AN017
2
tip of the probe. Minimize the ground lead effect by using a twisted pair or a short leaded
probe tip, and measure directly across the output capacitor. This reduces the erroneous noise
seen on the waveform.
Note 1:
The input voltage range of the evaluation card is limited by the 20V rating of the
input capacitor. For input voltages greater than 20V—up to the maximum input voltage of
36V—the user must change the input capacitor to one with a sufficient voltage rating for the
application. Keep in mind that circuit stability may be affected, and should be reevaluated.
Output Voltage Programming
V
OUT
= V
FB
* (1+R1/R2)
R1 = R2 * (V
OUT
/V
FB
-1)
Given: V
REF
= 0.79V Typ.
Factory Configuration: R1 = 8.98K, R2 = 10.0K
V
OUT
= 0.79 * (1+8.98K/10.0K) = 1.499V
Switching Frequency Programming
The operating frequency is programmed by the value of R
T
. The value for R
T
will vary from
187k at 200 kHz to 8.66k at 2.4 MHz based on the application requirements. Higher
operating frequencies require smaller inductor and capacitor values, but are less efficient,
have lower maximum input voltages, and higher dropout voltages. The flexibility of the
frequency adjustment also helps avoid issues with noise sensitive frequency bands.
F
SW(MAX)
=
V
D
+ V
OUT
t
ON(MIN)
(V
D
+ V
IN
– V
SW
)
Where:
F
SW(Max)
is the maximum switching frequency
V
D
is the catch diode voltage drop (~0.5V)
V
SW
is the internal switch drop (~0.3V at 1A)
R2
