max5974 Maxim Integrated Products, Inc., max5974 Datasheet - Page 18

• Manufacturer Part Number: max5974
• Description: Active-clamped, Spread-spectrum, Current-mode Pwm Controllers
• Manufacturer: Maxim Integrated Products, Inc.

Active-Clamped, Spread-Spectrum,
Current-Mode PWM Controllers
The ICs’ switching frequency is programmable between
100kHz and 600kHz with a resistor R
between RT and GND. Use the following formula to
determine the appropriate value of R
erate the desired output-switching frequency (f
where f
The current-sense resistor (R
Application Circuits), connected between the source
of the n-channel MOSFET and PGND, sets the current
limit. The current-limit comparator has a voltage trip level
(V
culate the value of R
where I
the transformer, which also flows through the MOSFET.
When the voltage produced by this current (through the
current-sense resistor) exceeds the current-limit com-
parator threshold, the MOSFET driver (NDRV) terminates
the current on-cycle, within 35ns (typ).
The devices implement 115ns of leading-edge blanking
to ignore leading-edge current spikes. These spikes
are caused by reflected secondary currents, current-
Figure 4. Hiccup Mode Timing Diagram
18
CS-PEAK
SW
PRI
) of 400mV. Use the following equation to cal-
SOFT-START
is the desired switching frequency.
is the peak current in the primary side of
VOLTAGE,
Oscillator/Switching Frequency
V
SS
R
CS
R
(BLANKED CS
RT
CS
VOLTAGE)
:
HICCUP
=
V
=
CSBL
8.7 10
400mV
t
SS
V
f
I
SS-HI
SW
PRI
×
Peak Current Limit
9
CS
RT
in the Typical
needed to gen-
RT
connected
SW
):
discharging capacitance at the FET’s drain, and gate-
charging current. Use a small RC network for additional
filtering of the leading-edge spike on the sense wave-
form when needed. Set the corner frequency between
10MHz and 20MHz.
After the leading-edge blanking time, the device moni-
tors V
400mV. The duty cycle is terminated immediately when
V
The devices protect the transformer against saturation
due to reverse current by monitoring the voltage across
R
is on.
When the device detects eight consecutive peak current-
limit events, both NDRV and AUXDRV driver outputs are
turned off for a restart period, t
device undergoes soft-start. The duration of the restart
period depends on the value of the capacitor at SS (C
During this period, C
rent of I
the restart period ends and the device initiates a soft-start
sequence. An internal counter ensures that the minimum
restart period (t
time required for C
1024 clock cycles. Figure 4 shows the behavior of the
device prior and during hiccup mode.
CS
CS
exceeds 400mV.
while the AUX output is low and the p-channel FET
CS
SS-DH
for any breaches of the peak current limit of
DISCHARGE WITH I
(2FA typ). Once its voltage reaches 0.15V,
RSTRT-MIN
Output Short-Circuit Protection
SS
SS
t
to discharge to 0.15V is less than
SS-DH
RSTRT
is discharged with a pulldown cur-
V
(400mV)
CS-PEAK
) is 1024 clock cycles when the
Reverse Current Limit
RSTRT
with Hiccup Mode
. After t
V
SS-DTH
RSTRT
, the
SS
).