LTC3375
26
3375fc
For more information www.linear.com/3375
applicaTions inForMaTion
Buck Switching Regulator Output Voltage and
Feedback Network
The output voltage of the buck switching regulators is
programmed by a resistor divider connected from the
switching regulator’s output to its feedback pin and is
given by V
OUT
= V
FB
(1 + R2/R1) as shown in Figure 5.
Typical values for R1 range from 40k to 1M. The buck
regulator transient response may improve with optional
capacitor C
FF
that helps cancel the pole created by the
feedback resistors and the input capacitance of the FB
pin. Experimentation with capacitor values between 2pF
and 22pF may improve transient response.
Figure 5. Feedback Components
BUCK
SWITCHING
REGULATOR
V
OUT
C
(OPTIONAL)
C
FF
R2
R1
FB
Buck Regulators
All eight buck regulators are designed to be used with
inductors ranging from 1µH to 3.3µH depending on the
lowest switching frequency that the buck regulator must
operate at. To operate at 1MHz a 3.3µH inductor should
be used, while to operate at 3MHz a 1µH inductor may be
used. Table 3 shows some recommended inductors for
the buck regulators.
The input supply needs to be decoupled with a 10µF
capacitor while the output needs to be decoupled with a
22µF capacitor. Refer to Capacitor Selection for details on
selecting a proper capacitor.
Each buck regulator can be programmed via I
2
C. To program
buck regulator 1 use sub-address 01h, buck regulator 2
sub-address 02h, buck regulator 3 sub-address 03h, buck
regulator 4 sub address 04h, buck regulator 5 sub-address
05h, buck regulator 6 sub-address 06h, buck regulator 7
sub-address 07h, and buck regulator 8 sub-address 08h.
The bit format is explained in Table 7.
Combined Buck Regulators
A single 2A buck regulator is available by combining two
adjacent 1A buck regulators together. Likewise a 3A or 4A
buck regulator is available by combining any three or four
adjacent buck regulators respectively. Tables 4, 5, and 6
show recommended inductors for these configurations.
The input supply needs to be decoupled with a 22µF capaci
-
tor while the output needs to be decoupled with a 47µF
capacitor
for
a 2A combined buck regulator. Likewise for
3A and 4A configurations the input and output capacitance
must be scaled up to account for the increased load. Refer
to Capacitor Selection in the Applications Information sec
-
tion for details on selecting a proper capacitor.
In many cases, any extra unused buck converters may be
used to increase the efficiency of the active regulators.
In general the efficiency will improve for any regulators
running close to their rated load currents. If there are
unused regulators, the user should look at their specific
applications and current requirements to decide whether
to add extra stages.
Table 3. Recommended Inductors for 1A Buck Regulators
PART NUMBER L (µH) MAX I
DC
(A) MAX DCR (MΩ)
SIZE IN mm (L × W × H)
MANUFACTURER
IHLP1212ABER1R0M-11 1.0 3 38
3 × 3.6 × 1.2
Vishay
1239AS-H-1R0N 1 2.5 65
2.5 × 2.0 × 1.2
Toko
XFL4020-222ME 2.2 3.5 23.5
4 × 4 × 2.1
CoilCraft
1277AS-H-2R2N 2.2 2.6 84
3.2 × 2.5 × 1.2
Toko
IHLP1212BZER2R2M-11 2.2 3 46
3 × 3.6 × 1.2
Vishay
XFL4020-332ME 3.3 2.8 38.3
4 × 4 × 2.1
CoilCraft
IHLP1212BZER3R3M-11 3.3 2.7 61
3 × 3.6 × 1.2
Vishay
(15 pages)
(36 pages)
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