NCP1252
www.onsemi.com
15
R
BOlo
+
V
BO
I
BO
ǒ
V
bulkon
* V
BO
V
bulkoff
* V
BO
* 1
Ǔ
(eq. 4)
R
BOup
can be also written independently of R
BOlo
by
substituting Equation 4 into Equation 3 as follow:
R
BOup
+
V
bulkon
* V
bulkoff
I
BO
(eq. 5)
From Equation 4 and Equation 5, the resistor divider value
can be calculated:
R
BOlo
+
1
10 m
ǒ
370 * 1
350 * 1
* 1
Ǔ
+ 5731 W
R
BOup
+
370 * 350
10 m
+ 2.0 MW
Short Circuit or Over Load Protection:
A short circuit or an overload situation is detected when
the CS pin level reaching its maximum level at 1 V. In that
case the fault status is stored in the latch and allows the
digital timer count. If the digital timer ends then the fault is
latched and the controller permanently stops the pulses on
the driver pin.
If the fault is gone before ending the digital timer, the
timer is reset only after 3 switching controller periods
without fault detection (or when the CS pin < 1 V during at
least 3 switching periods).
If the fault is latched the controller can be reset if a BO
reset is sensed or if V
CC
is cycled down to V
CC(off)
. The fault
timer is typically set to 15 ms for A/B/C and D versions but
is extended to 150 ms for the E version.
CS pin
(500 mV/div)
12 Vout
(5 V/div)
Time
(4 ms/div)
Short Circuit
Figure 38. Short Circuit Detection Example
Fault timer: 15 ms
Shut Down
There is one possibility to shut down the controller; this
possibility consists of grounding the BO pin as illustrated in
Figure 37.
Slope Compensation
Slope compensation is a known mean to cure
subharmonic oscillations. These oscillations take place at
half of the switching frequency and occur only during
Continuous Conduction Mode (CCM) with a duty−cycle
close to and above 50%. To lower the current loop gain, one
usually injects between 50 and 100% of the inductor
downslope. Figure 39 depicts how internally the ramp is
generated:
The compensation is derived from the oscillator via a
buffer. A switch placed between the buffered internal
oscillator ramp and R
ramp
disconnects the compensation
ramp during the OFF time DRV signal.