MAX15004_11 Datasheet, PDF (21/27 Pages) Maxim Integrated Products – 4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers

4.5V to 40V Input Automotive
Flyback/Boost/SEPIC Power-Supply Controllers
MOSFET, Diode, and Series Capacitor Selection
in a SEPIC Converter
For the SEPIC configuration, choose an n-channel
MOSFET with a V_DS rating at least 20% higher than the
sum of the output and input voltages. When operating
at a high switching frequency, the gate charge and
switching losses become significant. Use low gate-
charge MOSFETs. The RMS current of the MOSFET is:
I_MOS−RMS(A) =
⎡⎣ _{(ILPK )} ²
+
(I_LDC ) ²
+
(I_LPK
×
I_LDC)
⎤
⎦
×
D_MAX
3
where I_LDC = (I_LPK - ΔI_L).
Use Schottky diodes for higher conversion efficiency.
The reverse voltage rating of the Schottky diode must
be higher than the sum of the maximum input voltage
(V_IN-MAX) and the output voltage. Since the average
current flowing through the diode is equal to the output
current, choose the diode with forward current rating of
I_OUT-MAX. The current sense (R_S) can be calculated
using the current-limit threshold (0.305V) of
MAX15004/MAX15005 and I_LPK. Use a diode with a for-
ward current rating more than the maximum output cur-
rent limit if the SEPIC converter needs to be output
short-circuit protected.
R _CS
=
0.305
I_LPK
Select R_CS 20% below the value calculated above.
Calculate the output current limit using the following
equation:
( ) I_OUT−LIM
=
⎡
⎣⎢
D
(1− D)
×
I_LPK − ΔI_L
⎤
⎦⎥
where D is the duty cycle at the highest input voltage
(V_IN-MAX).
The series capacitor should be chosen for minimum rip-
ple voltage (ΔV_CP) across the capacitor. We recommend
using a maximum ripple ΔV_CP to be 5% of the minimum
input voltage (V_IN-MIN) when operating at the minimum
input voltage. The multilayer ceramic capacitor X5R and
X7R series are recommended due to their high ripple
current capability and low ESR. Use the following equa-
tion to calculate the series capacitor CP value.
CP
=
⎡
⎢
⎣
I_{OUT −MA X}
ΔV_CP ×
× D_MAX
f_OUT
⎤
⎥
⎦
where ΔV_CP is 0.05 x V_IN-MIN.
For a further discussion of SEPIC converters, go to
http://pdfserv.maxim-ic.com/en/an/AN1051.pdf.
Power Dissipation
The MAX15004/MAX15005 maximum power dissipation
depends on the thermal resistance from the die to the
ambient environment and the ambient temperature. The
thermal resistance depends on the device package,
PCB copper area, other thermal mass, and airflow.
Calculate the temperature rise of the die using following
equation:
T_J = T_C + (P_T x θ_JC)
or
T_J = T_A + (P_T x θ_JA)
where θ_JC is the junction-to-case thermal impedance
(3°C/W) of the 16-pin TSSOP-EP package and P_T is
power dissipated in the device. Solder the exposed
pad of the package to a large copper area to spread
heat through the board surface, minimizing the case-to-
ambient thermal impedance. Measure the temperature
of the copper area near the device (T_C) at worst-case
condition of power dissipation and use 3°C/W as θ_JC
thermal impedance. The case-to-ambient thermal
impedance (θ_JA) is dependent on how well the heat is
transferred from the PCB to the ambient. Use a large
copper area to keep the PCB temperature low. The θ_JA
is 38°C/W for TSSOP-16-EP and 90°C/W for TSSOP-16
package with the condition specified by the JEDEC51
standard for a multilayer board.
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