Datasheet

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.