Datasheet
LTC3630A
18
3630afa
For more information www.linear.com/LTC3630A
applicaTions inForMaTion
Next, verify that this value meets the L
MIN
requirement.
For this input voltage and peak current, the minimum
inductor value is:
L
MIN
=
24V • 150ns
1.2A
≅ 3µH
Therefore, the minimum inductor requirement is satisfied
and the 10μH inductor value may be used.
Next, C
IN
and C
OUT
are selected. For this design, C
IN
should
be sized for a current rating of at least:
I
RMS
= 500mA •
3.3V
24V
•
24V
3.3V
– 1≅ 175mA
RMS
The value of C
IN
is selected to keep the input from droop-
ing less than 240mV (1%):
C
IN
>
10µH • 1.2A
2
2 • 24V • 240mV
≅ 2.2µF
C
OUT
will be selected based on a value large enough to
satisfy the output voltage ripple requirement. For a 50mV
output ripple, the value of the output capacitor can be
calculated from:
C
OUT
>
10µH • 1.2A
2
2 • 3.3V • 50mV
≅ 47µF
C
OUT
also needs an ESR that will satisfy the output voltage
ripple requirement. The required ESR can be calculated
from:
ESR <
50mV
1.2A
≅ 40mΩ
A 47µF ceramic capacitor has significantly less ESR than
40mΩ.
Since an output voltage of 3.3V is one of the standard
output configurations, the LTC3630A can be configured
by connecting V
PRG1
to ground and V
PRG2
to the SS pin.
The undervoltage lockout requirement on V
IN
can be satis-
fied with a resistive divider from V
IN
to the RUN pin (refer
to Figure 9). Calculate R3 and R4 as follows:
R3 = 200kwhichis ≤
12V
40µA
R4 =
200k • 1.21V
12V – 1.21V + 200k • 4µA
= 20.9k
Choose standard values for R3 = 200k, R4 = 21k. Note
that the V
IN
falling threshold will be 10% less than the
rising threshold or 11V.
Since the maximum V
IN
is more than 4.5x the UVLO thresh-
old, a 4.7
V Zener diode in parallel with R4 is required to
keep the maximum voltage on the RUN pin less than the
absolute maximum of 6V.
The I
SET
pin should be left open in this example to select
maximum peak current (1.2A typical). Figure 11 shows a
complete schematic for this design example.
V
FB
SW
10µH
V
IN
RUN
200k
2.2µF
47µF
V
OUT
3.3V
500mA
V
IN
24V
21k
4.7V
3630a F11
SS
V
PRG2
V
PRG1
FBO
I
SET
GND
LTC3630A
Figure 11. 24V to 3.3V, 500mA Regulator at 200kHz