LTC3603_15 Datasheet, PDF (15/22 Pages) Linear Technology – 2.5A, 15V Monolithic Synchronous Step-Down Regulator

LTC3603
Applications Information
To prevent the LTC3603 from exceeding the maximum
junction temperature, the user will need to do some thermal
analysis. The goal of the thermal analysis is to determine
whether the power dissipated exceeds the maximum
junction temperature of the part. The temperature rise is
given by:
T_R = (P_D) • (q_JA)
where P_D is the power dissipated by the regulator and q_JA
is the thermal resistance from the junction of the die to
the ambient temperature.
Design Example
As a design example, consider using the LTC3603 in
an application with the following specifications: V_IN =
12V, V_OUT = 3.3V, I_OUT(MAX) = 2.5A, I_OUT(MIN) = 100mA,
f = 1MHz. Because efficiency is important at both high and
low load current, Burst Mode operation will be utilized.
First, calculate the timing resistor:
R_OSC
=
1.15 •10¹¹
1MHz
–
10k
=
105k
The junction temperature, T_J, is given by:
T_J = T_A + T_R
Next, calculate the inductor value for about 40% ripple
current at maximum V_IN:
where T_A is the ambient temperature.
As an example, consider the LTC3603 in dropout at an
input voltage of 8V, a load current of 2.5A and an ambient
temperature of 70°C. From the Typical Performance graph
of Switch Resistance, the R_DS(ON) of the top switch at 70°C
is approximately 85mW. Therefore, power dissipated by
the part is:
P_D = (I_LOAD2)(R_DS(ON)) = (2.5A)²(85mW) = 0.53W
L
=
⎛
⎝⎜
3.3V ^⎞
(1MHz )(1A ) ⎠⎟
•
⎛⎝⎜
1–
3.3V
12V
⎞⎠⎟
=
2.39µH
Using a 2.2µH inductor results in a maximum ripple cur-
rent of:
ΔI_L
=
⎛
⎝⎜
3.3V ^⎞
(1MHz )(2.2µH) ⎠⎟
• ^{⎛⎝⎜}
1–
3.3V
12V
⎞⎠⎟
=
1.1A
For the MSOP package, the q_JA is 45°C/W. Thus, the
junction temperature of the regulator is:
T_J = 70°C + (0.53W)(45°C/W) = 93.85°C
which is below the maximum junction temperature of
125°C.
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 DI_LOAD•(ESR), where ESR is the effective series
resistance of C_OUT. DI_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 the
front page application will provide adequate compensation
for most applications.
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. In this applica-
tion, a tantalum capacitor will be used to provide the bulk
capacitance and a ceramic capacitor in parallel to lower
the total effective ESR. For this design, a 100µF ceramic
capacitor will be used. C_IN should be sized for a maximum
current rating of:
I_RMS
=
2.5A
•
3.3V
12V
•
12V
3.3V
–
1
=
1.12A_RMS
Decoupling the PV_IN pin with a 22µF ceramic capacitor is
adequate for most applications.
The output voltage can now be programmed by choosing
the values of R1 and R2. Choose R1 = 105k and calculate
R2 as:
R2 = R1^{⎛⎝⎜}
V_OUT
0.6V
– 1^{⎞⎠⎟}
=
472.5k
3603fc
For more information www.linear.com/LTC3603
15