Datasheet

R2 =
- 1
V
TO
1.25
x R3
R5 =
- 1
V
OUT
0.8
x R6
R8 =
78000
F
SW
- 55
R7 =
x
C8
C9
I
OUT
V
OUT
+
-1
2 x D
f
SW
x L1
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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 Equation 7:
(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 LM20343 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.
6.9.1 Frequency Adjust Resistor: R8
The value for R8 will set the operating frequency of the device. For this evaluation board, a value of 49.9
kΩ was chosen that sets oscillator frequency to 750 kHz. The value of R8 can be adjusted to support
operating frequencies from 250 kHz to 1 MHz by using Equation 8:
(8)
Where, f
SW
is the switching frequency in kHz, and R8 is the frequency adjust resistor in kΩ.
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 LM20343 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 Equation 9:
(9)
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.
6.11 Programmable UVLO: R2 and R3
The resistors labeled R2 and R3 create a voltage divider from V
IN
to the enable pin that can be used to set
the turn-on threshold or UVLO of the voltage regulator. To allow evaluation of the device down to 4.5V
these components are not installed. To change the turn-on threshold of the device a 10 kΩ resistor is
recommended for R3 and the value of R2 can be calculated using Equation 10:
(10)
8
AN-1792 LM20343 Evaluation Board SNVA326B–May 2008–Revised May 2013
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