A question about motor driver ICs - Page 1
Forum section: electronics / beginners

author: Prithul0218
author_rank: Contributor
timestamp: January 08, 2019, 04:26:42 am
content: A typical h-bridge DC motor driver IC has two voltage pins. One is the logic voltage and the other is the voltage used to power the motors. Like - L293, L298, TB6612.Why do they need the voltage for motor to always be slightly (2-3V) higher than the logic voltage?I've attached datasheet for the L298 below.http://cloud.tapatalk.com/s/5c34267fca44c/L298_H_Bridge.pdf

author: spec
author_rank: Frequent Contributor
timestamp: January 08, 2019, 09:50:19 am
content: Hi Prithul0218Quote from: Prithul0218 on January 08, 2019, 04:26:42 amThere is no absolute reason why the two voltages cannot be independent. It is just that the designers probably found the chip design easier with the +2V3 stipulation.

author: Ian.M
author_rank: Super Contributor
timestamp: January 08, 2019, 10:08:02 am
content: Back in the old days before LiPOs there wasn't much demand for motor drivers with Vmotor <= Vlogic. If you needed one, then it was easy enough to build with discrete transistors or MOSFETs directly driven from the logic signals. Also the high On state saturation voltage of bipolar motor driver ICs made low motor supply voltage operation very inefficient.In recent years, this has changed considerably due to cellphones and other gadgets that operate of a single LiPO cell. However the new MOSFET motor driver IC for this market usually don't support high motor voltages. For many, 5V is more than their maximum!

author: Prithul0218
author_rank: Contributor
timestamp: January 08, 2019, 10:28:11 am
content: Quote from: spec on January 08, 2019, 09:50:19 amSo are there any motor driver ICs that can easily drive motors at lower voltage than logic voltage? I'm learning robotics and plan on using tiny 3V toy motors. My microcontroller runs on 5V logic.

author: Ian.M
author_rank: Super Contributor
timestamp: January 08, 2019, 10:39:11 am
content: Google:low voltage H-bridgethis one looks interesting:http://www.ti.com/lit/ds/symlink/drv8837.pdf

author: Prithul0218
author_rank: Contributor
timestamp: January 08, 2019, 10:50:46 am
content: Quote from: Ian.M on January 08, 2019, 10:39:11 amAnother little question. If it uses MOSFET, and MOSFETs usually have <100 milliohm on resistance, why does most of the motor driver have like 500m ohm resistance?

author: Ian.M
author_rank: Super Contributor
timestamp: January 08, 2019, 11:10:52 am
content: That one has 330mΩ max. for both high side and opposite low side in series (at VM = 5 V; VCC = 3 V), so well under the 500mΩ you claim is common.MOSFET on resistance depends on channel geometry, gate oxide thickness and gate drive voltage. In general, physically larger MOSFETS, driven harder have lower on resistance. However that costs more silicon die area and may be incompatible with the low threshold voltage (typ. less than Vlogic/2) requirement for direct logic drive. Also it does't take much power to run a cellphone vibrator motor so low on resistance isn't a priority in the main market for low voltage motor drivers.

author: exe
author_rank: Unknown
timestamp: January 08, 2019, 01:25:41 pm
content: Quote from: Ian.M on January 08, 2019, 11:10:52 amYeah, it's not that easy to find a motor driver that wouldn't be a heater. I'm pretty sure there are much better drivers, but can't find any atm. May be there are solutions with external fets and a charge pump?

author: Prithul0218
author_rank: Contributor
timestamp: January 10, 2019, 10:26:37 am
content: Quote from: exe on January 08, 2019, 01:25:41 pmTake a look at the VMH5019. It has ~18mΩ on resistance. But it can't drive low voltage motors.Sent from my Redmi 5 using Tapatalk