AAT1217
600mA, 1.2MHz, Micropower
Synchronous Step-Up Converter
Input Capacitor
∆I
+
DC
L
I
= I
PEAK
2
A surface mount 4.7µF or greater, X5R or X7R,
ceramic capacitor is suggested for the input capac-
itor. The input capacitor provides a low impedance
loop for the edges of pulsed current drawn by the
AAT1217. Low ESR/ESL X7R and X5R ceramic
capacitors are ideal for this function. To minimize
stray inductance, the capacitor should be placed
as close as possible to the IC. This keeps the high
frequency content of the input current localized,
minimizing EMI and input voltage ripple. Always
examine the ceramic capacitor DC voltage coeffi-
cient characteristics to get the proper value. For
example, the capacitance of a 10ꢀF, 6.3V, X5R
ceramic capacitor with 5.0V DC applied is actually
about 6ꢀF.
I
· V
OUT
OUT
I
DC
=
η · V
IN
VIN · D
L · FSW
VOUT - VIN
VOUT
∆IL =
; D =
VIN · (VOUT - VIN)
L · FSW · VOUT
∆IL
=
I
· V
V · (V
IN
- V )
OUT IN
OUT
OUT
I
=
+
PEAK
η · V
2L · F · V
SW OUT
IN
IPEAK Peak Inductor Current
IDC DC Component (Average) of the Inductor
Current
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the
evaluation board input voltage pins. The inductance
of these wires, along with the low-ESR ceramic input
capacitor, can create a high Q network that may
affect converter performance. This problem often
becomes apparent in the form of excessive ringing
in the output voltage during load transients which
can produce errors in loop phase and gain meas-
urements. Since the inductance of a short printed
circuit board (PCB) trace feeding the input voltage is
significantly lower than the power leads from the
bench power supply, most actual applications do not
exhibit this problem. In applications where the input
power source lead inductance cannot be reduced to
a level that does not affect the converter perform-
ance, a high ESR tantalum or aluminum electrolytic
capacitor should be placed in parallel with the low
ESR, ESL bypass input ceramic capacitor. The intro-
duction of the high ESR capacitor dampens the high
Q network and stabilizes the AAT1217.
∆IL Peak-Peak Inductor Ripple Current
IOUT Output (Load) Current
VOUT Output Voltage
VIN Input Voltage
η
AAT1217 Efficiency (consult the perform-
ance graphs in the “Typical Characteristics”
section of the data sheet)
D
Steady-State Duty Cycle
FSW Switching Frequency
L
Inductor Value
For a given chosen inductor value and application
conditions make sure the peak inductor current does
not exceed the maximum current rating of the select-
ed vendor’s inductor. For optimum load transient and
efficiency, low DCR inductors should be selected.
Table 2 lists some typical surface mount inductors
that are suitable for typical AAT1217 applications.
1217.2007.07.1.0
9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
