AAT1130 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – 2.5MHz 500mA Step-Down DC/DC Converter

DATA SHEET
AAT1130
2.5MHz 400mA Step-Down DC/DC Converter
Where t_SW is the switching period constant (typically
360ns) as defined by the on-time specification in the
Electrical Characteristics table, and t_DELAY is the current-
limit comparator delay (typically 150ns).
TON
ΔI_L
T_DELAY
I_OUT(MAX)
ILIMIT
Figure 1: Output Current to
Valley Current-Limit Relationship
Anti-Ringing Switch
The AAT1130 includes an anti-ringing switch that dissi-
pates any energy left in the inductor when the current is
approximately zero. The anti-ringing switch turns on
when both the p-channel switch and n-channel synchro-
nous rectifier are off and the inductor current is approx-
imately zero. The switch shorts the LX and VOUT nodes
together, effectively shorting the inductor. The low on-
resistance of the anti-ringing switch dissipates any
energy left in the inductor preventing ringing at light
loads. When either the switch or synchronous rectifier
are on, the anti-ringing switch remains off.
Over-Temperature
The AAT1130 includes thermal protection that automati-
cally turns off the regulator when the die temperature
exceeds a safe level. The thermal protection turns on at
a die temperature of 140°C and has a 15°C hysteresis.
Applications Information
Inductor Selection
The step-down converter uses a predictive on-time con-
trol scheme with internal slope and current compensa-
tion. The internal current compensation eliminates the
"minimum, output capacitor ESR" stability requirement
commonly required with this control architecture. However,
the current compensation requires that the inductor cur-
rent down slope to maintain a sufficient signal-to-noise
ratio. The inductor current down-slope is defined by:
di
dt
=
V_OUT
L
resulting in an inductor recommendation to keep the
inductance
value
equal
to
the
output
voltage
μH
L = V_{OUT V}
to
maintain the appropriate _ddti.
Manufacturer’s specifications list both the inductor DC
current rating, which is a thermal limitation, and the
peak current rating, which is determined by the satura-
tion characteristics. The inductor should not show any
appreciable saturation under normal load conditions.
Some inductors may meet the 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. See Table 2 for suggested inductor values
and vendors.
Input Capacitor
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. Always examine the ceramic capacitor DC
voltage coefficient characteristics when selecting the
proper value. For example, the capacitance of a 10μF,
6.3V, X5R ceramic capacitor with 5.0V DC applied is
actually about 6μF.
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT1130. Low
ESR/ESL X7R and X5R ceramic capacitors are ideal for
this function. To minimize stray inductance, the capaci-
tor should be placed as closely as possible to the IC. This
keeps the high frequency content of the input current
localized, minimizing EMI and input voltage ripple.
The proper placement of the input capacitor (C1) can be
seen in the evaluation board layouts in Figures 4 and 5.
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201977B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013