LTC3602_15 Datasheet, PDF (14/20 Pages) Linear Technology – 2.5A, 10V, Monolithic Synchronous Step-Down Regulator

LTC3602
APPLICATIONS INFORMATION
Thermal Considerations
In most applications, the LTC3602 does not dissipate much
heat due to its high efficiency. But, in applications where the
LTC3602 is running at high ambient temperature with low
supply voltage and high duty cycles, such as in dropout,
the heat dissipated may exceed the maximum junction
temperature of the part. If the junction temperature reaches
approximately 150°C, both power switches will be turned
off and the SW node will become high impedance.
To prevent the LTC3602 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) • (θ_JA)
where P_D is the power dissipated by the regulator and θ_JA
is the thermal resistance from the junction of the die to
the ambient temperature.
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 LTC3602 in dropout at an
input voltage of 8V, a load current of 2.5A and an ambi-
ent 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 120mΩ. Therefore, power dis-
sipated by the part is:
P_D = (I_LOAD2)(R_DS(ON)) = (2.5A)²(120mΩ) = 0.75W
For the TSSOP package, the θ_JA is 38°C/W. Thus the junc-
tion temperature of the regulator is:
T_J = 70°C + (0.75W)(38°C/W) = 98.5°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 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 the
front page application will provide adequate compensation
for most applications.
Design Example
As a design example, consider using the LTC3602 in
an application with the following specifications: V_IN =
8.4V, 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
Next, calculate the inductor value for about 40% ripple
current at maximum V_IN:
L
=
⎛
⎝⎜⎜
3.3V
(^1MHz ) (^1A )
⎞
⎠⎟⎟
•
⎛⎝⎜_1–
3.3V
8.4V
⎞
⎠⎟
=
2µH
Using a 2.2μH inductor results in a maximum ripple cur-
rent of:
ΔI_L
=
⎛
⎝⎜⎜
3.3V
(^1MHz ) (^2.2µH)
⎞
⎠⎟⎟
•
⎛
⎝⎜^1–
3.3V
8.4V
⎞
⎠⎟
=
0.91A
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
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