Keywords: How to use SSD1928, cmos camera interface to SSD1928,
PIC32 interface SSD1928, cmos camera interface to MCU
TechToys Company @ www.TechToys.com.hk
AN020513
Document Rev 02052013
Author John Leung
Page 4
Two power paths are possible. Readers are advised to look at the schematic for reference.
They are 5V DC with pin positive for development or a 3V CR123A lithium battery (at
BT500) for mobile applications. When 5V DC is applied at J500, the connection via R500 (1k)
pulls up the gate of Q500 (IRLML6402TR) to switch it off. This leaves only the forward
diode voltage at source pin of Q500 to source pin of Q504. This voltage at 3V (minus 0.2V)
meets 3.3V from KIA1117-3.3V regulator output therefore reverse biasing the diode built-in
Q500. As a result no battery drains when external power applied at J500.
When a 3V CR123A lithium battery is used, MOSFET (Q504: IRLML6402TR) is used to cut
off (or connect) the battery from the whole circuit. This ensures long battery life when the
device is left idle with battery inserted.
The switching action is explained as follows.
•
When switch SW500 is released, Q501 (MMBT3904) is switched off. Resistor R502
pulls gate and source of Q504 to the same voltage level. MOSFET Q504 is switched
off therefore cutting off current from the battery.
•
When SW500 is pressed and held, transistor Q501 (MMBT3904) conducts, pulling
gate of Q504 low to turn it on. Battery power drains from the source pin of Q500
and conducts via Q504 with a resistance as low as 0.065Ω. Q500 is used as a reverse
polarity protection device when the battery was inserted in wrong polarity out of
mistake. If it was not a development kit, Q500 could be omitted.
•
Now 3V is present at the inductor end of L500. DC-DC converter XC9104 switches
3V to 3.3V to supply the whole circuit including the MCU.
•
MCU boots up from POR. Software in MCU sets PWR_CTRL pin an output high to
turn on Q502, pulling gate of Q504 low to maintain its state. After this, the user may
release SW500 because the state of Q504’s gate is now maintained by software.
•
Power cut-off is possible with PWR_CTRL switched low after a press-and-hold on
SW500 like most mobile phones and digital cameras.
SW500
