Datasheet

LTC3634
13
3634fb
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Figure 3. Setting the Output Voltage
applicaTions inForMaTion
When RT is tied to INTV
CC
, the switching frequency will
default to approximately 2MHz, as set by an internal resis-
tor. This internal resistor is more sensitive to process and
temperature variations than an external resistor (see the
T
ypical Performance Characteristics section) and is best
used for applications where switching frequency accuracy
is not critical.
Output V
oltage Programming
Each regulator’s output voltage is set by an external resis
-
tive divider according to the following equation:
V
OUT
= V
FBREG
1+
R2
R1






where V
FBREG
is the reference voltage as specified in the
Electrical Characteristics Table. The reference voltage is
600mV for channel 1; for channel 2 the reference voltage
is equal to the VTTR pin voltage. The desired output volt
-
age is set by appropriate selection of resistors R1 and R2
as shown in Figure 3.
The buffered output voltage on the VTTR pin is nominally
equal to half of the VDDQIN voltage; thus configuring
V
OUT2
as a V
TT
bus termination supply for DDR memory
is as simple as shorting V
OUT2
to V
FB2
and connecting
VDDQIN directly to the V
OUT1
(the V
DDQ
supply).
Choosing large values for R1 and R2 will result in im-
proved zero-load efficiency but may lead to undesirable
noise coupling or phase margin reduction due to stray
capacitances at the V
FB
node. Care should be taken to
route the V
FB
trace away from any noise source, such as
the SW trace.
The LTC3634 controlled on-time architecture is optimized
for an output voltage range of 0.6V to 3V, which is suitable
for powering DDR memory. The LTC3634 is capable of regu
-
lating higher output voltages; however, controlled on-time
behavior is not ensured. When the output voltage is greater
than 3V, the step-down regulator is for
ced to increase the
switching frequency in order to achieve output regulation.
Furthermore, external clock synchronization is no longer
possible, and channel 2 cannot maintain 90°/180° phase
operation with respect to channel 1. In short, the LTC3634
will behave like a constant on-time regulator instead of a
controlled on-time regulator. Therefore, output voltages
greater than 3V should only be used in applications where
switching frequency and channel-to-channel phase-locking
are not critical performance characteristics.
Inductor Selection
For a given input and output voltage, the inductor value and
operating frequency determine the inductor ripple current.
More specifically, the inductor ripple current decreases
with higher inductor value or higher operating frequency
according to the following equation:
∆I
L
=
V
OUT
f •L






1−
V
OUT
V
IN






where ΔI
L
= inductor ripple current, f = operating frequency
and L = inductor value. A trade-off between component
size, efficiency and operating frequency can be seen from
this equation. Accepting larger values of ΔI
L
allows the
use of lower value inductors but results in greater inductor
LTC3634
R2
C
F
(OPTIONAL)
V
OUT
3634 F03
V
FB
SGND
R1
Figure 2. Switching Frequency vs R
T
0
FREQUENCY (kHz)
1000
2000
3000
5000
200
700600
3634 F02
0
6000
4000
100 300 400 500
R
T
RESISTOR (kΩ)