Rainbow Electronics MAX5071 - Manual (Page 15)

MAX5071A/MAX5071B/MAX5071C
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The MAX5071A/MAX5071B/MAX5071C are designed to
be used with either an external error amplifier when
designed into a nonisolated converter or an error ampli-
fier and optocoupler when designed into an isolated
power supply. The COMP input is level-shifted and
connected to the inverting terminal of the PWM com-
parator (CPWM). Connect the COMP pin to the output
of the external error amplifier for nonisolated design.
Pull COMP high externally to at least 5V (or VREF) and
connect the optocoupler transistor as shown in Figures
7 and 8. COMP can be used for soft-start and also as a
shutdown. See the Typical Operating Characteristics to
find the turn-off COMP voltage at different tempera-
tures. If the maximum external COMP voltage is below
4.9V, it may reduce the PWM current-limit threshold
below 1V. Use the following equation to calculate mini-
mum COMP voltage (V
COMP ) required for a guaranteed
peak primary current (I P-P ):
V COMP = (3 x I P-P x R CS ) + 1.95V
where R
CS
is a current-sense resistor.
Oscillator
The oscillator frequency is adjusted by adding an
external capacitor and resistor at R
T
/C
T
(see R
T
and C
T
in the Typical Application Circuits). R T is connected
from R T /C T to the 5V reference (VREF) and C T is con-
nected from R T /C T to GND. VREF charges C T through
R T until its voltage reaches 2.8V. C T then discharges
through an 8.3mA internal current sink until C
T
’s voltage
reaches 1.1V, at which time C
T
is allowed to charge
through R T again. The oscillator’s period will be the
sum of the charge and discharge times of C T . Calculate
the charge time as:
t C = 0.57 x R T x C T
The discharge time is then:
The oscillator frequency will then be:
For the MAX5070A/MAX5071A, the converter output
switching frequency (f
SW ) is the same as the oscillator
frequency (f OSC ). For the MAX5070B/MAX5071B/
MAX5071C, the output switching frequency is 1/2 the
oscillator frequency.
Reference Output
VREF is a 5V reference output that can source 20mA.
Bypass VREF to GND with a 0.1µF capacitor.
Current Limit
The MAX5070/MAX5071 include a fast current-limit com-
parator to terminate the ON cycle during an overload or a
fault condition. The current-sense resistor (R
CS ), connect-
ed between the source of the MOSFET and GND, sets
the current limit. The CS input has a voltage trip level
(V
CS ) of 1V. Use the following equation to calculate R CS :
I P-P is the peak current in the primary that flows through
the MOSFET. When the voltage produced by this current
(through the current-sense resistor) exceeds the current-
limit comparator threshold, the MOSFET driver (OUT) will
turn the switch off within 60ns. In most cases, a small RC
filter is required to filter out the leading-edge spike on the
sense waveform. Set the time constant of the RC filter at
50ns. Use a current transformer to limit the losses in the
current-sense resistor and achieve higher efficiency
especially at low input-voltage operation.
Synchronization (MAX5071A/MAX5071B)
SYNC
SYNC is a bidirectional input/output that outputs a syn-
chronizing pulse and accepts a synchronizing pulse
from other MAX5071A/MAX5071Bs (see Figures 7 and
9). As an output, SYNC is an open-drain p-channel
MOSFET driven from the internal oscillator and requires
an external pulldown resistor (R
SYNC ) from between
500Ω and 5kΩ. As an input, SYNC accepts the output
pulses from other MAX5071A/MAX5071Bs.
Synchronize multiple MAX5071A/MAX5071Bs by con-
necting their SYNC pins together. All devices connected
together will synchronize to the one operating at the
highest frequency. The rising edge of SYNC will precede
the rising edge of OUT by approximately the discharge
time (t
D
) of the oscillator (see the Oscillator section). The
pulse width of the SYNC output is equal to the time
required to discharge the stray capacitance at SYNC
through R
SYNC plus the C T discharge time t D . Adjust
R T /C T such that the minimum discharge time t D is 200ns.
R
V
I
CS
CS
P P
=
−
f
t t
OSC
C D
=
+
1
t
R C
R
D
T T
T
=
Ч Ч
Ч − Ч
10
4 88 1 8 10
3
3
. .
MAX5070/MAX5071
High-Performance, Single-Ended, Current-Mode
PWM Controllers
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