AAT1152 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
'AAT1152' PDF : 17 Pages View PDF
AAT1152
850kHz 1A Buck DC/DC Converter
Output Capacitor
Since there are no external compensation compo-
nents, the output capacitor has a strong effect on
loop stability. Larger output capacitance will reduce
the crossover frequency with greater phase mar-
gin. For the 1.5V 1A design using the 4.1μH induc-
tor, three 22μF 6.3V X5R capacitors provide a sta-
ble output. In addition to assisting stability, the out-
put capacitor limits the output ripple and provides
holdup during large load transitions. The output
capacitor RMS ripple current is given by:
1
IRMS
=
2
⋅
⋅ VOUT ⋅ (VIN - VOUT)
3
L ⋅ FS ⋅ VIN
For a ceramic capacitor, dissipation due to the
RMS current of the capacitor is not a concern.
Tantalum capacitors with sufficiently low ESR to
meet output voltage ripple requirements also have
an RMS current rating much greater than that
actually seen in this application.
Adjustable Output
For applications requiring an output other than
the fixed outputs available, the 1V version can be
externally programmed. Resistors R3 and R4 of
Figure 3 force the output to regulate higher than
1V. R4 should be 100 times less than the internal
1MΩ resistance of the FB pin. Once R4 is select-
ed, R3 can be calculated. For a 1.25V output with
R4 set to 10.0kΩ, R3 is 2.55kΩ.
R3 = (VO - 1) ⋅ R4 = 0.25 ⋅ 10.0kΩ = 2.55kΩ
Layout Considerations
Figures 5 and 6 display the suggested PCB layout
for the AAT1152. The most critical aspect of the
layout is the placement of the input capacitor C1.
For proper operation, C1 must be placed as close-
ly as possible to the AAT1152.
Thermal Calculations
There are two types of losses associated with the
AAT1152 output switching MOSFET: switching
losses and conduction losses. Conduction losses
are associated with the RDS(ON) characteristics of
the output switching device. At full load, assuming
continuous conduction mode (CCM), a simplified
form of the total losses is:
PLOSS
= IO2 ⋅ (RDS(ON)H ⋅ VO + RDS(ON)L ⋅
VIN
(VIN - VO))
+ tsw ⋅ FS ⋅ IO ⋅ VIN + IQ ⋅ VIN
Once the total losses have been determined, the
junction temperature can be derived from the ΘJA
for the MSOP-8 package.
1152.2006.09.1.7
13
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