APPLICATION NOTES
would be connected from the trim pin to the
Negative Sense Pin.
Output Over Voltage Protection:
The output voltage is monitored in two ways, by
the microcontroller which looks at the sensed signal
and a high-speed comparator that measures the
power pins. The microcontroller OVP allows the
OVP circuit to track the trimmed signal. However,
this circuit does not allow OVP signal detection in
the event the voltage sense pins are shorted
together. In that case, a redundant OVP set at a
fixed threshold will prevent excessive voltage.
Power Good Signal
The Leo generates a power good signal when the
output voltage falls in a 5% window of the nominal
value. The circuit tracks the trimmed voltage. The
circuit has a response time of approximately 1ms.
The output signal is derived from an internal 5V
power bus and can source up to 5mA. If interfacing
to other logic is required, the output can drive a
resistor divider to set a new high level. The voltage
is referenced to the VO(-) pin.
Thermal shutdown:
The printed circuit board temperature is measured
using a semiconductor sensor. If the maximum
rated temperature is exceeded, the converter is
shutdown until the temperature decreases to 90
degC. The time for this depends on the airflow and
heatsink mass.
Transient Response and Stability
The Leo uses a high-bandwidth control to keep
the output voltage in regulation. The crossover
frequency of the Leo is approximately 15kHz
(depending on the model) with greater than 60
degrees of phase margin. The control circuit
maintains high phase margin at lower frequencies
allowing the use of large amounts of external
capacitance to be applies without loss of stability.
If you require a high di/dt solution, Galaxy can
adjust the gain of the control to take advantage of
the on-board capacitance and improve the transient
performance up to 5X the nominal value.
If several Leos are used in parallel with current
sharing, the transient response is improved by the
number of Leos. For example, with two Leos a
60A step will give the same response as a single
Leo with a 30A step, or about 1/2 the peak
over/undershoot.
Please consult Galaxy Power for your special
needs.
Remote Sense:
The output voltage is regulated at the point where
the sense pins connect to the power output pins.
Total sense compensation should not exceed 0.4V
or 10% of Vout, whichever is greater.
If the unit is trimmed up, the application requires
that, under all conditions including current
transients, the output voltage must be kept less than
the redundant OVP, otherwise the unit will
shutdown.
Safety:
An external input fuse must always be used to
meet these safety requirements.
Trim:
External Output Trimming
The Leo converter has a novel regulation circuit
that uses a differential measurement technique to
eliminate voltage sense current. To trim the unit up,
a resistor is connected from the trim pin to the
Negative Sense pin. To trim the unit down, a
resistor is connected from the trim pin to the
Positive Sense pin. All models follow these trim
equations:
+ Sense
Trim
– Sense
RTRIM DOWN = 2250/D – 30kΩ
RTRIM UP = 750/DkΩ
where D is the percentage of trim (i.e. 10 = 10%).
For example, to trim up 10%, a 75kΩ resistor
Leo Series, Page 4
PRELIMINARY
Part No. 99–001 Rev. 4.0–120902
GALAXY [ GALAXY POWER, INC. ]
