Rainbow Electronics MAX5051 User Manual

Page 15

in control. Thus, the output-voltage slew rate is constant
at light or heavy loads. Once the soft-start ends, the
voltage on CSS regulates to 1.24V. Do not load CSS
with external circuitry. A suitable range of capacitors
connected to CSS is from 10nF to 0.1µF. Calculate the
required soft-start capacitor based on the total output-
voltage startup time as follows:

where C

CSS

is the capacitor connected to CSS, t

the soft-start time required for the output voltage to rise
from 0V to the rated output voltage. This only applies
when this amplifier is used for output voltage regulation.

PWM Ramp

The PWM ramp is generated at RCFF. Connect a
capacitor C

RCFF

from RCFF to ground and a resistor

RCFF

from RCFF to AVIN. The ramp generated on

RCFF is internally offset by 2.3V and applied to the non-
inverting input of the PWM comparator. The slope of the
ramp is part of the overall loop gain. The dynamic
range of RCFF is 0 to 3V, and so the ramp peak must
be kept below that. Assuming the maximum duty cycle
approaches 50% at minimum input voltage, use the fol-
lowing formula to calculate the minimum value of either
the ramp capacitor or resistor:

where V

INUVLO

is the minimum input supply voltage

(typically the PWM UVLO turn-on voltage), f

is the

switching frequency, and V

RPP

is the peak-to-peak

ramp voltage, typically 2V.

Allow the ramp peak to be as high as possible to maxi-
mize the signal-to-noise ratio. The low-frequency small-
signal gain of the power stage, Gps (the gain from the
inverting input of the PWM comparator to the output)
can be calculated by using the following formula:

where N

is the secondary-to-primary power trans-

former turns ratio.

Internal Regulators

The MAX5051 has two internal linear regulators that are
used to power internal and external control circuits. The
9V regulator, REG9, is primarily used to power the high-

and low-side gate drivers. Bypass REG9 with a 4.7µF
ceramic capacitor or any other high-quality capacitor;
use low-value ceramics in parallel as necessary. A 5V
regulator also is provided, REG5, primarily used to bias
the internal circuitry of the MAX5051. Bypass REG5 with
a 4.7µF ceramic capacitor similar to the one used for
REG9. Both of these regulators are always powered.
When using bootstrapped startup through a bleed resis-
tor, do not load these outputs while the MAX5051 is in
standby as it may fail to start. Any external loading to
this output should be such that the sum of their load and
the standby current through PVIN of the MAX5051 is
less than the current that the bleed resistor can supply.

Startup Modes

The MAX5051 can be configured for two different
startup modes, allowing operation in either boot-
strapped or direct power mode.

Direct Power Mode

In direct power mode, AVIN and PVIN are connected
directly to the input supply. This is typical in 12V to 24V
systems. The undervoltage lockout set at STT needs to
be adjusted down with an external resistor-divider to an
appropriate level.

Bootstrapped Startup

In bootstrap mode, a resistor is connected from the
input supply to PVIN, where a capacitor to GND is
charged towards the input supply. When this voltage
reaches the startup threshold, the device wakes up and
begins switching. A tertiary winding from the trans-
former is then used to sustain operation. The MAX5051
draws little current from PVIN before reaching the
threshold, which allows a large-value bootstrap resistor
and reduces its power dissipation after startup. A large
startup hysteresis helps the design of the bootstrap
circuit by providing longer running times during startup.

After coming out of standby and before initiating the
soft-start, the MAX5051 turns on the low-side FET to
charge up the boost capacitor. A voltage detector has
been incorporated in the high-side driver that prevents
the high-side switch from turning on with insufficient
voltage. It is also used to indicate when the boost
capacitor has been charged. Once the capacitor is
charged, soft-start commences. If the duty cycle is low,
the magnetizing energy in the transformer may be
insufficient to keep the bootstrap capacitor charged.
DRVB (see Figure 2 dotted lines) has been provided to
drive a small external FET connected between
XFRMRH and PGND, and is pulsed every cycle to keep
the capacitor charged.

sp RCFF

RCFF s

f V

RCFF

INUVLO

s RPP

≥

F s t

CSS

MAX5051

Parallelable, Clamped Two-Switch

Power-Supply Controller IC

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