HT1635A/B Wearable Sports Bracelet LED Display Application
11 / 19
AN0393E
Figure 10 Single RGB LED Driving Method
Each HT1635A or HT1635B device on the LED display board drives a 8×14 pattern RGB
LEDs (8 commons and 42 rows). Two devices combine to drive a 8×28 pattern RGB LED
matrix. This demo uses Everlight’s 19-237B RGB SMD LED, which has four pins and is a
common cathode type. A single LED size is 1.6mm×1.6mm, which is the equal to two
0603 SMD resistors. The LED-R’s V
F
is about 1.7V~2.2V, the LED-G’s and LED-B’s V
F
is
about 2.6~3.3V and the maximum I
F
is 25mA. In order to obtain a higher LED brightness
level, the HT1635A/HT1635B operating voltage is set to 5V,
a 150Ω resistor is connected
between the ROW I/O and the LED-R
anode, a 100Ω resistor is connected between
the
ROW I/O and the LED-G/LED-B anode. For those LEDs that are controlled by the same
COM, their cathodes are connected together to a NMOS (AO3400) drain terminal. The
HT1635A/HT1635B COM pins are set as PMOS open-drain outputs to control the
external NMOS on/off switching to provide a stronger COM driving capacity. When all
LEDs are on, the operating current will exceed 1A. If the COM pins are set as NMOS
open-drain outputs, a high current will directly flow into the HT1635A/HT1635B, which will
generate a device over temperature condition, which could cause damaged. To avoid
unexpected damage, this driving method is not suggested.
This demo uses a COM PMOS open-drain driving method together with an external
NMOS to drive the common cathode LED matrix. Since the COM pins are set as PMOS
open-drain outputs, when writing a logic “1” to these pins, their integrated PMOS will be
on and they will output a high level (LEDVDD). When writing logic “0” to them, their
integrated PMOS will be off and they will be in the high impedance state. Usually,
connecting a pull-low resistor in parallel between the external NMOS Gate and Source
terminals can provide a dissipation path for the NMOS source charge stored during the
COM pin high level output, by which the NMOS can be turned off. The part number for all
NMOS components
on the demo is AO3400. Connecting a 1K resistor in parallel between
the gate and source terminals can achieve normal NMOS on/off control.
Touch Key Detection Circuit
Figure 11 Touch Key Circuit
As shown in the above figure, if the host MCU key detection I/O’s internal pull-high resistor
is enabled, when the key is pressed, the detection I/O will be pulled-low. D1 is an ESD
protection component. This key is used to switch the RGB LED Matrix display functions.
LED-R
LED-G
LED-B
R1
R2
R3
R
1
G
1
B
1
10R
R85
1K
R86
NC
R87
COM1-1
COM1
Q1
AO3400
KEY
KEY
D1
TVS/SMAJ6.0CA
102
C11
