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NCP1252ADR2G

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NCP1252
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13
Time
Time
Time
Time
Time
BO pin
Time
SS pin
Time
DRV pin
Time
CASE #3 CASE #4
SS capacitor is
discharged
Soft start
Figure 34. Controller Shuts Down with the Brown Out Pin
V
BO
V
CC(on)
V
CC
pin
BO pin
SS pin
DRV pin
V
BO
V
CC(on)
V
CC
pin
When the BO pin is grounded, the controller is shut down
and the SS pin is internally grounded in order to discharge
the soft start capacitor connected to this pin (Case #3). If the
BO pin is released, when its level reaches the V
BO
level a
new soft start sequence happens.
Soft Start:
As illustrated by the following figure, the rising voltage on
the SS pin voltage divided by 4 controls the peak current
sensed on the CS pin. Thus as soon as the CS pin voltage
becomes higher than the SS pin voltage divided by 4 the
driver latch is reset.
LEB
CS
Rsense
S
R
Q
Rcomp
Clock
UVLO
Grand Reset
SS
Iss
Vdd
Fixed
Delay
120 ms
Soft start
+
−
Soft Start
Status
DRV
1/4
Figure 35. Soft Start Principle
Q
NCP1252
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14
The following figure illustrates a soft start sequence.
Soft Start pin
(2 V/div)
CS pin
(0.5 V/div)
Time
(4 ms/div)
Figure 36. Soft Start Example
V
SS
= 4 V
T
SS
= 13 ms
Brown−Out Protection
By monitoring the level on BO pin, the controller protects
the forward converter against low input voltage conditions.
When the BO pin level falls below the V
BO
level, the
controllers stops pulsing until the input level goes back to
normal and resumes the operation via a new soft start sequence.
The brown−out comparator features a fixed voltage
reference level (V
BO
). The hysteresis is implemented by
using the internal current connected between the BO pin and
the ground when the BO pin is below the internal voltage
reference (V
BO
).
BO
S
R
Q
shutdown
Vbulk
BOK
UVLO reset
Grand
Reset
+
−
VBO
IBO
RBOup
RBOlo
Figure 37. BO Pin Setup
Q
The following equations show how to calculate the
resistors for BO pin.
First of all, select the bulk voltage value at which the
controller must start switching (V
bulkon
) and the bulk
voltage for shutdown (V
bulkoff
) the controller.
Where:
• V
bulkon
= 370 V
• V
bulkoff
= 350 V
• V
BO
= 1 V
• I
BO
= 10 mA
When BO pin voltage is below V
BO
(internal voltage
reference), the internal current source (I
BO
) is activated. The
following equation can be written:
V
bulkON
+ R
BOup
ǒ
I
BO
)
V
BO
R
BOlo
Ǔ
) V
BO
(eq. 1)
When BO pin voltage is higher than V
BO
, the internal
current source is now disabled. The following equation can
be written:
V
BO
+
V
bulkoff
R
BOlo
R
BOlo
) R
BOup
(eq. 2)
From Equation 2 it can be extracted the R
BOup
:
R
BOup
+
ǒ
V
bulkoff
* V
BO
V
BO
Ǔ
R
BOlo
(eq. 3)
Equation 3 is substituted in Equation 1 and solved for
R
BOlo
, yields:
NCP1252
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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.
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NCP1252ADR2G

Mfr. #:
Buy NCP1252ADR2G
Manufacturer:
ON Semiconductor
Description:
Switching Controllers CURR MDE PWM CNTRLR FWD FLYBCK APPS
Lifecycle:
New from this manufacturer.
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