This design features an automotive backup battery system using a 1S Li battery for emergency operation.
The primary power input is rated for 6- to 18-V nominal (2.7- to 36-V peak) to comply with 12-V automotive requirements. The secondary power input is equipped with a fuel gauge to monitor a single cell Li battery and supports input voltages from 2 V to 4.35 V. The onboard learning load allows the easy implementation of an end-of-life detection for the backup battery.
Both power inputs are connected via ORing MOSFET switches to an 8.5-V boost converter, which can deliver up to 21.5-W continuous output power (30-W peak) with 2.5-V input voltage. At lower output currents, the input voltage of the boost converter can be as low as 2 V.
Due to the ORing MOSFET switches, this boost converter covers the emergency operation as well as cold crank situations, where the 12-V rail can drop down to 2.7 V at the primary input.
An ideal diode together with the bypass function of the boost stage minimizes losses during normal operation and allows low cold-crank voltages. Additionally, the ideal diode allows disconnecting the load on demand.
For ease of evaluation, an automated switchover between primary and secondary power input is implemented on this design.
The total power solution is completed by a linear battery charger for the Li battery, which can operate at input voltages as high as 18 V and withstand 40-V spikes during load dumps. The charger supports various cell chemistries.
The following images shown the top and bottom photos of the 4-layer PCB, FR4, with 35-µm copper.
Table 1-1 details the board connections, jumpers, and test points.
Connector | Description |
---|---|
J1 | Backup battery, positive terminal |
J2 | Backup battery, negative terminal |
J3 | Fuel gauge I²C interface (EV2400 connector) |
J4 | Fuel gauge NTC connector |
J5 | IO connector for status and control signals |
J6 | Battery charger NTC connector |
J7 | Battery charger voltage and timer selection |
J8 | Boost converter UVLO selection |
J9 | Positive output terminal |
J10 | Negative output terminal |
J11 | Boost converter SYNC/DITHER configuration |
J12 | Boost converter MODE selection |
J13 | Primary power input, positive terminal |
J14 | Primary power input, negative terminal |
J15 | Primary power input, voltage sense connector |
J16 | Automated switchover disconnect jumper |
TP1 | Frequency response analyzer connection, high impedance side |
TP2 | Frequency response analyzer connection, low impedance side |
TP3 | Frequency response analyzer connection, GND |
TP4 | Boost converter, tracking voltage probe point |
TP5 | Boost converter, analog GND |
TP6 | Ideal diode, always on voltage |
TP7 | Boost converter output voltage |
TP8 | GND |
TP9 | Primary power input voltage |
TP10 | Boost converter input voltage |
TP11 | GND |
Table 2-1 describes the pin functions of the individual connectors.
Connector | Pin | Description |
---|---|---|
J2 | 1 | GND |
2 | I²C – SCL | |
3 | I²C – SDA | |
4 | Vpu – 3.3-V pullup voltage for open drain IOs and I²C interface | |
J4 | 1 | 10-kΩ resistor – connect to pin 2 if no external NTC is used |
2 | BQ25171-Q1 NTC connection | |
3 | GND | |
J6 | 1 | 10-kΩ resistor – connect to pin 2 if no external NTC is used |
2 | BQ25171-Q1 NTC connection | |
3 | GND | |
J5 | 2 | BQ25171-Q1 Charge Enable. 10-k pulldown – pull high to disable charger |
4 | BQ34210-Q1 Alert output. 10-k pullup to Vpu | |
6 | Learning load EN. FET input. 10-k pulldown – pullup to enable learning load | |
8 | BQ25171-Q1 STAT2 output. 22-k pullup to Vpu | |
10 | BQ25171-Q1 STAT1 output. 22-k pullup to Vpu | |
12 | LM5152-Q1 enable. Pull high to enable boost converter | |
14 | LM5152-Q1 status output. 22k pullup to Vpu | |
18 | LM74800-Q1 enable input. Pull high to connect main power input to boost converter | |
20 | Backup battery enable. 4.7-k pullup to Vpu. Pull low for normal operation, pull high or leave open with applied pullup voltage for backup battery operation | |
All others | No connection | |
J7 | All uneven | GND |
20 | Connect to GND to disable charge timer | |
22 | Connect to GND to set charge timer to 10 h | |
24 | Connect to GND to set charge timer to 5 h | |
2 | Connect to GND to set charging voltage to 3.5 V | |
4 | Connect to GND to set charging voltage to 3.6 V | |
6 | Connect to GND to set charging voltage to 3.7 V | |
8 | Connect to GND to set charging voltage to 3.8 V | |
10 | Connect to GND to set charging voltage to 3.9 V | |
12 | Connect to GND to set charging voltage to 4.05 V | |
14 | Connect to GND to set charging voltage to 4.1 V | |
16 | Connect to GND to set charging voltage to 4.2 V | |
18 | Connect to GND to set charging voltage to 4.35 V | |
J12 | 1 | LM5152-Q1 internal VCC. For frequency response analysis, connect 5.5 V to this pin and pin 2 and populate R37 with 49.9 Ω. For normal operation, remove external voltage source and populate R37 with 0 Ω. |
2 | LM5152-Q1 MODE pin. Connect to pin 1 for FPWM operation, to pin 3 for Diode Emulation or leave open for skip mode | |
3 | LM5152-Q1 AGND | |
J15 | 1 | GND |
2 | Voltage sense output of resistive voltage divider. Divider can be disconnected by LM74800-Q1 to avoid leakage current. |
The switchover also occurs when the ideal diode is disabled by asserting the control input low.
The input voltage of the boost converter can be measured at TP10.
When powering down the board, remove the 12-V input first.
Power up the board:
When all voltages are applied, the ideal diode enable must be pulled high to deliver power to the boost converter.
Now the boost converter and the battery charger can be enabled or disabled using the corresponding control inputs. Switchover between the primary and the secondary power input happens automatically when the voltage at the primary power input drops below approximately 3 V. When the voltage rises above approximately 4.6 V, the boost converter is connected to the primary power input again.