MAX5974A MAXIM [Maxim Integrated Products], MAX5974A Datasheet - Page 21

• Manufacturer Part Number: MAX5974A
• Description: Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers
• Manufacturer: MAXIM [Maxim Integrated Products]

Typically, offline power supplies keep startup times to
less than 500ms even in low-line conditions (85V AC
input for universal offline or 36V DC for telecom applica-
tions). Size the startup resistor, R
maximum startup bias of the device (100FA) and the
charging current for C
within the desired 500ms time period. C
enough charge to deliver current to the device for at
least the soft-start time (t
approximate amount of capacitance required, use the
following formula:
where I
after startup, Q
n-channel and p-channel FETs, f
ing frequency, V
(13V typ), and t
culated as follows:
where V
the application (36V for telecom), V
strap UVLO wake-up level (20V), and I
supply current at startup (150FA max).
Choose a higher value for R
above if a longer startup time can be tolerated in order
to minimize power loss on this resistor.
Traditional clamp circuits prevent transformer saturation
by channeling the magnetizing current (I
former onto a dissipative RC network. To improve effi-
ciency, the active clamp circuit recycles I
magnetizing inductance and clamp capacitor. V
is given by:
where V
the duty cycle. To select n-channel and p-channel FETs
IN
S(MIN)
S
is the voltage of the power source and D is
is the ICs’ internal supply current (1.8mA)
is the minimum input supply voltage for
SS
R
GTOT
HYST
IN
C
is the soft-start time. R
IN
I
≅
V
G
CLAMP
V
=
is the bootstrap UVLO hysteresis
is the total gate charge for the
IN
=
S(MIN)
(I
SS
Q
. C
IN
GTOT SW
) set by C
I
V
IN
START
+
IN
HYST
=
Active Clamp Circuit
I )(t
G
−
must be charged to 20V
1 D
Active-Clamped, Spread-Spectrum,
than the one calculated
V
f
V
−
S
INUVR
SW
IN
SS
, to supply both the
SS
)
INUVR
is the ICs’ switch-
. To calculate the
START
M
M
Current-Mode PWM Controllers
IN
IN
) of the trans-
between the
is the boot-
is then cal-
must store
is the IN
CLAMP
with adequate breakdown voltages, use the maximum
value of V
voltage is at its minimum and the duty cycle is at its
maximum. V
tion is therefore:
where V
source, N
and V
n-channel, and p-channel FETs must have breakdown
voltages exceeding this level.
If feed-forward maximum duty-cycle clamp is used then:
Therefore, V
mum duty clamp is:
The AUX driver controls the p-channel FET through a
level shifter. The level shifter consists of an RC network
(formed by C
in the Typical Application Circuits. Choose R
C
D5 is a small-signal diode with a voltage rating exceed-
ing 25V.
Additionally, C
complex poles formed with magnetizing inductance
(L
bandwidth:
AUX
MAG
so that the time constant exceeds 100/f
O
) and C
V
CLAMP(MAX-NORMAL)
S(MIN)
is the output voltage. The clamp capacitor,
P
CLAMP
=
/N
D
V
CLAMP(MAX-NORMAL)



CLAMP(MAX-FF)
=
2
MAX-FF
1
S
CLAMP(MAX-FF)
AUX
π
2.43
−
CLAMP
CLAMP
is the primary to secondary turns ratio,
is the minimum voltage of the power
2.43 R
L
. V
V
MAG
S
and R
×
CLAMP(MAX)
(
=
×
R
1-D
are 2x to 4x away from the loop
should be chosen such that the
DCLMP1
V
×
2.43
R
MIN
DCLMP1
AUX
C
DCLMP2
CLAMP
R
) and diode D5, as shown
=
during feed-forward maxi-
=
DCLMP2
=



1 D
+
1
1
−
+
R
−
occurs when the input
−
DCLMP2
R
> ×
during normal opera-
V
N
MAX FF
DCLMP2
DCLMP
V
3 f
V
2.43
S
S
N
S(MIN)
×
P
−
BW
V
×
S(MIN)
)
V



O



SW
AUX
. Diode
and
21