SC1402(2001) View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC1402
(Rev.:2001)
(Rev.:2001)
Semtech Corporation
SC1402 Datasheet PDF : 18 Pages
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SC1402
POWER MANAGEMENT
Application Information (Cont.)
Step 6: From Above Calculations Choose:
Co ≥ 233uF
ESR= 0.022Ω
derating chart for temperature and frequency operation at
300KHz, two of these capacitors in parallel will suffice, as
calculated below:
The RMS ripple current is under a worse-case condition at full
load, 3A each when both SMPSs are on.
Step 7: Check the Ripple Current
If Co can handle the ripple current you’re done. Otherwise
increase the output capacitance to handle the ripple current at
maximum Vin and recheck the ESR using the equation for
determining the actual output capacitance.
If you are using tantalum or electrolytic capacitors you should
increase the capacitance to the level of approaching the
calculated ESR.
If you are using poly capacitors a small resistor in series with
the capacitor to bring the ESR to the desired level may be
necessary for stability due to their low ESR values.
If your ESR value varies significantly from the calculated value
and you don’t want to add more capacitance or add a series
resistor in the capacitor path as described above. We
recommend that you bench test the supply over temperature
to verify transient response and operation of the SMPS.
When the 5V output is at maximum ripple of 1.5A (D = 50%),
the 3.3V output adds 1.41A of ripple current.
The maximum ripple current is then calculated by:
IRMS(MAX) = 1.5 2 + 1.412 = 2.06A
Conversely:
When the 3V output is at maximum ripple 1.5A (D = 50%), the
5V output adds 1.29A of ripple current.
The worse case ripple current is then calculated by:
IRMS(MAX) = 1.5 2 + 1.29 2 = 1.98A
Clearly, the combined input capacitor bank must be chosen to
handle 2A of ripple current under worse-case conditions.
MOSFET Switches
Input Capacitor Selection
Input capacitor is selected based upon the input ripple current
demand of the converter. First determine the input ripple current
expected and then choose a capacitor to meet that demand.
The input RMS ripple current can be calculated as follows:
After selecting the voltage and current requirements of each
MOSFET device for the upper and lower switches, the next
step is to determine their power handling capability. For the
EVAL board the IRF7413 met the voltage and current
requirements. These are 30V, 9A FET’s. Based on 85°C ambient
temperature, 150°C junction temperature and thermal
resistance, their power handling is calculated as follows:
IRMS =
VOUT
•
(VIN
−
VOUT
)
•
IOUT
VIN
The worse case input RMS ripple current occurs at 50% duty
cycle (D = 0.5 or Vin = 2 Vout) and therefore under this condition
the IRMS ripple current can be approximated by:
IRMS
=
ILOAD
2
Therefore, for a maximum load current of 3.0A , the input
capacitors should be able to safely handle 1.5A of ripple current.
For the EVAL board there are two such regulators that operate
simultaneously. Each capable of 1.5A of ripple current, although
it is impossible for both regulators to be at 50% duty cycle at
the same time since they have different output voltages. For
the EVAL board, we chose four 10uF, 30V OS-CON capacitors,
two for each supply. Each capacitor has a ripple current
capability of 1.38A at 100KHz, 45°C. Following the capacitor-
Power Limit for Upper & Lower FET:
TJ = 150°C; TA = 85°C; θja = 50°C/W
PT
=
TJ − TA
θ JA
=
150 −
50
85
= 1.3W
Each FET must not exceed 1.3W of power dissipation. The
conduction losses for the upper & lower FET can be determined.
For the calculations below, a nominal input voltage of 12V, for
Vout = 3.3V, Iout = 3A and f = 300KHz. The Rdson value for the
upper & lower FET is 11mΩ. We will calculate the conduction
losses and switching losses for each FET. From the
calculations below we are well within the 1.3W dissipation limit
as calculated above.
2001 Semtech Corp.
14
www.semtech.com
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