A
FROM THE
BRIDGE B
/ n
(I
1A
1A
I
REF
I
1A
+I
2A
V
+
SA
) / n
I
2A
DISABLE
OUT
I
2A
2A
D(OFF)EN
POWER DMOS
/ n
OCD(OFF)
) to ensure a safe r.m.s. value
n cells
HIGH SIDE DMOSs OF
). The current begins to
THE BRIDGE A
), switches the internal
D01IN1337
POWER SENSE
EN
and C
1 cell
EN
are