Rainbow Electronics MAX5069 User Manual
Page 15
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
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 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.
Internal Regulators
Two internal linear regulators power the MAX5069 inter-
nal and external control circuits. V
CC
powers the exter-
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
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 MAX5069 is in the
power-up mode. Overloading the outputs may cause
the MAX5069 to fail upon startup.
Error Amplifier
The MAX5069 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
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
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 NDRVA and NDRVB off. In
normal operation, the N-channel MOSFETs turns off
when:
I
PRIMARY
x R
CS
> V
EA
– V
OFFSET
- V
SCOMP
where I
PRIMARY
is the current through the N-channel
MOSFETs, 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
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
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
optimally compensated, the current loop responds to
input-voltage transients within one cycle.
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
9
10
R
t
C
FLTINT
RT
FLTINT
.
≅
×
0 595
C
I
x t
V
FLTINT
FLTINT
SH
.
≅
2 8
MAX5069
High-Frequency, Current-Mode PWM Controller
with Accurate Oscillator and Dual FET Drivers
______________________________________________________________________________________
15