Rainbow Electronics MAX5068 User Manual
Page 15
where I
FLTINT
= 60µA, t
SH
is the desired fault-integra-
tion time during which current-limit events from the cur-
rent-limit comparator are ignored. For example, a 0.1µF
capacitor gives a fault-integration time of 4.7ms.
This is an approximate formula. Some testing may be
required to fine-tune the actual value of the capacitor. To
calculate the recovery time, use the following formula:
where t
RT
is the desired recovery time.
Choose t
RT
= 10 x t
SH
. Typical values for t
SH
range from
a few hundred microseconds to a few milliseconds.
Soft-Start
The MAX5068 soft-start feature allows the load voltage
to ramp up in a controlled manner, eliminating output-
voltage overshoot. Soft-start begins after UVLO is
deasserted. The voltage applied to the noninverting
node of the amplifier ramps from 0 to 1.23V in 2047
oscillator clock cycles (soft-start timeout period). Unlike
other devices, the MAX5068 reference voltage to the
internal amplifier is soft-started. This method results in
superior control of the output voltage under heavy- and
light-load conditions.
Internal Regulators
Two internal linear regulators power the MAX5068 inter-
nal and external control circuits. V
CC
powers the exter-
nal N-channel MOSFET and is internally set to
approximately 9.5V. The REG5 5V regulator has a 1mA
sourcing capability and may be used to provide power
to external circuitry. Bypass V
CC
and REG5 with 1µF
and 0.1µF high quality capacitors, respectively. Use
lower value ceramics in parallel to bypass other
unwanted noise signals. Bootstrapped operation
requires startup through a bleed resistor. Do not exces-
sively load the regulators while the MAX5068 is in the
power-up mode. Overloading the outputs can cause
the MAX5068 to fail upon startup.
N-Channel MOSFET Switch Driver
NDRV drives an external N-channel MOSFET. The NDRV
output is supplied by the internal regulator (V
CC
), which
is internally set to approximately 9.5V. For the universal
input-voltage range, the MOSFET used must be able to
withstand the DC level of the high-line input voltage plus
the reflected voltage at the primary of the transformer.
For most applications that use the discontinuous flyback
topology, a MOSFET rated at 600V is required. NDRV
can source/sink in excess of 650mA/1000mA peak cur-
rent. Therefore, select a MOSFET that yields acceptable
conduction and switching losses.
Error Amplifier
The MAX5068 includes an internal error amplifier that
can regulate the output voltage in the case of a noniso-
lated power supply (Figure 1). Calculate the output volt-
age using the following equation:
where V
REF
= 1.23V. The amplifier’s noninverting input
internally connects to a digital soft-start reference voltage.
This forces the output voltage to come up in an orderly
and well-defined manner under all load conditions.
Slope Compensation (MAX5068C/D/E/F)
The MAX5068C/D/E/F use an internal-ramp generator
for slope compensation. The internal-ramp signal resets
at the beginning of each cycle and slews at the rate
programmed by the external capacitor connected at
SCOMP and the resistor at RT. Adjust the MAX5068
slew rate up to 90mV/µs using the following equation:
where R
RT
is the external resistor at RT that sets the oscil-
lator frequency and C
SCOMP
is the capacitor at SCOMP.
PWM Comparator
The PWM comparator uses the instantaneous current,
the error amplifier, and the slope compensation to
determine when to switch NDRV off. In normal opera-
tion, the N-channel MOSFET turns off when:
I
PRIMARY
x R
CS
> V
EA
– V
OFFSET
- V
SCOMP
where I
PRIMARY
is the current through the N-channel
MOSFET, V
EA
is the output voltage of the internal
amplifier, V
OFFSET
is the 1.6V internal DC offset and
V
SCOMP
is the ramp function starting at zero and slew-
ing at the programmed slew rate (SR). When using the
MAX5068 in a forward-converter configuration, the fol-
lowing conditions must be met to avoid current-loop
subharmonic oscillations:
where K = 0.75 and N
S
and N
P
are the number of turns
on the secondary and primary side of the transformer,
respectively. L is the secondary filter inductor. When
S
P
CS
OUT
N
N
K
R
V
L
SR
Ч
Ч
Ч
=
SR
R
C
mV
s
RT
SCOMP
(
/
)
=
Ч
Ч
−
165
10
6
µ
V
R
R
x V
OUT
REF
=
+
1
8
9
R
t
C
FLTINT
RT
FLTINT
.
≅
×
0 595
MAX5068
High-Frequency, Current-Mode PWM Controller
with Accurate Programmable Oscillator
______________________________________________________________________________________
15