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FAQs

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What function has the case pin?

The case pin is an electrical connection to the metal case or baseplate. It can be used to improve the EMC performance in certain applications by tying the case to ground or to Vin+. If not used, it should be left open.

What function have the sense pins?

The sense pins (Sense + and Sense-) are used by the DC/DC converter to regulate the output voltage as delivered to the load, not as measure directly across the output pins. The converter uses four connections, Vout+ and Vout- delivering a high current and two low current connections, Sense+ and Sense- for feedback.

The advantage of using the sense pins compared with simply trimming up the output voltage to compensate for the volt-drop along the connection is that sense pins regulate the load voltage at both low and high load currents, thus avoiding too high a load voltage when lightly loaded.

Sense pins can also be used by some load sharing controllers to allow two or more DC/DC converters to be connected in parallel to increase the power.

Can the output voltage of a switching regulator be increased by adding a resistor to the GND connection?

No. Switching regulators function differently than linear regulators and this “trick” does not work. They need a very good ground connection to function properly.

Can the output voltage be trimmed by an external voltage or current rather than by a resistor?

In theory, yes, but in practice it is very non-linear. It is not really recommended.

What function has the trim pin?

The trim pin (if fitted) can be used to increase or decrease the regulated output voltage over a limited range (typically +/-10% or -20%, +10%).  Some switching regulators offer a Vadj. pin which can adjust the output voltage over a wider range (up to +/-50%).

Connecting a resistor between trim (or Vadj,) and the Vout+ pin will reduce the output voltage. Connecting a resistor between trim (or Vadj,) and the Vout- or Gnd pin will increase the output voltage.

Trimming is typically used to help compensate for the voltage drop along a long cable or PCB track by increasing the output voltage, or to reduce the output voltage to avoid over-voltage stress on the load under worst-case conditions. Voltage trim is also useful to match different battery chemistries. A 12V lead acid battery can be trickle charged using a 12V converter trimmed up to 13.2V (+10%) or a LiPo Battery safely charged from a 5V converter trimmed down to 4.6V (-8%).

What is the maximum control pin voltage?

The maximum allowed control pin voltage varies from converter series to converter series. Most DC/DC converters allow up to 5V and some up to 12V or more.  Refer to the datasheets for guidance. Do not connect an unused control pin to +Vin unless this is expressly permitted in the datasheet.

What is multi-level control pin function?

Some RECOM converters, like the R-78AA series, have a two level control pin function. If the control pin voltage drops below 2.6V, the main power stage is switched off, but the internal oscillator and voltage regulator is kept running. This allows a very rapid restart from standby to full power. For ultra-low power mode, the control pin voltage must be below 1.6V. Then the main oscillator is also turned off and the converter draws only 20µA from the input. Start-up will be slower from deep sleep than from standby.

What is the difference between quiescent or standby current, shutdown current and control pin current?

Quiescent or standby current is the current drawn by the converter from the supply when it is unloaded (the converter is active and has an output voltage, but no output current). The shutdown current is the residual current drawn by the converter from the supply while it is disabled by using the control pin. The control pin current is the current drawn by converter through the control pin in order to keep it in the disabled state.

What is control pin voltage hysteresis?

When a control pin voltage is rising, the switching point (threshold) is higher than when the voltage is falling. The difference between the rising voltage trigger point and the falling voltage trigger point is the hysteresis. For example, a converter with negative control logic could switch on as soon as the control pin voltage exceeds 3V but once started, only switches off again when the voltage drops below 1V. The 2V difference is the hysteresis and stops the converter from switching on-and-off erratically with a slowly rising or falling control voltage.

What is positive or negative control pin logic?

Negative control logic means that logic 0 (low) enables the converter and logic 1 (high) disables the converter. If the pin is left unconnected, it will be logic 0 and the converter will start up as soon as power is applied.

Positive control logic means that logic 0 (low) disables the converter and logic 1 (high) enables the converter. If the pin is left unconnected, it will be logic 0 and the converter will not start up when power is applied, but wait for a positive signal before starting. For many safety-critical systems, this is an important feature.

What function has the CTRL pin?

The Remote On/Off or Control pin is commonly used for the following reasons:

  1. To control a high-power converter using a low-power signal. The input power of a control pin is typically only a few milliwatts, but it can enable or disable a converter of as much as a hundred watts. This means that the low power output of a microcontroller or logic IC can be used to control a system without the need of extra amplifiers or bulky relays.

  2. To power-up or power-down a system of several converters in the right sequence. Many complex power supplies need to start up or shut down in a certain order to be safe. An example could be a computer-based controller where the microprocessor should be up and running before the peripherals are powered up.  Another example is where one power supply feeds another. The primary power supply often should have reached a stable output voltage before the secondary power supplies are turned on.

  3. To save energy. A control pin can be used to turn off the power completely to parts of a circuit during standby mode, while leaving a central watchdog circuit still active. This is especially important for battery powered circuits because all DC/DC converters draw some power even when they are not loaded.

  4. To reduce the inrush current. In a system of several parallel sub-systems, it is often useful to stagger the start-up of each sub-system so as not to overload the primary power supply or cause the main fuse to trip or blow on switch-on.

What happens if I use an unbalanced load on a dual output converter?

Some unregulated converters feature power sharing, where all or some of the load can be taken from just one output pin.

Regulated dual output converters regulate the difference between Vout+ and Vout- and allow the common pin to float. So if a +/-15V is asymmetrically loaded with, say +80%, -20%, then the output voltage difference will stay 30V, but the common pin will drift so that the output voltage will measure +13, -17V. If a balanced output is required with unbalanced load, then use post-regulation to stabilize the outputs.

What are the output options of DC/DC converters?

The main output options are:

  1. Single output, with Vout+ and Vout- pins. This is the most commonly used option.

  2. Dual (bipolar) output, with Vout+, Com and Vout- pins, e.g. +/-15V. Useful for generating bipolar supply rails from a single input voltage rail, e.g, to supply an op-amp.

  3. Dual (asymmetric) output , with Vout+, Com and Vout- pins, e.g. +18, -9V. Useful for IGBT driver applications which uses asymmetric supply voltages.

  4. Dual (independent) output, with Vout1+, Vout1- and Vout2+, Vout2-, where the outputs are isolated both from the input and from each other. Useful for supplying a two-channel application using only one converter.

  5. Triple output, with a main Vout+ and  Aux+, Com and Aux- pins, e.g. +5V and +/-12V. Useful for applications requiring a single high current supply and an auxiliary supply to power peripherals.

Does it matter of the input is positive or negative voltage compared to the output voltage?

An isolated DC/DC converter has no electrical connection between the input and output. So it does not matter if Vin+ is connected to a positive supply and Vin- is connected to ground or if Vin+ was connected to ground and Vin- was connected to a negative supply. This is useful in the telecommunications industry, for example, where a standard -48V supply can be used to generate a +5V output (Vin+ = ground, Vin- = -48V, Vout + = 5V, Vout- = ground).

This does not apply to non-isolated switching regulators, but the R-78 series can be configured to generate a negative output voltage from a positive input voltage (refer to the application notes).

What happens if I connect the Vin+ and Vin- the wrong way around?

DC/DC converters are not reverse polarity protected. They will be irreparably damaged if connected the wrong way around. If it is possible or likely that the converter could be reverse polarity connected, then a diode must be used to protect the converter (refer to the application notes).

What is the difference between a 1:1, 2:1 and 4:1 DC/DC converter?

The ratio refers to the input voltage range. A 24V input DC/DC converter with 1:1 input range is specified with 24Vinput +/-10% (21.6V to 26.4V). A 24V input DC/DC converter with 2:1 input range is specified over a two-to-one input voltage range of 18V-36V and a 24V input DC/DC converter with 4:1 input range is specified over a four-to-one input voltage range of 9V-36V.

What pin functions does an isolated DC/DC converter have?

A DC/DC converter can have the following connections:

  1. The supply pins, Vin+ and Vin –
  2. The output pins, Vout + and Vout- (plus a common pin for +/- outputs)
  3. A remote on/off, enable or control pin
  4. A trim pin
  5. Sense Pins
  6. Case pin
What is the function of an isolated DC/DC converter?

An isolated DC/DC converter will translate a DC input voltage to the same or a different DC output voltage which is electrically isolated from the input via an internal transformer. It is commonly used for the following reasons:

  1. To match the load to the input. For example to convert an input voltage which is higher or lower that what is required by the load. A common requirement is to convert a 24VDC supply to an isolated 5V output or to generate +/-15V supply rails from a 5V source.

  2. To stabilise a power supply. Battery-powered equipment has to work with a variable input but many circuits require a stable supply voltage. For example, a typical 12V-to-12V DC/DC converter can deliver a stable 12V output from an input voltage range from 9V up to 18V.

  3. To isolate a supply. An isolated DC/DC converter can be used for safety (for example in medical applications) to protect the user from power supply faults, or it can be used to break earth-loop connections to reduce noise and interference  (for example, a DC/DC used in a motor controller circuit can provide a low-noise, stable output from a noisy DC supply). For applications with multiple channels, isolating each channel power supply with a separate DC/DC converter means that if any channel is faulty or short-circuited, the remaining channels are unaffected.

  4. To simplify a power supply. An application requiring many different board-level voltages can be made simpler and more reliable by using a single main supply voltage followed by DC/DC converters at the point-of-load to generate local power.

What is the function of a non- isolated DC/DC converter?

A non-isolated or switching regulator converter can efficiently reduce or boost a DC input voltage to a lower or higher output voltage. A switching regulator has the following advantages over a simple linear regulator:

  1. As a transformer is not used and the input and output share a common ground, the efficiencies can be very high (>97%) compared with a linear regulator (typically 20% when dropping 24V down to 5V, for example).

  2. A switching regulator varies the internal on/off duty cycle to compensate for changes in load and/or input voltage. Thus the converter can work efficiently over a wide input voltage range (7:1) and wide load range (100:1).

  3. A switching regulator is a power converter, so for a fixed load, the input current reduces as the input voltage increases. This allows for high output currents without overloading the input.