LOAD SWITCHES
Load Switches
| Date Published: | |
| Last Modified: |
MOSFET Based
The following image shows a MOSFET based high-side switch:
BJT Current Sink Driving P-Channel MOSFET Load Switch
A simple resistor divider can be used to provide the correct \(V_{GS}\) to turn on a P-channel MOSFET based load switch, however that only works well if the \(V_{IN}\) is known and stays at fixed voltage. If it doesn’t, then the resistor divider provides a varying \(V_{GS}\), which could either turn the switch the MOSFET off at lower input voltages, or exceed \(V_{GS(max)}\) at higher input voltages (\(V_{GS(max)} = \pm 20V\) for most MOSFETs).
A better option in this case is to use a BJT current sink to set the desired \(V_{GS}\) across a resistor, as shown in the following diagram:
Rather than using a simple resistor divider to provide the necessary gate-source voltage to turn on a P-channel load switch, you can use a BJT current sink, which has the added benefit of providing a constant Vgs over a wide range of input voltages.
We assume the BJT (\(Q_1\)) is switched with \(+3.3V\) coming from a microcontroller or similar. The BJT is configured to be a simple current sink, with the current given by:
\begin{align} I_C &= \frac{V_B - 0.7V}{R_E} \\ &= \frac{3.3V - 0.7V}{2.7k\Omega} \\ &= 1mA \end{align}
This current goes through \(R_1\), which provides the necessary \(V_{GS}\) to turn the P-channel MOSFET (\(Q_2\)) on:
\begin{align} V_{GS} &= -I \cdot R_1 \\ &= -1mA \cdot 10k\Omega \\ &= -10V \end{align}
\(R_G\) is added as good standard practise to limit gate current and gate voltages. In the above example, \(V_{IN}\) can vary from approx. 11V right up to the maximum allowed drain-source or collector-emitter voltages (for example, \(48V\)), whilst keeping \(V_{GS} = -10V\).
IC Based
The following image shows an IC based high-side switch.
Some load-switches have reverse-polarity protection. More information of how they exactly implement reverse-protection with only the one MOSFET can be found in the The Substrate (Body) Connection section of the MOSFET page.
