coupled voltage used to develop primary side voltages will also drop, thereby
shutting down the PWM controller. Following a time-out period, the PWM will
restart, causing the output voltage to begin ramping up to its appropriate value.
If the short-circuit condition persists, another shutdown cycle will initiate. This
rapid on/off cycling is called “hiccup mode”. The hiccup cycling reduces the
average output current, thereby preventing excessive internal temperatures
and/or component damage. A short circuit can be tolerated indefi nitely.
The “hiccup” system differs from older latching short circuit systems
because you do not have to power down the converter to make it restart. The
system will automatically restore operation as soon as the short circuit condi-
tion is removed.
Remote Sense Input
Use the Sense inputs with caution. Sense is normally connected at the load.
Sense inputs compensate for output voltage inaccuracy delivered at the load.
This is done by correcting IR voltage drops along the output wiring and the
current carrying capacity of PC board etch. This output drop (the difference be-
tween Sense and Vout when measured at the converter) should not be allowed
to exceed 0.5V. Consider using heavier wire if this drop is excessive. Sense
inputs also improve the stability of the converter and load system by optimizing
the control loop phase margin.
Note: The Sense input and power Vout lines are internally connected through
low value resistors to their respective polarities so that the converter can
operate without external connection to the Sense. Nevertheless, if the Sense
function is not used for remote regulation, the user should connect +Sense to
+Vout and –Sense to –Vout at the converter pins.
The remote Sense lines carry very little current. They are also capacitively
coupled to the output lines and therefore are in the feedback control loop to
regulate and stabilize the output. As such, they are not low impedance inputs
and must be treated with care in PC board layouts. Sense lines on the PCB
should run adjacent to DC signals, preferably Ground. In cables and discrete
wiring, use twisted pair, shielded tubing or similar techniques.
Any long, distributed wiring and/or signifi cant inductance introduced into the
Sense control loop can adversely affect overall system stability. If in doubt, test
your applications by observing the converter’s output transient response dur-
ing step loads. There should not be any appreciable ringing or oscillation. You
may also adjust the output trim slightly to compensate for voltage loss in any
external fi lter elements. Do not exceed maximum power ratings.
Please observe Sense inputs tolerance to avoid improper operation:
[Vout(+) −Vout(-)] − [Sense(+) −Sense(-)] ≤ 10% of Vout
Output overvoltage protection is monitored at the output voltage pin, not the
Sense pin. Therefore excessive voltage differences between Vout and Sense
together with trim adjustment of the output can cause the overvoltage protec-
tion circuit to activate and shut down the output.
Power derating of the converter is based on the combination of maximum
output current and the highest output voltage. Therefore the designer must
insure:
(Vout at pins) x (Iout)
≤ (Max. rated output power)
Trimming the Output Voltage
The Trim input to the converter allows the user to adjust the output voltage
over the rated trim range (please refer to the Specifi cations). In the trim equa-
tions and circuit diagrams that follow, trim adjustments use either a trimpot or
a single fi xed resistor connected between the Trim input and either the +Sense
or –Sense terminals. (On some converters, an external user-supplied precision
DC voltage may also be used for trimming). Trimming resistors should have a
low temperature coeffi cient (±100 ppm/deg.C or less) and be mounted close
to the converter. Keep leads short. If the trim function is not used, leave the
trim unconnected. With no trim, the converter will exhibit its specifi ed output
voltage accuracy.
There are two CAUTIONs to be aware of for the Trim input:
CAUTION: To avoid unplanned power down cycles, do not exceed EITHER
the maximum output voltage OR the maximum output power when setting the
trim. Be particularly careful with a trimpot. If the output voltage is excessive,
the OVP circuit may inadvertantly shut down the converter. If the maximum
power is exceeded, the converter may enter current limiting. If the power is
exceeded for an extended period, the converter may overheat and encounter
overtemperature shut down.
CAUTION: Be careful of external electrical noise. The Trim input is a senstive
input to the converter’s feedback control loop. Excessive electrical noise may
cause instability or oscillation. Keep external connections short to the Trim
input. Use shielding if needed.
Figure 4. Remote Sense Circuit Confi guration
LOAD
5
8
7
6
9
Contact and PCB resistance
losses due to IR drops
Contact and PCB resistance
losses due to IR drops
+VOUT
+SENSE
TRIM
−
SENSE
-VOUT
−
V
IN
ON/OFF
CONTROL
+
V
IN
1
3
4
Sense Current
I OUT
Sense Return
I OUT Return
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Single Output UCH Models
Isolated, “Half-Brick”
1.8−15V Output DC/DC Converters
MDC_UCH Models.C01 Page 16 of 18
