ISR2805DDRH データシートの表示（PDF） - M.S. Kennedy
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ISR2805DDRH Datasheet PDF : 9 Pages
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APPLICATION NOTES CONT'D
BASIC ARCHITECTURE:
MSK power converters utilize two DC-DC converters to opti-
mize power efficiency and to provide internal housekeeping power
while magnetically isolating the power bus from the load. A small
flyback converter, referenced to input return, is used to provide
electrical input-output isolation and will generate nominal +10VDC
bias voltages for the input and output control circuits of the power
supply. A single transistor forward converter, referenced to out-
put return, provides the main output voltage.
FLYBACK CONVERTER:
The flyback converter uses peak current-mode Pulse Width
Modulator (PWM) control and operates in discontinuous conduc-
tion-mode. Regulation is achieved by sampling the input bias volt-
age directly and feeding it back to the control circuit of the flyback
converter, which is referenced to input return.
FORWARD CONVERTER:
The forward converter uses voltage Mode Pulse Width Modu-
lator (PWM) control with input voltage feed-forward and operates
in continuous conduction-mode. Regulation is achieved by sam-
pling the output voltage directly and feeding it back to the control
circuit of the forward converter, which is referenced to output
return. Since the main output voltage is directly regulated without
the use of magnetically-coupled feedback, superior voltage regula-
tion can be achieved. A single-stage L-C output filter reduces the
typical output ripple to less than 25mVp-p.
SYNCHRONOUS RECTIFICATION:
Synchronous rectification is used to achieve high-conversion
efficiency over a half to full output load range and allows con-
verter operation in continuous conduction-mode down to zero out-
put load. This capability results in far superior dynamic perfor-
mance when the load is stepped to or from a zero load condition,
and is particularly critical in pulsed Phased Array Antenna and
modern microprocessor applications which have high peak current
demands and fast load transients.
SOFT-START:
Soft-start circuitry associated with both the flyback and for-
ward converters is guaranteed to prevent output voltage over-
shoot during power-up for both the auxiliary and main output volt-
ages.
USER-ADJUSTABLE MAIN OUTPUT:
The +VOUT output voltage is user-adjustable over a range of
3.3VDC to 5VDC. Setting the positive output voltage is accom-
plished by using a single external resistor. The output voltage
may be programmed by the user to any desired voltage within this
range. The table below shows the value of resistance necessary
to set several common output voltages. The equation below may
be used to determine the resistor value required for other set points.
The output voltage may be adjusted up to 6V and down to 3V, but
some performance specifications will not be met.
Output Voltage Adjust Resistor Value (in KΩ)
f(Vout) =
2.5
0.04975Vout-0.1484166
-6.7
f(3.0) = 2.98 x 10³
f(3.3) = 141.95
f(5.0) = 8.22
f(5.2) = 5.97
OVERVOLTAGE PROTECTION (OVP):
Overvoltage protection is provided for both the negative and
positive output voltages of the power supply. The overvoltage
protection circuit for the negative output voltage limits peak volt-
age to approximately 120% of nominal. The overvoltage limit for
the positive output voltage is programmable with a single external
resistor. It is recommended that this overvoltage limit be set at
approximately 120% of nominal.
The overvoltage protection is programmable over a limited range
using only a single external resistor. The OVP voltage may be
programmed by the user to any desired voltage within this range.
The OVP set point should be coordinated with the positive output
voltage and set to approximately 110% to 120% of the nominal
output. The table below shows the value of resistance necessary
to set the OVP to 120% for several common output voltages. The
equation below may be used to determine the resistor value re-
quired for other set points.
INPUT EMI FILTER:
The input EMI filter is comprised of a multi-stage L-C filter
which provides attenuation of the differential-mode emissions from
the power supply. A balun, in conjuction with very small line-to-
chassis and return-to-chassis capacitors, provides attenuation of
the common-mode emissions from the power supply. Together
these components enable the power supply to meet the conducted
emission requirements of MIL-STD-461.
Output Voltage Over-Voltage Protection (OVP) Adjust Resistor Value
(in KΩ)
g(OVP) =
10.4667
OVP-3.55667
-3.44
g(3.6) = 238.12
g(3.96) = 22.51
g(6) = 0.84
g(6.24) = 0.46
INRUSH CURRENT LIMITING:
Inrush current limiting circuitry limits initial peak input charg-
ing current at turn-on to less than two times maximum steady-
state input current. Consequently, even when the input voltage to
VOLTAGE SEQUENCING:
Output voltage sequencing is provided to protect T/R modules
the power supply is instantaneously stepped, peak charging cur-
from damage when used in PAA applications. During power-up,
rent is well controlled.
the positive output voltage remains off until the negative output
voltage has reached normal regulation limits. During power-down,
UNDERVOLTAGE LOCKOUT:
a crowbar command is available to ensure that the positive output
voltage falls to 10% or less of its steady state value before the
An undervoltage lockout circuit prevents the power supply from
negative output voltage falls more than 20% from its nominal value.
operating when the input line voltage is too low to maintain the
During fault mode the crowbar command allows user control of
output voltage. The converter will not start until the line voltage
the crowbar circuit on the positive output to ensure proper voltage
rises to approximately 17V and will shut down when the input
sequencing for application specific requirements.
voltage drops below approximatley 16V. The approximate 1V of
hysteresis reduces the possibility of line noise interfereing with
the converter during power-up and power-down.
6
PRELIMINARY Rev. - 3/01
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