25-40W, SINGLE OUTPUT DC/DC CONVERTERS
On/Off Control (Standard)
The On/Off Control pin (pin 5) may be used for remote on/off operation. As
shown in Figure 6, the control pin has an internal 10kΩ pull-up resistor to
approximately 10V. The converter is designed so that it is enabled when the
control pin is left open (normal mode) and disabled when the control pin is
pulled low (to less than +0.8V relative to –Input, pin 2).
Dynamic control of the on/off function is best accomplished with a mechani-
cal relay or an open-collector/open-drain drive circuit (optically isolated if
appropriate). The drive circuit should obviously be able to sink approximately
1mA when activated and withstand more than 10 Volts when deactivated.
Applying an external voltage to pin 5 when no input power is applied to the
converter can cause permanent damage to the converter. The on/off control
function, however, is designed such that the converter can be disabled (pin 5
pulled low) while input power is ramping up and then "released" once the
input has stabilized. Under these circumstances, it takes approximately 30ms
for the output of the fully loaded DC/DC to ramp up and settle to within ±1%
of its final value after the converter has been turned on.
In critical applications employing multiple switching DC/DC converters, it may
be desirable to intentionally synchronize the switching of selected converters
(so the system noise can be reduced with notch filtering) or to purposely
desynchronize the converters (to lessen the current-carrying requirements on
intermediate dc buses). UMP DC/DC Converters have been designed so that
the On/Off Control function on pin 5 can be replaced with a Sync function. This
change has to be implemented by DATEL during the product assembly
process. Contact our Applications Engineering Group for additional details.
To synchronize the switching of multiple UMP converters configured with the
Sync function, an external clock can be applied to pin 5 of each converter.
The clock should be a TTL square wave referenced to –Input (logic high =
+2 to +5 Volts, 250µA max.; logic low = 0 to +0.8 Volts, 70µA max.) with a
maximum 1µsec "high" duration. The frequency of the synchronizing clock
should be higher than that of any individual converter. Therefore, it should be
VOUT may be trimmed ±5% via a single trimpot or fixed resistor. The trimpot
should be connected between +Output (pin 9) and Common (pin 8) with its
wiper connected to pin 10 (Trim). A trimpot can also be used to determine the
value of a single fixed resistor which can be connected between pin 10 (Trim)
and pin 9 (+Output) to trim "down" the output voltages, or between pins 10
(Trim) and 6 (–Output) to trim "up" the output voltages. Fixed resistors should
be metal-film types with absolute TCR's less than 100ppm/°C to ensure
Figure 6. Driving the On/Off Control Pin
Unlike most other DC/DC converters, UMP DC/DC's do not have their metal
case connected to one of their input pins. The "uncommitted" case is
connected to pin 4 which, depending on your system configuration, should be
connected to either +Input (pin 3) or –Input (pin 2).