AN2946 datasheet(11/38 Pages) STMICROELECTRONICS

11 / 38 page

AN2946

Hardware design

Doc ID 15473 Rev 2

11/38

Figure 8.

Noise filtering circuit for VDD and VDDA

3.1.2

Electricity power collection

In Figure 9, C1, and C4//C5//C6 are used to reject the high switching frequency interference

from the charger so that only the "clean" current flows through the solar cells (P1). When

photons hit the solar cells, P1 releases electrons which flow along the metal contacts and

stores electricity to C1 and C4/C5/C6 through R1//R2//R3//R4 and Q1. R1~R4 are current

sense resistors which are used to sense the solar module current. An operational amplifier

U1 (LM258D) is used to amplify and smooth the sense signal, then feedback to the MCU.

When solar cells charge the battery with high current, Q1 is turned on in order to minimize

the power losses. Q1 is turned off if solar cells voltage falls below the battery voltage. Q1

also works as a polarity protection diode, preventing that the solar module is reversely

connected. The gate driving signal (PWM_Input) of Q1 is given by the MCU through U4. In

order to properly drive Q1, the 3.3 V PWM signal from the MCU must be sent to U4

(comparator TS391). Q11 and Q13 are configured as the push-pull totem to turn on/off Q1

perfectly.

Figure 9.

Solar module control circuit

A hardware solution to protect the battery from being overcharged is important. When

battery voltage exceeds 15 V (example of 12 V battery in system), D3 in Figure 10 is

triggered and SCR (Q10) is turned on. The battery provides latch-current to Q10 and the

fuse (F1 in Figure 11) is blown. Then battery is protected.

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