While blocking diodes are easy to understand and apply, their forward drop results in significant power dissipation, making them unsuitable in both low voltage and high current applications. In low voltage applications, the forward voltage drop becomes a limiting factor for a circuit’s operating range, even when using a Schottky barrier diode. At least 500mV of supply headroom is lost across a series diode—a substantial degradation in 12V automotive systems where the supply can drop to as low as 4V during cold crank.
Since diodes operate at a fixed voltage drop of 400mV to 700mV minimum, regardless of current rating, power dissipation becomes an issue in the 1A–2A range, for surface mount applications. In applications greater than 5A, power dissipation becomes a major issue, requiring elaborate thermal layouts or costly heat sinks to keep the diode cool. Circuit designers need a better solution.
One solution is to replace diodes with MOSFET switches. The MOSFET is connected so that its body diode points in the same direction as the diode it replaces, but during forward conduction the MOSFET is turned on, shorting the body diode with a low loss path through the MOSFET channel. When the current reverses, the MOSFET is turned off, and the body diode blocks the flow of current, thus maintaining the diode behavior. The forward drop and power dissipation are reduced by as much as a factor of 10. This forms the basis of an “ideal” diode, when compared to conventional p-n or Schottky barrier diodes.
The LTC4357 and LTC4359 are ideal diode controllers, designed to drive N-channel MOSFETs in a wide variety of power supply reverse blocking, ORing and holdup applications. MOSFETs with RDS(ON) specifications as low as 1mΩ are readily available, so ideal diodes can be built to handle currents in excess of 50A using a single pass device while maintaining voltage and power loss levels 10 times better than any diode solution.
The LTC4357 and LTC4359 both replace a diode, but the latter has a wider operating range down to 4V and its quiescent current is a quarter of the former. The LTC4359’s /SHDN pin reduces the quiescent current and turns the LTC4359 solution into a load switch, a feature the LTC4357 and diode solutions do not have. Table 1 highlights the features of the LTC4357 and LTC4359.
The LTC4359 is a low quiescent current controller with a wide operating range of 4V–80V. The 4V end of the operating range is particularly important in low voltage applications, where the diode drop is not tolerable, while the 80V rating allows it to operate and survive transients in 48V telecom systems and automotive environments. The LTC4359 protects downstream circuitry from reverse inputs down to −40V, seen when battery terminals are misconnected.