ncp5217a ON Semiconductor, ncp5217a Datasheet - Page 11

• Manufacturer Part Number: ncp5217a
• Description: Single Synchronous Step-down Controller
• Manufacturer: ON Semiconductor

Overcurrent Protection (OCP)
event occurs. The current is continuously monitored by the
differential current sensing circuit. The current limit
threshold voltage VOCSET can be programmed by resistor
Rocset connecting at the IDRP/OCP pin. However, fixed
default VOCSET can be achieved if Rocset is less than 2 kW.
continuously, the top gate driver will be turned off cycle by
cycle. If it happens over consecutive 16 clock cycles time
(16 x 1/f
bottom gate drivers are off. EN resets or power recycle the
device can exit the fault. The following diagram shows the
typical behavior of OCP.
acquiring current information. In addition, the threshold
OCP voltage can be programmed to some desired value by
setting the programming resistor Rocp.
The NCP5217A protects power system if over current
If the inductor current exceeds the current threshold
The NCP5217A uses lossless inductor current sensing for
Vo
SW
L
DCR
L
Vo
DCR
Figure 20. Overcurrent Protection
), the device is latched off such that top and
Top to Bottom : SWN, V
Figure 21. OCP Configurations
Rt
Rb
Rs1
Cs
Rt
Rb
Rs1
Cs
Rocp
Rs2
IDRP/OCP
IDRP/OCP
Rocp
Rs2
CS+
CS−
CS+
CS−
FB
FB
+
−
+
−
Without AVP
Vref
Vref
With AVP
o
+
+
+
+
, PGOOD, I
G i
G i
I DRP
I DRP
o
COMP
COMP
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Rocp resistor. If Adaptor Voltage Position (AVP) is used, the
Rocp should be connected to FB pin. If AVP is not used, the
Rocp should be connected to ground. At the IDRP/OCP pin,
there is a constant current(24 mA typ.) flowing out during the
programming stage at system start up. This is used to sense
the voltage level which is developed by a resistor R
to program the overcurrent detection threshold voltage. For
typical application, the V
typ) by setting R
pin to ground. It has the benefit of saving one component at
application board. For other programming values of V
please refer to the electrical table of “Overcurrent Protection
Setting” section.
Guidelines for selecting OCP Trip Component
Overvoltage Protection (OVP)
V
At that moment, the top gate drive is turned off and the
bottom gate drive is turned on until the V
under voltage (UV) threshold and bottom gate drive is
FB
It should be noted that there are two configurations for
When V
1. Choose the value of R
2. Define the DC value of OCP trip point (I
3. Calculate the inductor peak current (I
4. Check with inductor datasheet to find out the value
5. Select Cs value between 100 nF to 200 nF.
6. Calculate Rs1 value by the equation:
7. Calculate Rs2 value by the equation:
8. Hence, all the current sense components Rs1, Rs2,
9. If Rs2 is not used (open), set k = 1, at that
voltage for over 1.5 ms blanking time, an OV fault is set.
(typical is 0 W for V
that you want. The typical value is 1.5 to 1.8 times
of maximum loading current. For example, if
maximum loading is 10 A, then set OCP trip point
at 15 A to 18 A.
estimated by the equation:
of inductor DC resistance DCR, then calculate the
RS1, RS2 dividing factor k based on the equation:
Typically, 100 nF will be used.
Cs have been found for target I
moment, the I
FB
I
pk
voltage is above 115% (typical) of the nominal
+ I
ocp
Rs1 +
OCP_DC
= 0 W, or directly short the IDRP/OCP
pk
Rs2 +
k +
I
will be restricted by:
pk
octh
k * DCR * Cs
+
)
I
octh
pk
V
is set as default value (40 mV
k * Rs1
V
ocp
2 * V
DCR
V
* DCR
1 * k
OCth
OCth
O
= 40 mV typical)
L
for V
* (V
IN
* f
IN
octh
OCP_DC
SW
* V
* L
selection.
FB
O
O
pk
)
below lower
.
)which is
OCP_DC
ocp
(eq. 4)
(eq. 5)
(eq. 6)
(eq. 7)
(eq. 8)
so as
octh
)
,