lt1683 Linear Technology Corporation, lt1683 Datasheet - Page 16

• Manufacturer Part Number: lt1683
• Description: Slew Rate Controlled Ultralow Noise Push-pull Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1683
So if we wanted to turn on at 20V with 2V of hysteresis:
Resistor values could be altered further by adding Zeners
in the divider string. A resistor in series with SHDN pin
could further change hysteresis without changing turn on
voltage.
Frequency Compensation
Loop frequency compensation is accomplished by way of
a series RC network on the output of the error amplifier
(V
Referring to Figure 6, the main pole is formed by capacitor
C
(approximately 400kΩ). The series resistor R
“zero” which improves loop stability and transient re-
sponse. A second capacitor C
size of the main compensation capacitor, is sometimes
used to reduce the switching frequency ripple on the V
pin. V
attenuated by the output divider and multiplied by the error
amplifier. Without the second capacitor, V
where V
gm = Error amplifier transconductance
R
V
16
VC
OUT
VC
C
RA
RB
V
pin).
CPINRIPPLE
= Series resistor on V
and the output impedance of the error amplifier
= DC output voltage
C
=
=
RIPPLE
pin ripple is caused by output voltage ripple
2
2
24
V
V
• .
• .
µ
1 39
1 39
24
A
= Output ripple (V
=
• (
µ
0.01µF
1 25
V
V
20
A
.
U
C
−
• .
−
V
VC
R
1 39
VC
2k
20
20
−
•
V
Figure 6
V
V
1 39
RIPPLE
U
V
.
C
• .
• .
V
pin
0 1
0 1
OUT
V
VC2
V
V
)
C
4.7nF
•
, typically one-tenth the
P-P
=
=
VC2
gm R
1 75
W
23 4
V
)
.
C
1683 F06
•
.
PIN
k
VC
k
C
pin ripple is:
VC
U
creates a
C
To prevent irregular switching, V
kept below 50mV
maximum output load current and will also be increased if
poor quality (high ESR) output capacitors are used. The
addition of a 0.0047µF capacitor for C
switching frequency ripple to only a few millivolts. A low
value for R
margin may be inadequate.
Setting Current Limit
The sense resistor sets the value for maximum operating
current. When the CS pin voltage is 0.1V the gate drivers
will immediately go low (no slew control). Therefore the
sense resistor value should be set to R
where I
I
values and tolerances. Certainly it should be set below the
saturation current value for the transformer.
If CS pin voltage is 0.22V in addition to the drivers going
low, V
provide additional protection in the event of a short circuit.
By discharging V
as hard on subsequent cycles since the current trip will be
set lower.
Turn off of the MOSFETs will normally be inhibited for about
100ns at the start of every turn on cycle. This is to prevent
noise from interfering with normal operation of the control-
ler. This current sense blanking does not prevent the out-
puts from be turned off in the event of a fault. Slewing of
the gate voltage effectively provides additional blanking.
Traces to the SENSE resistor should be kept short and
wide to minimize resistance and inductance. Large
interwinding capacitance in the transformer or high ca-
pacitance on the drain of the MOSFETs will produce a
current pulse through the sense resistor during drain volt-
age slewing. The magnitude of the pulse is C • dV/dt where
C is the capacitance and dV/dt is the voltage slew rate
which is controlled by the part. This pulse will increase the
sensed current on switch turn on and can cause premature
MOSFET turn off. If this occurs, the transformer may need
a different winding technique (see AN39) or alternatively,
a blanking circuit can be used. Please contact the LTC
applications group for support if required.
SW(PEAK)
C
SW(PEAK)
and SS will be discharged to ground. This is to
will depend on the topology and component
VC
will also reduce V
C
P-P
is the peak current in the MOSFETs.
and SS the MOSFET will not be stressed
. Worst-case V
C
pin ripple, but loop phase
C
C
pin ripple should be
pin ripple occurs at
S
= 0.1V/I
VC2
pin reduces
SW(PEAK)
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