AAT3183 Datasheet, PDF (13/16 Pages) Advanced Analogic Technologies – 300mA Inductorless Step-Down Converter

AAT3183
300mA Inductorless Step-Down Converter
Input and Output Voltage Ripple:
Charge Pump Operation
The AAT3183 minimizes switching noise with PFM
control. PFM switches only when required to main-
tain the output load, reducing the total switching
noise. PFM control generates a small amount of
V_IN and V_OUT regulation ripple (ΔV_PFM) due to the
charge and discharge of the input and output
capacitors. Additional voltage ripple is due to the
parasitic resistance and inductance distributed on
circuit traces and within the input, fly, and output
capacitors themselves; see Figure 3 for the graph-
ic illustration of the AC parasitic components of a
AAT3183 typical application circuit.
During the charge pump switching events, an AC
current path (I_AC) is established from the voltage
source (V_IN) and input capacitor (C_IN) through the
flying capacitor (C_FLY) to the output capacitor (C_OUT)
and returning through the ground plane (GND).
The AC voltage ripple signal is measured across C_IN
and C_OUT and is highest at full load and high V_IN.
These AC currents charge and discharge the flying
capacitor and flow through the ESR and ESL, which
are parasitic elements within the capacitors. Circuit
board traces can add to ESR and ESL and will con-
tribute to the AC voltage ripple. Proper component
selection and good layout practice are critical in pro-
viding low ripple, low EMI performance. These para-
sitic elements should be minimized to optimize loop
transient response and achieve stable operation.
The I_AC current from the flying capacitor flows
through parasitic ESR and ESL. Voltage ripple
across the input and output capacitors due to ESR
and ESL are approximated:
ΔV_ESR = ESR_TOT · I_AC
ΔV_ESL
=
(ESL_TOT · I_AC)
Δt_RISE-FALL
The total AC voltage ripple (V_RIPPLE) is the sum of
the individual AC voltage ripple signals.
V_RIPPLE = ΔV_ESR + ΔV_ESL + ΔV_PFM
Due to fast switching, a large amount of AC switch-
ing noise due to the parasitic ESL within the C_IN
and C_OUT ceramic capacitors is seen on the output
ripple. This noise may be attenuated with a small
amount of input and output filtering.
L_TRACE
ESL
V_IN
CIN
ESR
CFLY
ESL
ESR
AAT3183
IAC
GND
L_TRACE
ESL
C_OUT
ESR
VOUT
R_LOAD
Figure 3: AC Parasitic Components of an AAT3183 Typical Application Circuit.
3183.2007.07.1.2
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