lt1738 Linear Technology Corporation, lt1738 Datasheet - Page 15

• Manufacturer Part Number: lt1738
• Description: Slew Rate Controlled Ultralow Noise Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Setting Current Limit
The sense resistor sets the value for maximum operating
current. When the CS pin voltage is 0.1V the gate driver
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 inductor.
If the CS pin voltage is 0.22V in addition to the driver 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 MOSFET 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 being 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 MOSFET will produce a current pulse through
the sense resistor during drain voltage slewing. The mag-
nitude 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 if large enough can cause premature MOSFET
turn off. If this occurs, the inductor transformer may need
a different winding technique (see AN39) or alternatively,
a blanking circuit can be used. Please contact the LTC ap-
plications group for support if required.
Soft-Start
The soft-start pin is used to provide control of switching
current during startup. The maximum voltage on the V
pin is approximately the voltage on the SS pin. A current
source will linearly charge a capacitor on the SS pin. The
V
time for the voltage on these pins to ramp up is
(1.31V/9 A) • C
SW(PEAK)
C
pin voltage will thus ramp up also. The approximate
C
SW(PEAK)
and SS will be discharged to ground. This is to
will depend on the topology and component
SS
C
is the peak current in the MOSFET.
and SS the MOSFET will not be stressed
or approximately 146ms per F.
U
U
W
S
= 0.1V/I
U
SW(PEAK)
C
,
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
pin. The V
including: 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
2) V5 RMS sink current times 5V
3) The gate drive’s RMS discharge current times voltage.
Because of the strong V
to operate the LT1738 at as low a V
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 destruc-
tion but this should not replace careful thermal design.
The thermal shutdown feature does not protect the exter-
nal MOSFET. A separate analysis must be done for this
device to insure that it is operating within safe limits.
Once IC power dissipation, P
tion temperature is then computed as:
where T
thermal resistance. For the 20-pin SSOP,
Choosing The Inductor
For a boost converter, inductor selection involves trade-
offs of size, maximum output power, transient response
and filtering characteristics. Higher inductor values pro-
vide more output power and lower input ripple. However,
they are physically larger and can impede transient re-
sponse. Low inductor values have high magnetizing cur-
rent, which can reduce maximum power and increase
input current ripple.
T
J
= T
AMB
AMB
IN
is ambient temperature and
+ P
current depends on a number of factors
IN
current times V
DIS
•
JA
IN
component it is advantageous
DIS
IN
, is determined die junc-
IN
as possible.
JA
JA
is the package
LT1738
is 100 C/W.
15
1738fa
IN