13
LTC1148
LTC1148-3.3/LTC1148-5
114835fd
APPLICATIO S I FOR ATIO
WUU U
To prevent stray pickup a 100pF capacitor is suggested
across R1 located close to the LTC1148.
For Figure 1 applications with V
OUT
below 2V, or when
R
SENSE
is moved to ground, the current sense comparator
inputs operate near ground. When the current comparator
is operated at less than 2V common mode, the off time
increases approximately 40%, requiring the use of a
smaller timing capacitor C
T
.
Auxiliary Windings – Suppressing Burst Mode
Operation
The LTC1148 synchronous switch removes the normal
limitation that power must be drawn from the inductor
primary winding in order to extract power from auxil-
iary windings. With synchronous switching, auxiliary
outputs may be loaded without regard to the primary
output load, providing that the loop remains in continu-
ous mode operation.
Burst Mode
operation can be suppressed at low output
currents with a simple external network which cancels the
25mV minimum current comparator threshold. This tech-
nique is also useful for eliminating audible noise from
certain types of inductors in high current (I
OUT
> 5A)
applications when they are lightly loaded.
An external offset is put in series with the SENSE
–
pin to
subtract from the built-in 25mV offset. An example of this
technique is shown in Figure 6. Two 100Ω resistors are
inserted in series with the leads from the sense resistor.
and δ
P
= δ
N
= 0.007(63 – 25) = 0.27. The required R
DS(ON)
for each MOSFET can now be calculated:
P-Ch R
DS(ON)
=
12(0.25)
5(2)
2
(1.27)
= 0.12Ω
N-Ch R
DS(ON)
=
12(0.25)
7(2)
2
(1.27)
= 0.085Ω
The P-channel requirement can be met by a Si9430DY,
while the N-channel requirement is exceeded by a
Si9410DY. Note that the most stringent requirement for
the N-channel MOSFET is with V
OUT
= 0 (i.e., short circuit).
During a continuous short circuit, the worst-case
N-channel dissipation rises to:
P
N
= I
SC(AVG)
2
(R
DS(ON)
)(1 + δ
N
)
With the 0.05Ω sense resistor I
SC(AVG)
= 2A will result,
increasing the 0.085Ω N-channel dissipation to 450mW at
a die temperature of 73°C.
C
IN
will require an RMS current rating of at least 1A at
temperature, and C
OUT
will require an ESR of 0.05Ω for
optimum efficiency.
Now allow V
IN
to drop to its minimum value. At lower input
voltages the operating frequency will decrease and the
P-channel will be conducting most of the time, causing its
power dissipation to increase. At V
IN(MIN)
= 7V:
f
MIN
= (1/2.92µs)[1 – (5V/7V)] = 98kHz
P
P
=
5V(0.12Ω)(2A)
2
(1.27)
7V
= 435mW
This last step is necessary to assure that the power
dissipation and junction temperature of the P-channel are
not exceeded.
LTC1148 Adjustable Applications
When an output voltage other than 3.3V or 5V is required,
the LTC1148 adjustable version is used with an external
resistive divider from V
OUT
to V
FB
Pin 9 (see Figure 9). The
regulated voltage is determined by:
V
OUT
= 1.25
)
)
1 +
R2
R1
Figure 6. Suppression of Burst Mode Operation
R
SENSE
1000pF
R2
100Ω
R1
100Ω
R3
+
C
OUT
V
OUT
SENSE
+
(PIN 8)
SENSE
–
(PIN 7)
LTC1148 • F06
