Rainbow Electronics MAX5074 - Manual (Page 10)

MAX5074
Maximize the signal-to-noise ratio by setting the ramp
peak as high as possible. Calculate the low-frequency,
small-signal gain of the power stage (the gain from the
inverting input of the PWM comparator to the output)
using the following formula:
G PS = N SP x R RCFF x C RCFF x f S
where N SP is the secondary to primary power trans-
former turns ratio.
Oscillator
The MAX5074 oscillator is externally programmable
through a resistor connected from RTCT to REGOUT and
a capacitor connected from RTCT to GND. The PWM
frequency will be 1/2 the frequency at RTCT with a 50%
duty cycle. Use the following formula to calculate the
oscillator components:
where C PCB is the stray capacitance on the PC board
(14pF, typ), V TH is the RTCT peak trip level, and f S is
the switching frequency.
Integrating Fault Protection
The integrating fault protection feature allows the
MAX5074 to ignore transient overcurrent conditions for a
programmable amount of time, giving the power supply
time to behave like a current source to the load. This can
happen, for example, under load-current transients when
the control loop requests maximum current to keep the
output voltage from going out of regulation. Program the
ignore time externally by connecting a capacitor to
FLTINT. Under sustained overcurrent faults, the voltage
across this capacitor ramps up toward the FLTINT shut-
down threshold (typically 2.7V). When FLTINT reaches
the threshold, the power supply shuts down. A high-
value bleed resistor connected in parallel with the
FLTINT capacitor allows the capacitor to discharge
toward the restart threshold (typically 1.9V). Crossing the
restart threshold soft-starts the supply again.
The ILIM comparator provides cycle-by-cycle current
limiting with a typical threshold of 156mV. The fault inte-
gration circuit works by forcing an 80µA current into
FLTINT for one clock every time the current-limit com-
parator ILIM (Figure 1) trips. Use the following formula
to calculate the approximate capacitor needed for the
desired shutdown time:
where I FLTINT is typically 80µA, and t SH is the desired
ignore time during which current-limit events from the
current-limit comparator are ignored.
This is an approximate formula; some testing may be
required to fine tune the actual value of the capacitor.
Calculate the approximate bleed resistor needed for
the desired recovery time using the following formula:
where t RT is the desired recovery time.
Choose at least t RT = 10 x t SH . Typical values for t SH
range from a few hundred microseconds to a few mil-
liseconds.
Shutdown
Shut down the MAX5074 by driving UVLO to GND
using an open-collector or open-drain transistor con-
nected to GND. The IC will be internally shut down if
REGOUT is below its UVLO level. The MAX5074 also
features internal thermal shutdown using a temperature
sensor that monitors the high-power area. A thermal
fault arises from excessive dissipation in the power
MOSFETs or in the regulator. When the temperature
limit is reached (+160°C), the temperature sensor ter-
minates switching and shuts down the regulator. The
integration of thermal shutdown and the power
MOSFETs results in a very robust power circuit.
Applications Information
Isolated Telecom Power Supply
Figure 3 shows a typical application circuit of an isolat-
ed power supply with a 30V to 60V input. This power
supply is fully protected and can sustain a continuous
short circuit at its output terminals.
R
t
C
FLTINT
RT
FLTINT
≅
⎛
⎝⎜ ⎞
⎠⎟
ln
.
.
2 7
1 9
C
I t
FLTINT
FLTINT SH ≅
1 4 .
R
f C C
V
V V
RTCT
S RTCT PCB
REGOUT
REGOUT TH
≅
+
⎛
⎝⎜ ⎞
⎠⎟
−
1
2 ( )ln
Power IC with Integrated MOSFETs for Isolated IEEE
802.3af PD and Telecom Power-Supply Applications
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