max5069aauet Maxim Integrated Products, Inc., max5069aauet Datasheet - Page 15

• Manufacturer Part Number: max5069aauet
• Description: Max5069 High-frequency, Current-mode Pwm Controller With Accurate Oscillator And Dual Fet Drivers
• Manufacturer: Maxim Integrated Products, Inc.

Note that cycle-by-cycle current limiting is provided at
all times by CS with a threshold of 314mV (typ). The
fault-integration circuit forces a 60µA current onto
FLTINT each time that the current-limit comparator is
tripped (see the Functional Diagram). Use the following
formula to calculate the value of the capacitor neces-
sary for the desired shutdown time of the circuit:
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 MAX5069 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 MAX5069 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 MAX5069 inter-
nal and external control circuits. V
nal N-channel MOSFETs 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 MAX5069 is in the
power-up mode. Overloading the outputs may cause
the MAX5069 to fail upon startup.
High-Frequency, Current-Mode PWM Controller
with Accurate Oscillator and Dual FET Drivers
RT
FLTINT
RT
is the desired recovery time.
= 10 x t
R
FLTINT
C
= 60µA, t
FLTINT
______________________________________________________________________________________
SH
≅
. Typical values for t
≅
0 595
SH
.
I
FLTINT
is the desired fault-integra-
Internal Regulators
×
2 8
CC
t
.
RT
C
V
CC
x t
and REG5 with 1µF
FLTINT
SH
powers the exter-
SH
Soft-Start
range from
The MAX5069 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 MAX5069 uses an internal-ramp generator for
slope compensation. The internal-ramp signal resets at
the beginning of each cycle and slews at the rate pro-
grammed by the external capacitor connected at
SCOMP and the resistor at RT. Adjust the MAX5069
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 NDRVA and NDRVB off. In
normal operation, the N-channel MOSFETs turns off
when:
where I
MOSFETs, V
amplifier, V
V
ing at the programmed slew rate (SR). When using the
MAX5069 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
optimally compensated, the current loop responds to
input-voltage transients within one cycle.
SCOMP
I
PRIMARY
PRIMARY
RT
REF
is the ramp function starting at zero and slew-
is the external resistor at RT that sets the oscil-
SR
OFFSET
N
N
= 1.23V. The amplifier’s noninverting input
EA
S
P
V
=
OUT
x R
×
is the output voltage of the internal
is the current through the N-channel
R
CS
K
(Figure
RT
is the 1.6V internal DC offset, and
165
SCOMP
=
×
> V
S
×
⎛
⎜
⎝
and N
R
1
×
EA
CS
C
+
1). Calculate the output volt-
10
SCOMP
L
is the capacitor at SCOMP.
– V
Slope Compensation
R
×
R
P
−
10
6
9
OFFSET
are the number of turns
V
PWM Comparator
⎞
⎟
⎠
OUT
x V
(
Error Amplifier
mV
REF
=
- V
/
µ
SR
SCOMP
s
)
15