LTC1147-3.3_15 Datasheet, PDF (12/16 Pages) Linear Technology – High Efficiency Step-Down Switching Regulator Controllers

LTC1147-3.3
LTC1147-5/LTC1147L
APPLICATIO S I FOR ATIO
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)(10⁴) = 220pF
L = (5.1)(10⁵)(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:
5(0.25)
P-Ch R_DS(ON) = _{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:
12
) ^{fMIN =}
1
2.61µs
1–
3.3
4.5
= 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.
3.3V
0V
Figure 6a. Continuous Mode Operation C_T Waveform
3.3V
Figure 6b. Burst Mode Operation C_T Waveform
0V
LTC1147 • F06
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.
sn1147 1147fds