We create a VRM model of the regulator in Hyperlynx for the power source for Power Integrity simulations. We need to generate a lumped equivalent circuit. Let us look at creation of VRM model for TLV7A03.
The TLV7A03 is an ultra-small, ultra-low quiescent
current low-dropout linear regulator (LDO) that
sources 200mA with excellent transient performance.
The first requirement is to get the step load for the LDO. Below is the load transient response of the LDO from the datasheet.
Get the output capacitor value as recommended by the LDO datasheet. Datasheet recommends a 1uF output capacitor for excellent transient response. This capacitor will anyway comes form your design.
Calculate the di/dt:
From the above graph, the current rise to 200mA within the 5 us.So, it is 0.2A/5us
Variation of the VOUT on load transient:
The VOUT varies up to 100mV on a sudden load transient which actually looks huge variation.
Calculate Series Inductance:
Inductance can be calculated using the equation, V = L* (di/dt)
L = V/(di/dt) = 100m/(0.2/0.000005) = 2.5uH
Rise Time of the output pulse:
The rise time of the output is ~4uS.
Calculate the Series Resistance:
Series resistance of the output, Rout = L/(1.5*tr)
Rout = 2.5u/(1.5*4u) = 2.4 ohms
VRM model:
VRM model (lumped equivalent circuit) now looks like,
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