Driver for 7 segment led display.
In one of product we are using MSP430F149 as main controller. This
product has 4 row 4 digit [ total 16 digits ] 7 segment LED
display. The LED displays are common cathode type and we use
multiplexed driving. The displays are RED Led type with Vf=1.7V
and require 4.5mA each segment. The segments are driven (sourced)
using SN74LS245 and the common cathodes of each digit goes to
SN74LS145(sinking); Multiplexing is done by varying input of
74LS145. SN74LS245 and LS145 are powered using 5V supplies.
Inputs of 74LS245 as well as 74LS145 are directly connected to
MSP430F149 port pins. Now the problem is 74LS145 requires very
large quiscent current around 60mA we intend to reduce this power
consumption at the same time we dont want to compromise in qulaity.
For achieving this,
1. Can we eliminate 74LS245 all together and drive the segments of
LED displays directly using MSP430F149 port pins will it be able to
source 4.5mA per port pin without any harm.
2. Can we use 74HCT541 in this application since quiscent current of
HCT series is much lower. Will HC541 be able to source 4.5mA.
3. Is there any alternate solution ?
PROBLEM - 2
This is regarding another product this product too uses MSP430F149
and has 16 digit multiplexed LED display like above. But in this
case the Display segements are TWO LEDs IN SERIES with Vf=3.4V and
requires 13mA current each segment. In this case MSP430F149 port
pins cant be used for sure since Vf is too high. We have come up
with a transisitor solution which is attached with this mail please
have a look and do let us know How robust it is ? Can we use any
other alternate in this case ?