NCP5212A, NCP5212T
http://onsemi.com
15
programming stage at system start up. This is used to sense
the voltage level which is developed by a resistor Rocp so as
to program the overcurrent detection threshold voltage. For
typical application, the V
octh
is set as default value(40 mV
typ) by setting Rocp = 0 W, or directly short the IDRP/OCP
pin to ground. It has the benefit of saving one component at
application board. For other programming values of V
octh
,
please refer to the electrical table of “Overcurrent Protection
Setting” section.
Guidelines for selecting OCP Trip Component
1. Choose the value of Rocp for V
octh
selection.
2. Define the DC value of OCP trip point(I
OCP_DC
)
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.
3. Calculate the inductor peak current (I
pk
)which is
estimated by the equation:
I
pk
+ I
OCP_DC
)
V
o
*(V
IN
* V
o
)
2*V
IN
*f
SW
*L
o
(eq. 4)
4. Check with inductor datasheet to find out the value
of inductor DC resistance DCR, then calculate the
RS1, RS2 dividing factor k based on the equation:
k +
V
octh
I
pk
* DCR
(eq. 5)
5. Select C
S
value between 100 nF to 200 nF.
Typically, 100 nF will be used.
6. Calculate Rs1 value by the equation:
Rs1 +
L
k * DCR * Cs
(eq. 6)
7. Calculate Rs2 value by the equation:
Rs2 +
k*Rs1
1 * k
(eq. 7)
8. Hence, all the current sense components Rs1, Rs2,
Cs had been found for taget I
OCP_DC
.
9. If Rs2 is not used (open), set k = 1, at that
moment, the I
pk
will be restricted by:
I
pk
+
V
octh
DCR
(eq. 8)
Overvoltage Protection (OVP)
When V
FB
voltage is above OVPth+ of the nominal V
FB
voltage for over 1.5 ms blanking time, an OV fault is set. At
that moment, the top gate drive is turned off and the bottom
gate drive is turned on until the V
FB
below lower under
voltage (UV) threshold and bottom gate drive is turned on
again whenever V
FB
goes above upper UV threshold. EN
resets or power recycle the device can exit the fault. The
following diagram shows the typical waveform when OVP
event occurs.
Top to Bottom : SWN, DL, Vo, PGOOD
Figure 25. Overvoltage Protection
Undervoltage Protection (UVP)
An UVP circuit monitors the V
FB
voltage to detect under
voltage event. The under voltage limit is 80% (typical) of the
nominal V
FB
voltage. If the V
FB
voltage is below this
threshold over consecutive 8 clock cycles, an UV fault is set
and the device is latched off such that both top and bottom
gate drives are off. EN resets or power recycle the device can
exit the fault.
Top to Bottom : SWN, Vo, PGOOD
Figure 26. Undervoltage Protection
Thermal Shutdown
The IC will shutdown if the die temperature exceeds
150°C. The IC restarts operation only after the junction
temperature drops below 125°C.
