LTC3565_1 Datasheet, PDF (17/22 Pages) Linear Technology – 1.25A, 4MHz, Synchronous Step-Down DC/DC Converter

LTC3565
Applications Information
losses including diode conduction losses during dead-time
and inductor core losses, which generally account for less
than 2% total additional loss.
Thermal Considerations
In a majority of applications, the LTC3565 does not dis-
sipate much heat due to its high efficiency. However, in
applications where the LTC3565 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 avoid the LTC3565 from exceeding the maximum junc-
tion 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_RISE = 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_RISE + T_AMBIENT
As an example, consider the case when the LTC3565 is
in dropout at an input voltage of 3.3V with a load current
of 1A. From the Typical Performance Characteristics
graph of Switch Resistance, the R_DS(ON) resistance of the
P‑channel switch is 0.160Ω. Therefore, power dissipated
by the part is:
P_D = I_OUT2 • R_DS(ON) = 160mW
The MSE package junction-to-ambient thermal resistance,
θ_JA, will be in the range of about 40°C/W. Therefore, the
junction temperature of the regulator operating in a 70°C
ambient temperature is approximately:
T_J = 0.16 • 40 + 70 = 76.4°C
Remembering that the above junction temperature is
obtained from an R_DS(ON) at 25°C, we might recalculate
the junction temperature based on a higher R_DS(ON) since
it increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
Design Example
As a design example, consider using the LTC3565 in a
portable application with a Li-Ion battery. The battery pro-
vides a V_IN = 2.5V to 4.2V. The load requires a maximum
of 1.25A in active mode and 10mA in standby mode. The
output voltage is V_OUT = 2.5V. Since the load still needs
power in standby, Burst Mode operation is selected for
good low load efficiency.
First, calculate the timing resistor for 1MHz operation:
R_T = 1.21 • 10⁶ (10³)^–1.2674 = 190.8k
Use a standard value of 191k. Next, calculate the inductor
value for about 40% ripple current at maximum V_IN:
L
=
2.5V
1MHz • 500mA
•
_1−

2.5V^
4.2V
=
2µH
Choosing the closest inductor from a vendor of 2.2µH,
results in a maximum ripple current of:
ΔI_L
=
2.5V
1MHz • 2.2µH
•
_1−

2.5V^
4.2V


=
460mA
For cost reasons, a ceramic capacitor will be used. C_OUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
C_OUT
≈
2.5
1MHz
1.25A
• (5% •
2.5V)
=
25µF
The closest standard value is 22µF. Since the output
impedance of a Li-Ion battery is very low, C_IN is typically
22µF. In noisy environments, decoupling SV_IN from PV_IN
with an R6/C8 filter of 1Ω/0.1µF may help, but is typically
not needed.
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