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LT1683EG#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LT1683
16
1683fd
APPLICATIONS INFORMATION
So if we wanted to turn on at 20V with 2V of hysteresis:
RA =
2V •1.39V − 20V • 0.1V
24µA •1.39V
= 23.4k
RB=
2V •1.39V − 20V • 0.1V
24µA •(20V − 1.39V)
= 1.75k
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
C
pin).
V
C
PIN
1683 F06
R
VC
2k
C
VC
0.01µF
C
VC2
4.7nF
Figure 6
Referring to Figure 6, the main pole is formed by capaci-
tor C
VC
and the output impedance of the error amplifier
(approximately 400kΩ). The series resistor, R
VC
, creates a
“zero” which improves loop stability and transient response.
A second capacitor, C
VC2
, typically one-tenth the size of
the main compensation capacitor, is sometimes used to
reduce the switching frequency ripple on the V
C
pin. V
C
pin ripple is caused by output voltage ripple attenuated
by the output divider and multiplied by the error amplifier.
Without the second capacitor, V
C
pin ripple is:
V
CPINRIPPLE
=
1.25• V
RIPPLE
• gm •R
VC
V
OUT
where V
RIPPLE
= Output ripple (V
P-P
)
gm = Error amplifier transconductance
R
VC
= Series resistor on V
C
pin
V
OUT
= DC output voltage
To prevent irregular switching, V
C
pin ripple should be
kept below 50mV
P-P
. Worst-case V
C
pin ripple occurs at
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
VC2
pin reduces
switching frequency ripple to only a few millivolts. A low
value for R
VC
will also reduce V
C
pin ripple, but loop phase
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
S
= 0.1V/I
SW(PEAK)
, where
I
SW(PEAK)
is the peak current in the MOSFETs. I
SW(PEAK)
will depend on the topology and component 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
C
and SS will be discharged to ground. This is to
provide additional protection in the event of a short cir-
cuit. By discharging V
C
and SS, the MOSFET will not be
stressed 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 controller. This current sense blanking does not
prevent the outputs 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
capacitance on the drain of the MOSFETs will produce
a current pulse through the sense resistor during drain
voltage 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 pre-
mature 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.
LT1683
17
1683fd
Soft-Start
The soft-start pin is used to provide control of switching
current during start-up. The max voltage on the V
C
pin is
approximately the voltage on the SS pin. A current source
will linearly charge a capacitor on the SS pin. The V
C
pin
voltage will thus ramp also. The approximate time for the
voltage on these pins to ramp is (1.31V/9µA) • C
SS
or
approximately 146ms per µF.
The soft-start current will be initiated as soon as the part
turns on. Soft-start will be reinititated after a short-circuit
fault.
Thermal Considerations
Most of the IC power dissipation is derived from the V
IN
pin. The V
IN
current depends on a number of factors in-
cluding: oscillator frequency; loads on V5; slew settings;
gate charge current. Additional power is dissipated if V5
sinks current and during the MOSFET gate discharge.
The power dissipation in the IC will be the sum of:
1) The RMS V
IN
current times V
IN
2) V5 RMS sink current times 5V
3) The gate drive’s RMS discharge current times voltage
Because of the strong V
IN
component it is advantageous
to operate the LT1683 at as low a V
IN
as possible.
It is always recommended that package temperature be
measured in each application. The part has an internal
thermal shutdown to minimize the chance of IC destruction
but this should not replace careful thermal design.
The thermal shutdown feature does not protect the external
MOSFETs. A separate analysis must be done for those
devices to ensure that they are operating within safe limits.
Once IC power dissipation, P
DIS
, is determined die junction
temperature is then computed as:
T
J
= T
AMB
+ P
DIS
• θ
JA
where T
AMB
is ambient temperature and θ
JA
is the package
thermal resistance. For the 20-pin SSOP, θ
JA
is 100°C/W.
APPLICATIONS INFORMATION
Magnetics
Design of magnetics is dependent on topology. The fol-
lowing details the design of the magnetics for a push-pull
converter. In this converter the transformer usually stores
little energy. The following equations should be considered
as the starting point to building a prototype.
The following definitions will be used:
V
IN
= Input supply voltage
R
ON
= Switch-on resistance
I
SW
= Maximum switch current
V
OUT
= Desired output voltage
I
OUT
= Output current
f = Oscillator frequency
V
F
= Forward drop of the rectifier
Duty cycle is the major defining equation for this topology.
Note that the output L and C basically filter the chopped
voltage so duty cycle controls output voltage. N is the
turns ratio of the transformer. The turns ratio must be
large enough to ensure that the transformer can put out
a voltage equal to the output voltage plus the diode under
minimum input conditions. Note the transformer operates
at half the oscillator frequency (f).
N=
V
OUT
+ V
F
2•DC
MAX
( )
V
IN(MIN)
−I
SW
R
ON
+R
SENSE
( )
DC
MAX
is the maximum duty cycle of each driver with
respect to the entire cycle, which consists of two periods
(A on and B on). So the effective duty cycle is 2 • DC
MAX
.
The controller, in general, determines maximum duty
cycle. A 44% maximum duty cycle is a guaranteed value
for this part.
Remember to add sufficient margin in the turns ratio to
account for IR drops in the transformer windings, worst-
case diode forward drops and switch-on voltage. Also at
very slow slew rates the effective DC may be reduced.
LT1683
18
1683fd
There are a number of ways to choose the inductance
value for L. We suggest as a starting point that L be selected
such that the converter is continuous at I
OUT(MAX)
/4. If your
minimum I
OUT
is higher than this or your components can
handle higher peak currents then use a higher number.
APPLICATIONS INFORMATION
1:N
L
PRI
D2
D1
L
C R
OUT
R
SENSE
V
OUT
V
IN
1683 F07
Figure 7. Push-Pull Topology
Continuous operation occurs when the current in the
inductor never goes to zero. Discontinuous operation
occurs when the inductor current drops to zero before the
start of the next cycle and can occur with small inductors
and light loads. There is nothing inherently bad about
discontinuous operation, however, converter control and
operation are somewhat different. The inductor is smaller
for discontinuous operation but the peak currents in the
switch, the transformer, the diodes, inductor and capacitor
will be higher which may produce greater losses.
For continuous operation the inductor ripple current must
be less than twice the output current. The worst case for
this is at maximum input (lowest duty cycle, DC
MIN
) but
in the following we will evaluate at nominal input since
the I
OUT
/4 is somewhat arbitrary.
Note when both inputs are off, the inductor current splits
between both secondary outputs and the diode common
goes to 0V.
Looking at the inductor current during off time, output
ripple current is:
∆I
OUT
= 2 •I
OUT(MIN)
I
OUT(MIN)
= I
OUT(MAX)
/ 4
L =
V
OUT(MIN)
+ V
F
( )
• 1− 2 • DC
( )
∆I
OUT
• f
The inductance of the transformer primary should be such
that L, when reflected into the primary, dominates the
input current. In other words, we want the magnetizing
current of the transformer small with respect to the current
going through the transformer to L. In general, then, the
inductance of the primary should be at least five times that
of L reflected to the input. This ensures that most of the
power will be passed through the transformer to the load.
It also increases the power capability of the converter and
reduces the peak currents that the switch will see.
L
PRI
=
5•L
N
2
If the magnetizing current is small, say below 100mA, then
a smaller L can be used with a higher percentage of the
switch current generated by the magnetizing current.
With the value of L set, the ripple in the inductor is:
∆I
L
=
V
OUT
+ V
F
( )
• 1− 2 •DC
( )
L • f
However, the peak inductor current is evaluated at maxi-
mum load and maximum input voltage (minimum DC).
I
L(MAX)
=I
OUT(MAX)
+
∆I
L(MAX)
2
The magnetizing ripple current can be shown to be:
∆I
MAG
=
V
OUT
+ V
F
N•L
PRI
• f
and the peak current in the switch is:
I
SW(PEAK)
= N • I
L(MAX)
+ ∆I
MAG
This current should be less than the current limit.
Worst-case switch ripple is:
∆I
SW(PEAK)
= N • ∆I
L(MAX)
+ ∆I
MAG
In the push-pull converter the maximum switch voltage
will be 2 • V
IN
. Because voltage is slew-controlled, the
leakage spikes are small. So, the MOSFET should have a
maximum rated switch voltage at least 20% higher than
2 • V
IN
.
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LT1683EG#TRPBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators SR Controlled Ultralow N PP DC/DC Cntr
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