Rainbow Electronics MAX5095A User Manual

Page 15

MAX5095_Feedback

The MAX5095A/MAX5095B/MAX5095C use either an
external error amplifier when designed into a nonisolat-
ed converter or an error amplifier and optocoupler
when designed into an isolated power supply. The
COMP input is level-shifted and connected to the
inverting terminal of the PWM comparator (CPWM).
Connect the COMP input to the output of the external
error amplifier for nonisolated design. Pull COMP high
externally to 5V (or REF) 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 temperatures.

Oscillator

The oscillator frequency is programmed by adding an
external capacitor and resistor at R

(see R

and C

in the Typical Application Circuits). R

is connected

from R

to the 5V reference (REF) and C

is con-

nected from R

to GND. REF charges C

through R

until its voltage reaches 2.8V. C

then discharges

through an 8.3mA internal current sink until C

’s voltage

reaches 1.1V, at which time C

is allowed to charge

through R

again. The oscillator’s period will be the

sum of the charge and discharge times of C

. Calculate

the charge time as

= 0.57 x R

x C

The discharge time is then

The oscillator frequency will then be

For the MAX5094A/MAX5094C/MAX5095A, the convert-
er output switching frequency (f

) is the same as the

oscillator frequency (f

OSC

). For the MAX5094B/

MAX5094D/MAX5095B/MAX5095C, the output switch-
ing frequency is 1/2 the oscillator frequency.

Reference Output

REF is a 5V reference output that can source 20mA.
Bypass REF to GND with a 0.1µF capacitor.

Current Limit

The MAX5094/MAX5095 include a fast current-limit com-
parator to terminate the ON cycle during an overload or a
fault condition. The current-sense resistor (R

), connect-

ed between the source of the MOSFET and GND, sets
the current limit. The CS input has a voltage trip level
(V

) of 1V (MAX5094A/B) or 0.3V (MAX5094C/D,

MAX5095_). Use the following equation to calculate R

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 (MAX5095A/MAX5095B)

SYNC

SYNC is a bidirectional input/output that outputs a syn-
chronizing pulse and accepts a synchronizing pulse
from other MAX5095A/MAX5095Bs (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

) between 500

Ω

and 5k

Ω. As an input, SYNC accepts the output pulses

from other MAX5095A/MAX5095Bs.

Synchronize multiple MAX5095A/MAX5095Bs 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

) 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

discharge time t

. Adjust

such that the minimum discharge time t

is 200ns.

P P

−

OSC

−

4 88

1 8 10

MAX5094A/B/C/D/MAX5095A/B/C

High-Performance, Single-Ended, Current-Mode

PWM Controllers

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