Texas Instruments TPS76933, User Manual

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External Capacitor Requirements – ESR
3-7 Circuit Design
Figure 3–3. Calculated and Measured Maximum Output Current vs Input Voltage Without
Cooling for TPS76933 in Test Setup
0
50
100
150
200
250
3 4
Vin [V]
±
3% Vout – Tolerance Area
I out
[mA]
300 mA Current limit
V
out
[V]
300
350
2
2.2
2.4
2.6
2.8
3
3.2
3.4
5 6 7 8 9 10 11 12 13 14
Iout, max vs. Input Voltage,
calculated Values
Iout, max vs. Input Voltage,
measured Values
Vout at Iout, max
Vout at Iout = 100 mA
3.3 External Capacitor Requirements – ESR
Besides its capacitance, every capacitor also contains parasatic resistances.
These parasatic resistances are ohmic resistances as well as inductive
impedances. The ohmic resistances are called equivalent series resistance
(ESR), and the inductive impedances are called equivalent series inductance
(ESL). L is the designator for inductors. The equivalent schematic diagram of
any capacitor can therefore be drawn as shown in Figure 3–4.
Figure 3–4. ESR and ESL
R
ESR
L
ESL
C
In most cases one can neglect the very small inductive impedance ESL.
Therefore the following description focuses mainly on the parasitic resistance
ESR.
Figure 3–5 shows the output capacitor and its parasitic resistances in a typical
LDO output stage.