ESR stands for Equivalent Series Resistance.
A capacitor is not ideal — it behaves like it has a small resistor in series with its capacitance. This resistor is called ESR.
1. Definition
ESR capacitor = a capacitor with low equivalent series resistance, designed to supply or absorb high-frequency current spikes efficiently.
Mathematically: ESR = Resistive component in series with the capacitor
In circuit models: Capacitor → [ Ideal Capacitance C ] — [ ESR ] — [ Inductance ]
2. Why ESR Matters
When a capacitor is used for power supply smoothing, such as powering a SIMCom module:
- The module draws high peak current during transmission bursts (up to ~2A).
-
The ESR creates a voltage drop:
V_drop = I_peak × ESR
-
High ESR means more drop → poor voltage stability → possible module reboot.
I_peak = 2 A ESR = 0.5 Ω V_drop = 2 × 0.5 = 1 V drop
If VBAT is 4.2 V, a 1 V drop means the module sees 3.2 V → unstable operation.
3. Low ESR Capacitor
A low ESR capacitor has:
- Very small ESR (often < 0.1 Ω).
- Better performance for high-frequency pulses.
- Less heat generated under load.
Typical types:
| Type | ESR | Application |
|---|---|---|
| Ceramic MLCC | < 0.01 Ω | High frequency decoupling |
| Tantalum | 0.1 – 0.5 Ω | Bulk filtering in low-voltage |
| Low-ESR Electrolytic | 0.01 – 0.1 Ω | Bulk power supply smoothing |
4. Application in GSM/4G Modules
For GSM/4G power stability:
- Use low ESR capacitors close to VBAT pin.
-
Typical combination:
- 470 µF – 1000 µF low ESR electrolytic/tantalum
- 10 µF ceramic MLCC
- 0.1 µF ceramic MLCC