1. Set heating frequency with UCC28C43-Q1: R1>5K, arbitrarily obtain R1=20K if frequency is 1KHz. Then C1 = 0.1µF.
   Equation 2. ${\mathit{F}}_{\mathit{H}\mathit{E}\mathit{A}\mathit{T}\mathit{I}\mathit{N}\mathit{G}}\approx \frac{1.72}{\mathit{R}1\times \mathit{C}1}$
2. Configure EA of UCC28C43-Q1. According to internal EA structure in UCC28C43-Q1, this is standard inverse amplifier with 2.5V biased, we can configure UCC28C43-Q1 EA as simplest 1x inverse amplifier (R9 and R8), since there is 2.5V biased reference voltage on the non-inverting input port, so the final calculation equation can be changed to:
   Equation 3. ${\mathit{V}}_{\mathit{C}\mathit{O}\mathit{M}\mathit{P}\_\mathit{U}\mathit{C}2843\mathit{L}}={\mathit{V}}_{\mathit{R}\mathit{E}\mathit{F}}\times \left(1+\frac{\mathit{R}8}{\mathit{R}9}\right)-{\mathit{V}}_{\mathit{F}\mathit{B}\_\mathit{U}\mathit{C}2843\mathit{L}}\times \frac{\mathit{R}8}{\mathit{R}9}$
3. Arbitrarily get R8=R9=10K
   Equation 4. ${\mathit{V}}_{\mathit{C}\mathit{O}\mathit{M}\mathit{P}\_\mathit{U}\mathit{C}2843\mathit{L}}=2\times {\mathit{V}}_{\mathit{R}\mathit{E}\mathit{F}}-{\mathit{V}}_{\mathit{F}\mathit{B}\_\mathit{U}\mathit{C}2843\mathit{L}}$
4. Estimate duty cycle for heating.
   Equation 5. ${\mathit{D}\mathit{u}\mathit{t}\mathit{y}}_{\mathit{H}\mathit{e}\mathit{a}\mathit{t}\mathit{i}\mathit{n}\mathit{g}}=\frac{{\mathit{V}}_{\mathit{C}\mathit{O}\mathit{M}\mathit{P}}}{{\mathit{V}}_{\mathit{C}\mathit{S}}}=\frac{{(\mathit{V}}_{\mathit{C}\mathit{O}\mathit{M}\mathit{P}\_\mathit{U}\mathit{C}2843\mathit{L}}-1.4)/3}{{\mathit{V}}_{\mathit{S}\mathit{A}\mathit{W}}\times \mathit{R}2/(\mathit{R}6+\mathit{R}2)}=\frac{(2\times {\mathit{V}}_{\mathit{R}\mathit{E}\mathit{F}}-{\mathit{V}}_{\mathit{F}\mathit{B}}-1.4)/3}{{\mathit{V}}_{\mathit{S}\mathit{A}\mathit{W}}\times \mathit{R}2/(\mathit{R}6+\mathit{R}2)}$
   Here:
   • V_SAW=1.72V
   • V_REF means the voltage reference in UCC28C43-Q1, this is 2.5V
   • 1.4V means the forward voltage of 2 diodes that in UCC28C43-Q1 EA unit
   • 1/3 is coefficient for resistor divider R/(2R+R) in UCC28C43-Q1 EA unit

   From Equation 5when VFB_UC2843L increases, duty cycle decreases accordingly, or when temperature increases, duty cycle decreases accordingly.

5. Set heating and stop heating threshold with ISOTMP35-Q1
   • Heating threshold: < -30℃, output voltage from ISOTMP35-Q1=200mV
   • Stop heating threshold: > -15℃, analog output voltage from ISOTMP35-Q1=350mV
6. Set amplify coefficient of OPA333-Q1, or R3, R4 setting according to the duty cycle equation.

Here:

• V_SAW = 1.72V
• Arbitrarily get R6=R2=1K
• V_REF means the voltage reference in UCC28C43-Q1, this is 2.5V
• 1.4V means the forward voltage of 2 diodes that in UCC28C43-Q1 EA unit
• 1/3 is coefficient for resistor divider R/(2R+R) in UCC28C43-Q1 EA unit

When Duty cycle is 1, 0, then V_{FB_UC2843L} is 1.02V and 3.6V respectively

• For stop heating, the threshold is -15℃ or 350mV, the amplified ratio for OPA333-Q1 is: Stop heating means duty cycle is 0, so VFB at least needs to be more than 3.6V, according to Equation 2, or 3.6V/0.35V= 10.3, this means if ratio is more than 10, this can trigger stop heating threshold.
• For start heating, the threshold is -30℃ or 200mV, the amplified ratio for OPA333-Q1 is: Start heating means duty cycle is more than 0, VFB needs to be less than 3.6V, according to Equation 2, 3.6V/0.2V=18, this means if ratio is less than 18, this can trigger start heating threshold.

Consider both stop and start amplified ratio data, 10 is an option for this design, but stop threshold can be slightly higher than expected, according to Equation 2, arbitrarily get R3=9K, so R4=1K.

Here:

1. Signal Lo-T to Hi-T source is used to simulate the analog output of ISOTMP35-Q1.
2. IO__PTC means the current that flows into PTC, or heating current.
3. PTC_PWM means PWM driving signal to PTC_MOSFET, or Q2.

Notes:

1. For R and C component in this design, the component needs to meet AEQ grade for automotive field.
2. For 48V EV system, please use buck LMR38010FSQDDARQ1, maximum input voltage is high up to 80V, provide fixed 12V power rail to UCC28C43-Q1, LDO TPS71550QDCKRM3Q1 for fixed 5V power rail (from 12V power rail of buck) for OPA333-Q1 and ISOTMP45-Q1
3. For 12V EV system, since the input range can be 9V to 16V, this needs buck-boost to generate 12V, TPS55160QPWPRQ1 can meet target application, LDO TPS71550QDCKRM3Q1 for fixed 5V from 12V bus directly.
4. If designers need more features, such as power on or power off, temperature hysteresis, PTC SCP or open detection, please refer to the note description in simulation schematic.

Simulation Waveform: -35℃ - -20℃, or 150mV - 300mV, start heating to continue heating

Simulation Waveform: -35℃ - -12℃, or 150mV - 380mV, continue heating to stop heating

Simulation Waveform: -35℃ - -12℃, or 150mV - 300mV, heating to PTC SCP