Datel 20A Removal of Soldered USQs from PCBs, Input Source Impedance, Input Overvoltage Shutdown

USQ Series 2 0 A , S I N G L E O U T P U T D C / D C C O N V E R T E R S

Absolute Maximum Ratings

Input Voltage:	24V models	48V models
Continuous:	39 Volts	81 Volts
Transient (100msec)	50 Volts	100 Volts
Input Reverse-Polarity Protection	Input Current must be <5A. 1 minute
	duration. Fusing recommended.
Output Current	Current limited. Devices can withstand
	an indefinite output short circuit.
On/Off Control (Pin 2) Max. Voltages
Referenced to –Input (pin 1)	–0.3 to +7 Volts
Storage Temperature	–40 to +125°C
Lead Temperature (Soldering, 10 sec.)	+300°C

These are stress ratings. Exposure of devices to any of these conditions may adversely affect long-term reliability. Proper operation under conditions other than those listed in the Performance/Functional Specifications Table is not implied, nor recommended.

T E C H N I C A L N O T E S

Removal of Soldered USQ's from PCB's

Should removal of the USQ from its soldered connection be needed, it is very important to thoroughly de-solder the pins using solder wicks or de-soldering tools. At no time should any prying or leverage be used to remove boards that have not been properly de-soldered first.

Input Source Impedance

impedance as highly inductive source impedance can affect system stability. In Figure 2, CBUS and LBUS simulate a typical dc voltage bus. Your specific system configuration may necessitate additional considerations.

In critical applications, output ripple/noise (also referred to as periodic and random deviations or PARD) can be reduced below specified limits using filtering techniques, the simplest of which is the installation of additional external output capacitors. Output capacitors function as true filter elements and should be selected for bulk capacitance, low ESR, and appropriate frequency response. In Figure 3, the two copper strips simulate real-world pcb impedances between the power supply and its load. Scope measurements should be made using BNC connectors or the probe ground should be less than ½ inch and soldered directly to the fixture.

All external capacitors should have appropriate voltage ratings and be located as close to the converter as possible. Temperature variations for all relevant parameters should be taken into consideration. OS-CONTMorganic semiconductor capacitors (www.sanyo.com) can be especially effective for further reduction of ripple/noise.

The most effective combination of external I/O capacitors will be a function of line voltage and source impedance, as well as particular load and layout conditions. Our Applications Engineers can recommend potential solutions and discuss the possibility of our modifying a given device’s internal filtering to meet your specific requirements. Contact our Applications Engineering Group for additional details.

USQ converters must be driven from a low ac-impedance input source. The DC/DC’s performance and stability can be compromised by the use of highly inductive source impedances. The input circuit shown in Figure 2 is a practical solution that can be used to minimize the effects of inductance in the input traces. For optimum performance, components should be mounted close to the DC/DC converter. The 24V models can benefit by increasing the 33µF external input capacitors to 100µF, if the application has a high source impedance.

+SENSE

+OUTPUT

–OUTPUT

–SENSE

7 COPPER STRIP

8

C1C2

4

5

COPPER STRIP

SCOPE

RLOAD

I/O Filtering, Input Ripple Current, and Output Noise

All models in the USQ Series are tested/specified for input ripple current (also called input reflected ripple current) and output noise using the circuits and layout shown in Figures 2 and 3.

	TO	CURRENT
OSCILLOSCOPE
		PROBE	3	+INPUT
	+	LBUS
VIN	CBUS	CIN
	–
			1	–INPUT

CIN = 33µF, ESR < 700mΩ @ 100kHz

CBUS = 220µF, ESR < 100mΩ @ 100kHz

LBUS = 12µH

Figure 2. Measuring Input Ripple Current

External input capacitors (CIN in Figure 2) serve primarily as energy-storage elements. They should be selected for bulk capacitance (at appropriate frequencies), low ESR, and high rms-ripple-current ratings. The switching nature of DC/DC converters requires that dc voltage sources have low ac

C1 = 1µF CERAMIC

C2 = 10µF TANTALUM

LOAD 2-3 INCHES (51-76mm) FROM MODULE

Figure 3. Measuring Output Ripple/Noise (PARD)

Input Overvoltage Shutdown

Standard USQ DC/DC converters do not feature overvoltage shutdown. They are equipped with this function, however. Many of our customers need their devices to withstand brief input surges to 100V without shutting down. Consequently, we disabled the function. Please contact us if you would like it enabled, at any voltage, for your application.

Start-Up Threshold and Undervoltage Shutdown

Under normal start-up conditions, the USQ Series will not begin to regulate properly until the ramping input voltage exceeds the Start-Up Threshold. Once operating, devices will turn off when the applied voltage drops below the Undervoltage Shutdown point. Devices will remain off as long as the undervoltage condition continues. Units will automatically re-start when the applied voltage is brought back above the Start-Up Threshold. The hyster- esis built into this function avoids an indeterminate on/off condition at a single input voltage. See Performance/Functional Specifications table for actual limits.

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