Technical Specification
Quarter
Brick
Figure B graphs this relationship between R
trim-down
and
∆
.
The output voltage can be trimmed down as much as 20%.
10,000
48Vin 1.8Vout 25A
Note: the TRIM feature does not affect the voltage at which
the output over-voltage protection circuit is triggered.
Trimming the output voltage too high may cause the over-
voltage protection circuit to engage, particularly during tran-
sients.
TOTAL DC VARIATION OF Vout:
For the converter to
meet its full specifications, the maximum variation of the dc
value of Vout, due to both trimming and remote load voltage
drops, should not be greater than +10%/-20%
Trim Resistance
(kOhms)
1,000
100
PROTECTION FEATURES
10
0
2
4
6
8
10
12
14
16
18
20
% Decrease in Output Voltage
Figure B:
Trim Down Graph for 1.8Vout
A resistor connected between Pin 6 and Pin 7 will increase
the output voltage. For a desired increase of
∆
percent of the
nominal output voltage, the value of this resistor should be
Input Under-Voltage Lockout:
The converter is
designed to turn off when the input voltage is too low, help-
ing avoid an input system instability problem, described in
more detail below. The lockout circuitry is a comparator with
dc hysteresis. When the input voltage is rising, it must
exceed a typical value of 33V before the converter will turn
on. Once the converter is on, the input voltage must fall
below a typical value of 29.5V before the converter will turn
off.
Output Current Limit:
The current limit does not change
appreciably as the output voltage drops. However, once the
impedance of the short across the output is small enough to
make the output voltage drop below approximately 60% of
its nominal value, the converter turns off.
The converter then enters a mode where it repeatedly turns
on and off at a 5 Hz (nominal) frequency with a 5% duty
cycle until the short circuit condition is removed. This pre-
vents excessive heating of the converter or the load board
(see Figure 18).
Output Over-Voltage Limit:
If the voltage across the
output pins exceeds the O.V. threshold, the converter will
immediately stop switching. This prevents damage to the
load circuit due to 1) a sudden unloading of the converter,
2) a release of a short-circuit condition, or 3) a release of a
current limit condition. Load capacitance determines exactly
how high the output voltage will rise in response to these
conditions. After 200 ms the converter will automatically
restart.
R
trim-up
=
where
(
5.11V
OUT
(
100+
∆
%
)
_
511
_
10.22
1.225
∆
%
∆
%
)
(kΩ)
V
OUT
=
Nominal Output Voltage
Figure C graphs this relationship between R
trim-up
and
∆
.
The output voltage can be trimmed up as much as 10%.
10,000
Trim Resistance
(kOhms)
1,000
100
10
0
1
2
3
4
5
6
7
8
9
10
% Increase in Output Voltage
Figure C:
Trim Up Graph for 1.8Vout
Product # PQ48018QNA25
Phone 1-888-567-9596
Thermal Shutdown:
The
PowerQor
series has a temper-
ature sensor located such that it senses the average temper-
ature of the converter. The thermal shutdown circuit is
designed to turn the converter off when the temperature at
Doc.# 005-2QN418D_B
9/04/01
Page 10
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