NCP1397A, NCP1397B
R18
Figure 1. Typical Application Example
PIN FUNCTION DESCRIPTION
Pin # Pin Name
Function
1
CSS(dis)
Soft−Start Discharge
2
Fmax
Maximum frequency clamp
3
Ctimer
Timer duration
4
Rt
Minimum frequency clamp
5
BO
Brown−Out
6
FB
7
DT
8 Skip/Disable
Feedback
Deadtime
Skip or Disable input
9
Fault
Fault detection input
10
GND
11
Mlower
12
VCC
13
NC
14
HB
15
Mupper
16
Vboot
Analog ground
Low side output
Supplies the controller
Not connected
Half−bridge connection
High side output
Bootstrap pin
Pin Description
Soft−start capacitor discharge pin. Connect to the soft−start capacitor to reset it
before startup or during overload conditions.
A resistor sets the maximum frequency excursion
Sets the timer duration in presence of a fault
Connecting a resistor to this pin, sets the minimum oscillator frequency reached
for VFB = 1 V.
Detects low input voltage conditions. When brought above Vlatch (4 V typically), it
fully latches off the controller.
Injecting current into this pin increases the oscillation frequency up to Fmax.
A simple resistor adjusts the dead−time width
Upon release, a clean startup sequence occurs if VFB < 0.3 V. During the skip
mode, when FB doesn’t drop below 0.3 V, the IC restarts without soft−start
sequence.
When asserted, the external timer starts to countdown and shuts down the
controller at the end of its time duration. Simultaneously the Soft−Start discharge
switch is activated so the converter operating frequency goes up to protect
application power stage. This input features also second fault comparator with
higher threshold (1.5 V typically) that:
A) Speeds up the timer capacitor charging current 8 times – NCP1397A
B) latches off the IC permanently – NCP1397B
In both versions the second fault comparator helps to protect application in case
of short circuit on the output or transformer secondary winding.
−
Drives the lower side MOSFET
The controller accepts up to 20 V
Increases the creepage distance
Connects to the half−bridge output
Drives the higher side MOSFET
The floating VCC supply for the upper stage
http://onsemi.com
2