Datasheet
Table Of Contents
- FEATURES
- APPLICATIONS
- CONTENTS
- DESCRIPTION
- DEVICE RATINGS
- ABSOLUTE MAXIMUM RATINGS
- RECOMMENDED OPERATING CONDITIONS
- ELECTROSTATIC DISCHARGE (ESD) PROTECTION
- PACKAGE DISSIPATION RATINGS
- ELECTRICAL CHARACTERISTICS
- TYPICAL CHARACTERISTICS
- DEVICE INFORMATION
- BLOCK DIAGRAM
- APPLICATION INFORMATION
- FUNCTIONAL DESCRIPTION
- Voltage Reference
- Oscillator
- Input Undervoltage Lockout (UVLO) and Startup
- Enable and Timed Turn On of the Outputs
- Output Voltage Sequencing
- Soft Start
- Output Voltage Regulation
- Feedback Loop and Inductor-Capacitor (L-C) Filter Selection
- Inductor-Capacitor (L-C) Selection
- Maximum Output Capacitance
- Minimum Output Capacitance
- Modifying The Feedback Loop
- Example: TPS54386 Buck Converter Operating at 12-V Input, 3.3-V Output and 400-mA(P-P) Ripple Current
- Bootstrap for the N-Channel MOSFET
- Light Load Operation
- SW Node Ringing
- Output Overload Protection
- Operating Near Maximum Duty Cycle
- Dual Supply Operation
- Cascading Supply Operation
- Multiphase Operation
- Bypass and FIltering
- Over-Temperature Protection and Junction Temperature Rise
- Power Derating
- PowerPAD Package
- PCB Layout Guidelines
- FUNCTIONAL DESCRIPTION
- DESIGN EXAMPLES
- ADDITIONAL REFERENCES
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( )
( )
2
2
OUTPUTx
RMS(outputx) OUTPUTx
I
I D I
12
æ ö
æ ö
D
ç ÷
ç ÷
= ´ +
ç ÷
ç ÷
ç ÷
è ø
è ø
(16)
2
D(cond) RMS(outputx) DS(on)
P I R= ´
(17)
2
P
D(SW)
=
(V ) C f
IN J S
2
´ ´
(18)
D D(cond)output1 D(SW )output1 D(cond)output2 D(SW )output2 IN
P P P P P V Iq= + + + + ´
(19)
( )
J A D TH(pkg) TH(pad amb)
T T P
-
= + ´ q + q
(20)
Power Derating
TPS54383 , , TPS54386
SLUS774B – AUGUST 2007 – REVISED OCTOBER 2007
where
• D is the duty cycle
• I
OUTPUTx
is the dc output current
• Δ I
OUTPUTx
is the peak ripple current in the inductor for Outputx
Notice the impact of the operating duty cycle on the result.
Multiplying the result by the R
DS(on)
of the MOSFET gives the conduction loss.
The switching loss is approximated by:
where
• where C
J
is the prallel capacitance of the rectifier diode and snubber (if any)
• f
S
is the switching frequency
The total power dissipation is found by summing the power loss for both MOSFETs plus the loss in the internal
regulator.
The temperature rise of the device junction depends on the thermal impedance from junction to the mounting pad
(See the Package Dissipation Ratings table), plus the thermal impedance from the thermal pad to ambient. The
thermal impedance from the thermal pad to ambient depends on the PCB layout (PowerPAD interface to the
PCB, the exposed pad area) and airflow (if any). See the PCB Layout Guidelines, Additional References section.
The operating junction temperature is shown in Equation 20 .
The TPS5438x delivers full current at ambient temperatures up to +85 ° C if the thermal impedance from the
thermal pad maintains the junction temperature below the thermal shutdown level. At higher ambient
temperatures, the device power dissipation must be reduced to maintain the junction temperature at or below the
thermal shutdown level. Figure 36 illustrates the power derating for elevated ambient temperature under various
airflow conditions. Note that these curves assume that the PowerPAD is properly soldered to the recommended
thermal pad. (See the References section for further information.)
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