LTC3413IFE Datasheet, PDF (11/16 Pages) Linear Technology – Monolithic Synchronous Regulator

LTC3413
APPLICATIONS INFORMATION
The junction temperature, T_J, is given by:
T_J = T_A + T_R
where T_A is the ambient temperature.
As an example, consider the LTC3413 in dropout at an
input voltage of 3.3V, a load current of 3A and an ambi-
ent temperature of 70°C. From the Typical Performance
graph of switch resistance, the R_DS(ON) of the P-channel
switch at 70°C is approximately 97mΩ. Therefore, power
dissipated by the part is:
P_D = (I_LOAD2)(R_DS(ON)) = (3A)²(97mΩ) = 0.87W
For the TSSOP package, the θ_JA is 38°C/W. Thus the junc-
tion temperature of the regulator is:
T_J = 70°C + (0.87W)(38°C/W) = 103°C
which is below the maximum junction temperature of
125°C.
Note that at higher supply voltages, the junction temperature
is lower due to reduced switch resistance (R_DS(ON)).
Checking Transient Response
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V_OUT immediately shifts by an amount
equal to ΔI_LOAD(ESR), where ESR is the effective series
resistance of C_OUT. ΔI_LOAD also begins to charge or dis-
charge C_OUT generating a feedback error signal used by the
regulator to return V_OUT to its steady-state value. During
this recovery time, V_OUT can be monitored for overshoot
or ringing that would indicate a stability problem. The I_TH
pin external components and output capacitor shown in
Figure 1a will provide adequate compensation for most
applications.
Output Voltage Tracking of V_REF
For applications in which the V_REF pin is connected to
the V_IN pin, the output voltage will be equal to one-half
of the voltage on the V_IN pin. Because the output voltage
will track the input voltage, any disturbance on V_IN will
appear on V_OUT. For example, a load step transient could
cause the input voltage to drop if there is insufficient bulk
capacitance at the V_IN pin. The corresponding drop in the
output voltage during the load step transient is caused by
the V_OUT tracking of V_IN and should not be confused with
poor load regulation.
Design Example
As a design example, consider using the LTC3413 in an
application with the following specifications: V_IN = 2.5V,
V_OUT = 1.25V, I_OUT(MAX) = ±3A, f = 1MHz.
First, calculate the timing resistor:
R_OSC
=
3.23 • 10¹¹
1• 10⁶
–
10kΩ
=
313kΩ
Use a standard value of 309k. Next, calculate the inductor
value for about 40% ripple current:
L
=
⎛ 1.25V ⎞
⎝⎜1MHz • 1.2A⎠⎟
⎛⎝⎜_1–
1.25V⎞
2.5V ⎠⎟
=
0.47μH
Using a 0.47μH inductor results in a maximum ripple
current of:
ΔI_L
=
⎛ 1.25V ⎞
⎝⎜1MHz • 0.47μH⎠⎟
⎛⎝⎜_1–
1.25V⎞
2.5V ⎠⎟
=
1.33A
C_OUT will be selected based on the ESR that is required
to satisfy the output voltage ripple requirement and the
bulk capacitance needed for loop stability. For this design,
two 100μF ceramic capacitors will be used. C_IN should be
sized for a maximum current rating of:
I_RMS
=
3A
⎛ 1.25V⎞
⎝⎜ 2.5V ⎠⎟
2.5V
1.25V
–
1
=
1.5A_RMS
Decoupling the PV_IN pins with two 100μF capacitors is
adequate for most applications. Connect the V_REF pin
directly to SV_IN. Connecting the V_FB pin directly to V_OUT
will set the output voltage equal to one-half of the volt-
age on the V_REF pin. The complete circuit for this design
example is illustrated in Figure 3.
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