MAX5068 Maxim Integrated Products, MAX5068 Datasheet - Page 15

• Manufacturer Part Number: MAX5068
• Description: Current-Mode PWM Controller
• Manufacturer: Maxim Integrated Products

where I
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
Choose t
a few hundred microseconds to a few milliseconds.
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.
Two internal linear regulators power the MAX5068 inter-
nal and external control circuits. V
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
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.
NDRV drives an external N-channel MOSFET. The NDRV
output is supplied by the internal regulator (V
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-
High-Frequency, Current-Mode PWM Controller
RT
FLTINT
RT
is the desired recovery time.
R
= 10 x t
FLTINT
N-Channel MOSFET Switch Driver
= 60µA, t
______________________________________________________________________________________
SH
with Accurate Programmable Oscillator
≅
. Typical values for t
0 595
.
SH
is the desired fault-integra-
×
t
Internal Regulators
RT
CC
C
FLTINT
CC
and REG5 with 1µF
powers the exter-
SH
Soft-Start
range from
CC
), which
rent. Therefore, select a MOSFET that yields acceptable
conduction and switching losses.
The MAX5068 includes an internal error amplifier that
can regulate the output voltage in the case of a noniso-
lated power supply
age using the following equation:
where V
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.
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
lator frequency and C
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:
where I
MOSFET, V
amplifier, V
V
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
on the secondary and primary side of the transformer,
respectively. L is the secondary filter inductor. When
SCOMP
Slope Compensation (MAX5068C/D/E/F)
I
PRIMARY
PRIMARY
RT
REF
is the ramp function starting at zero and slew-
SR
is the external resistor at RT that sets the oscil-
N
N
OFFSET
EA
= 1.23V. The amplifier’s noninverting input
S
P
V
=
OUT
×
x R
is the output voltage of the internal
is the current through the N-channel
R
K
CS
RT
(Figure 1). Calculate the output volt-
is the 1.6V internal DC offset and
165
SCOMP
=
×
> V
S
×
⎛
⎜
⎝
R
and N
1
×
CS
EA
C
+
L
10
SCOMP
is the capacitor at SCOMP.
– V
×
R
R
−
P
8
9
6
V
OFFSET
are the number of turns
⎞
⎟
⎠
OUT
PWM Comparator
x V
(
Error Amplifier
mV
REF
=
- V
/
µ
SR
s
SCOMP
)
15