25 EN/LZT 146 049 R4A © Ericsson Power Modules, March 2007
PMB 4518T WP Datasheet
Input And Output Impedance
Operating Information
Output Voltage Adjust (Vadj)
The output voltage can be set by means of an external
resistor, connected to the Vadj pin. Nominal output voltage
0.75 V is set by leaving the Vadj pin open. Adjustment can
only be made to increase the output voltage setting.
To increase:
Connect a resistor between (Vadj) and (Gnd). The output
voltage increases with decreasing resistor value as shown
in the table below. Note that the maximum output voltage
3.63 V may not be exceeded.
Rext up (kohm) = (21.007 / (VO - 0.75225)) - 5.1
Circuit configuration for output voltage adjust
Increase
+Out
GND
Vadj Load
Radj
Sense
Output Voltage (V) Resistor (ohm)
0.75 Open
1.0 79.691 k
1.2 41.817 k
1.5 22.990 k
1.8 14.949 k
2.5 6.919 k
3.3 3.145 k
Current Limit Protection
The PMB 4000 Series DC/DC regulators include current
limiting circuitry that allows them to withstand continuous
overloads or short circuit conditions on the output. The cur-
rent limit is of hick-up mode type.
The regulator will resume normal operation after removal
of the overload. The load distribution system should be
designed to carry the maximum output short circuit current
specified.
Input And Output Impedance
The impedance of both the power source and the load will
interact with the impedance of the DC/DC regulator. It is
most important to have a low characteristic impedance,
both at the input and output, as the regulators have a low
energy storage capability. Use capacitors across the input if
the source inductance is greater than 4.7 µH. Suitable input
capacitors are 22 µF - 220 µF low ESR ceramics.
Maximum Capacitive Load
When powering loads with significant dynamic current
requirements, the voltage regulation at the load can be
improved by addition of decoupling capacitance at the load.
The most effective technique is to locate low ESR ceramic
capacitors as close to the load as possible, using several
capacitors to lower the total ESR. These ceramic capacitors
will handle short duration high-frequency components
of dynamic load changes. In addition, higher values of
capacitors (electrolytic capacitors) should be used to handle
the mid-frequency components. It is equally important
to use good design practice when configuring the DC
distribution system.
Low resistance and low inductance PCB layouts and
cabling should be used. Remember that when using remote
sensing, all resistance (including the ESR), inductance and
capacitance of the distribution system is within the feedback
loop of the regulator. This can affect on the regulators
compensation and the resulting stability and dynamic
response performance.
Very low ESR and high capacitance must be used with
care. A “rule of thumb” is that the total capacitance must
never exceed typically 500-700 µF if only low ESR (< 2 mΩ)
ceramic capacitors are used. If more capacitance is needed,
a combination of low ESR type and electrolytic capacitors
should be used, otherwise the stability will be affected.
The PMB 4000 series regulator can accept up to 8 mF of
capacitive load on the output at full load. This gives <500
µF/A of IO. When using that large capacitance it is important
to consider the selection of output capacitors; the resulting
behavior is a combination of the amount of capacitance and
ESR.
Minimum Required External Capacitors
External input capacitors are required to increase the
lifetime of the internal capacitors and to further reduce the
input ripple. A minimum of 2x22 µF external input capaci-µF external input capaci-F external input capaci-
tance with low ESR should be added.
A minimum of 150 µF external output capacitance, low ESR,µF external output capacitance, low ESR,F external output capacitance, low ESR,
should be added for the converter to operate properly at full
load.