Datasheet

12
LTC1147-3.3
LTC1147-5/LTC1147L
sn1147 1147fds
APPLICATIO S I FOR ATIO
WUU
U
f
MIN
=
3.3
4.5
) )
1 –
1
2.61µs
= 102kHz
P
P
=
3.3(0.125Ω)(1A)
2
(1.27)
4.5
= 116mW
This last step is necessary to assure that the power
dissipation and junction temperature of the P-channel are
not exceeded.
Troubleshooting Hints
Since efficiency is critical to LTC1147 series applications,
it is very important to verify that the circuit is functioning
correctly in both continuous and Burst Mode operation.
The waveform to monitor is the voltage on the timing
capacitor Pin 2.
In continuous mode (I
LOAD
> I
BURST
) the voltage on the C
T
pin should be a sawtooth with a 0.9V
P-P
swing. This
voltage should never dip below 2V as shown in Figure 6a.
When load currents are low (I
LOAD
< I
BURST
) Burst Mode
operation occurs. The voltage on the C
T
pin now falls to
ground for periods of time as shown in Figure 6b. During
this time the LTC1147 series are in sleep mode with the
quiescent current reduced to 160µA.
The inductor current should also be monitored. Look to
verify that the peak-to-peak ripple current in continuous
mode operation is approximately the same as in Burst
Mode operation.
Other losses including C
IN
and C
OUT
ESR dissipative
losses, MOSFET switching losses, and inductor core losses,
generally account for less than 2% total additional loss.
Design Example
As a design example, assume V
IN
= 5V (nominal), V
OUT
=
3.3V, I
MAX
= 1A, and f = 130kHz; R
SENSE
, C
T
and L can
immediately be calculated:
R
SENSE
= 100mV/1A = 0.1Ω
t
OFF
= (1/130kHz)[1 – (3.3/5)] = 2.61µs
C
T
= 2.61µs/(1.3)(10
4
) = 220pF
L = (5.1)(10
5
)(0.1Ω)(220pF)(3.3V) = 33µH
Assume that the MOSFET dissipation is to be limited to
P
P
= 250mW.
If T
A
= 50°C and the thermal resistance of the MOSFET is
50°C/W, then the junction temperatures will be 63°C and
δ
P
= 0.007(63 – 25) = 0.27. The required R
DS(ON)
for the
MOSFET can now be calculated:
P-Ch R
DS(ON)
=
5(0.25)
3.3(1)
2
(1.27)
= 0.3Ω
The P-channel requirement can be met by a Si9430DY.
Note that the most stringent requirement for the Schottky
diode is with V
OUT
= 0 (i.e., short circuit). During a
continuous short circuit, the worst-case Schottky diode
dissipation rises to:
P
D
= I
SC(AVG)
(V
D
)
With the 0.1Ω sense resistor I
SC(AVG)
= 1A will result,
increasing the 0.4V Schottky diode dissipation to 0.4W.
C
IN
will require an RMS current rating of at least 0.5A at
temperature, and C
OUT
will require an ESR of 0.1Ω 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 the
power dissipation to increase. At V
IN(MIN)
= 4.5V, the
frequency will decrease and the P-channel will be con-
ducting most of the time causing its power dissipation to
increase. At V
IN(MIN)
= 4.5V:
If Pin 2 is observed falling to ground at high output
currents, it indicates poor decoupling or improper ground-
ing. Refer to the Board Layout Checklist.
3.3V
0V
LTC1147 • F06
Figure 6b. Burst Mode Operation C
T
Waveform
3.3V
0V
Figure 6a. Continuous Mode Operation C
T
Waveform