AAT1150_06 Datasheet, PDF (11/17 Pages) Advanced Analogic Technologies – 1MHz 1A Step-Down DC/DC Converter

AAT1150
1MHz 1A Step-Down DC/DC Converter
3.5
3
V_CC = V_P = 5.0V
2.5
V_O = 3.3V
Figure 2 Schematic
2
1.5
1
V_CC = V_P = 3.6V
V_O = 1.5V
0.5
Figure 1 Schematic
0
0
0.5
1
1.5
2
2.5
Output Current (A)
Figure 3: Current Limit Characteristics.
Inductor
The output inductor is selected to limit the ripple
current to some predetermined value, typically
20% to 40% of the full load current at the maximum
input voltage. Manufacturer's specifications list
both the inductor DC current rating, which is a ther-
mal limitation, and the peak current rating, which is
determined by the saturation characteristics. The
inductor should not show any appreciable satura-
tion under normal load conditions. During overload
and short-circuit conditions, the average current in
the inductor can meet or exceed the I_LIMIT point of
the AAT1150 without affecting converter perform-
ance. Some inductors may have sufficient peak
and average current ratings yet result in excessive
losses due to a high DCR. Always consider the
losses associated with the DCR and its effect on
the total converter efficiency when selecting an
inductor.
For a 1.0A load and the ripple set to 30% at the
maximum input voltage, the maximum peak-to-
peak ripple current is 300mA. The inductance
value required is 3.9μH.
L
=
I_O
V_OUT
⋅ k ⋅ F_S
⋅
⎛⎝₁
-
V_OUT ⎞
V_IN ⎠
L
=
1.0A
⋅
1.5V
0.3 ⋅
830kHz
⋅ ^⎛⎝1
-
1.5V ⎞
4.2V ⎠
L = 3.9μH
The factor "k" is the fraction of full load selected for
the ripple current at the maximum input voltage.
The corresponding inductor RMS current is:
I_RMS =
⎝⎛^Io2
+
ΔI²âŽž
12⎠
≈ I_o = 1.0A
ΔI is the peak-to-peak ripple current which is fixed by
the inductor selection above. For a peak-to-peak
current of 30% of the full load current, the peak cur-
rent at full load will be 115% of the full load. The
4.1μH inductor selected from the Sumida
CDRH5D18 series has a 57mΩ DCR and a 1.95A
DC current rating. At full load, the inductor DC loss
is 57mW which amounts to a 3.8% loss in efficiency.
Input Capacitor
The primary function of the input capacitor is to pro-
vide a low impedance loop for the edges of pulsed
current drawn by the AAT1150. A low ESR/ESL
ceramic capacitor is ideal for this function. To mini-
mize stray inductance, the capacitor should be
placed as closely as possible to the IC. This keeps
the high frequency content of the input current
localized, minimizing radiated and conducted EMI
while facilitating optimum performance of the
AAT1150. Ceramic X5R or X7R capacitors are
ideal for this function. The size required will vary
depending on the load, output voltage, and input
voltage source impedance characteristics. A typi-
cal value is around 10μF. The input capacitor RMS
1150.2006.09.1.5
11