3.3.2.1
VCC soft start
At VCC generation switch on, VCC is typically decoupled by a 100 to 200 µF capacitor on the
If the regulator is switched on abruptly, an inrush current is generated that cannot be sustained by the upfront DC-
DC converter, which then enters current limitation.
Since the +20 V is generated by cas5 V and 15 V, the current limitation also impacts the +5 V supply
(hence the MCU).
When the MCU supply drops below the
PowerOnReset
threshold, the MCU resets and the board reboots.
To avoid this behavior, the linear regulator U8 is switched on via a
soft start
using a PWM enable signal (which
limits the current on the upfront DC-DC).
When VCC has reached a stable value (that is, the VCC capacitor is charged), the enable signal remains in the
steady-state condition (always on, so always low).
Soft start phase usually lasts about 120 ms.
Figure 5.
VCC ramp-up typical waveform
3.3.2.2
NVM programming
The
interface board provides a VCC voltage to the target device that is high enough for an
NVM programming operation.
programming requirements are +18 V and 35 mA max. current.
If the VCC on the target device is < 17 V, the programming VCC can be simply connected to the target VCC
through a couple of OR-ing diodes.
The 19 V supply current delivered is limited to 100 mA by the on-board LDO (U8).
UM2342
Power supply
UM2342
-
Rev 1
page 6/57
