Rainbow Electronics MAX5043 - Manual (Page 17)

Maximize the signal-to-noise ratio by setting the ramp
peak as high as possible. Calculate the low-frequency,
small-signal gain of the power stage (the gain from the
inverting input of the PWM comparator to the output)
using the following formula:
G PS = N SP ✕ R RCFF ✕ C RCFF ✕ f S
where N SP = the secondary to primary power transformer
turns ratio.
Current-Sense Amplifier and Current-Mode Control
The MAX5042/MAX5043 can also be programmed for
current-mode control (see Figure 6). This control
method offers beneficial advantages for certain appli-
cations. Current-mode control reduces the order of the
output filter, allowing easier control-loop compensation.
In current-mode control, the voltage across the current-
sense resistor at SRC is amplified by the internal gain-
of-10 amplifier IAMP. The cycle-by-cycle current-limit
threshold is 156mV. This is the peak voltage amplified
by IAMP. A 200mV offset is added to this voltage. The
voltage at the output of the current-sense amplifier is:
V
CSOUT
= 2 + 10(V
CSP
- V
CSN
)
The low-frequency, small-signal gain of the power
stage (the gain from the inverting input of the PWM
comparator to the output) can be calculated using the
following formula:
where N PS = the primary to secondary power trans-
former turns ratio,
R L = the low-frequency output impedance,
R SENSE = the primary current-sense resistor value.
Oscillator and Synchronization
Program the MAX5042/MAX5043 oscillator using an RC
network at RCOSC with the resistor connected to REG5
and the capacitor connected to PWMNEG. The PWM
frequency is half the frequency at RCOSC.
Use the following formula to calculate the oscillator
components:
where C PCB = 14pF,
REG5 = 5V,
f S = switching frequency,
V TH = RCOSC peak trip level.
The delay programmed by the resistor at DRVDEL lim-
its the power MOSFET’s maximum duty cycle to less
than 50 percent.
SYNC allows synchronization of the MAX5042/MAX5043
to an external clock. For proper synchronization, set the
external SYNC frequency 15% to 20% higher than the
programmed free-running frequency of the MAX5042/
MAX5043’s internal oscillator. The actual switching
frequency will be half the synchronizing frequency.
Integrating Fault Protection
The integrating fault protection feature allows the
MAX5042/MAX5043 to ignore transient overcurrent
conditions for a programmable amount of time, giving
the power supply time to behave like a current source
to the load. This can happen, for example, under load-
current transients when the control loop requests maxi-
mum current to keep the output voltage from going out
of regulation. Program the ignore time externally by
connecting a capacitor to FLTINT. Under sustained
overcurrent faults, the voltage across this capacitor
ramps up toward the FLTINT shutdown threshold (typi-
cally 2.7V). When FLTINT reaches the threshold, the
power supply shuts down. A high-value bleed resistor
connected in parallel with the FLTINT capacitor allows
the capacitor to discharge toward the restart threshold
(typically 1.8V). Crossing the restart threshold soft-
starts the supply again.
The ILIM comparator provides cycle-by-cycle current
limiting with a typical threshold of 156mV. The fault inte-
gration circuit works by forcing an 80µA current out of
FLTINT for one clock cycle every time the current-limit
comparator (Figures 3 and 4, ILIM) trips. Use the fol-
lowing formula to calculate the approximate capaci-
tance (C
FLTINT ) needed for the desired shutdown time.
R
f C C
V
V V
RCOSC
S RCOSC PCB
REG
REG TH
ln
=
+ ( )






−
1
2
5
5
G N
R
R
PS PS
L
SENSE
= ×
MAX5042/MAX5043
Two-Switch Power ICs with Integrated
Power MOSFETs and Hot-Swap Controller
______________________________________________________________________________________ 17
CSP
PWMNEG
CSN
PWMPNEG
SRC
RAMP
CSOUT
OPTO
MAX5042/MAX5043
RS
50mΩ
(APPROXIMATELY
35W TO 40W)
Figure 6. Simplified Connection Diagram for Current-Mode
Control