Datasheet

R5 =
- 1
V
OUT
0.8
x R6
R7 =
x
C8
C9
I
OUT
V
OUT
+
-1
2 x D
f
SW
x L1
t
SS
=
0.8V x C6
I
SS
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6.4 Soft-Start Capacitor: C6
A soft-start capacitor can be used to control the startup time of the LM20333 voltage regulator. The startup
time of the regulator when using a soft-start capacitor can be estimated by the following equation:
(6)
For the LM20333, I
SS
is nominally 5 µA. For the evaluation board, the soft-start time has been designed to
be roughly 15 ms, resulting in a C
SS
capacitor value of 100 nF.
6.5 VCC Bypass: C5
The capacitor C5 is used to bypass the internal 4.5V sub-regulator. A value of 1 µF is sufficient for most
applications.
6.6 Boot Capacitor: C4
C4 is the boot capacitor which is used to provide the charge needed to drive the high-side FET. An
optimal value for this capacitor is 0.1 µF.
6.7 Compensation Capacitor: C8
The capacitor C8 is used to set the crossover frequency of the LM20333 control loop. Since this board
was optimized to be stable over the full input and output voltage range, the value of C8 was selected to be
1.5 nF. Once the operating conditions for the device are known, the transient response can be optimized
by reducing the value of C8 and calculating the value for R7 as outlined in the next section.
6.8 Compensation Resistor: R7
Once the value of C8 is known, resistor R7 is used to place a zero in the control loop to cancel the output
filter pole. This resistor can be sized according to the equation:
(7)
For stability purposes the device should be compensated for the maximum output current expected in the
application.
6.9 Optional Compensation Capacitor: C7
A second compensation capacitor C7 can be used in some designs to improve noise immunity for low
duty cycle conversions, as well as, provide a high frequency pole, useful for cancelling a possible zero
introduced by the ESR of the output capacitor. For the LM20333 evaluation board, a 10pF capacitor is
populated for C7. Higher value capacitors can be used to improve low duty cycle performance at the
expense of phase margin.
6.10 Feedback resistors: R4, R5, and R6
The resistors labeled R5 and R6 create a voltage divider from V
OUT
to the feedback pin that is used to set
the output of the voltage regulator. Nominally, the output of the LM20333 evaluation board is set to 3.3V,
giving resistor values of R5 = 30.9 kΩ and R6 = 10 kΩ. If a different output voltage is required, the value
of R5 can be adjusted according to the equation:
(8)
R6 does not need to be changed from its value of 10 kΩ. Resistor R4 has a value of 348Ω and is provided
as an injection point for loop stability measurements, as well as, a way to further tweak the output voltage
accuracy to account for resistor tolerance values differing from ideal calculated values.
8
AN-1791 LM20333 Evaluation Board SNVA325B–May 2008–Revised May 2013
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