LTC3676/LTC3676-1
17
3676fe
For more information www.linear.com/LTC3676
OPERATION
Operating Modes
The buck regulators can operate in either pulse-skipping,
Burst Mode operation, or forced continuous mode. In
pulse-skipping setting the regulator will skip pulses at
light loads but will operate at constant frequency. In Burst
Mode setting the regulator operates in Burst Mode opera
-
tion at light loads and in constant frequency PWM mode
at higher load. In forced continuous setting the inductor
current is allowed to be less than zero over the full range
of duty cycles. In forced continuous operation the buck
regulator has the ability to sink output current. Because
the regulator is switching every cycle regardless of output
load, forced continuous mode results in the least output
voltage ripple at light load.
Output Voltage Programming
Each of the step-down converters uses a dynamically slew
-
ing DAC for its reference. The output voltage of the DAC
reference
is selectable using a 5-bit I
2
C command register.
The output voltage is set by using a resistor divider con-
nected from
the step-down switching regulator output to
its
feedback pin as shown in Figure 3. The output voltage
is calculated using the following formula:
V
OUT
= 1+
R1
R2
⎛
⎝
⎜
⎞
⎠
⎟
• DVBx •12.5 + 412.5
( )
mV
( )
DVBx is the decimal value of the 5-bit binary number in
the I
2
C command registers. The default DAC input code is
11001 (25 in decimal) which corresponds to a reference
voltage of 725mV. Typical values for R1 are in the range
of 40k to 1M. Capacitor C
FB
cancels the pole created by
the feedback resistors and the input capacitance on the
FB pin and helps to improve load step transient response.
A value of 10pF is recommended.
Inductor Selection
The choice of step-down switching regulator inductor
influences the efficiency and output voltage ripple of the
converter. A larger inductor improves efficiency since the
peak current is closer to the average output current. Larger
inductors generally have higher series resistance that
counters the efficiency advantage of reduced peak current.
Inductor ripple current is a function of switching frequency,
inductance, V
IN
, and V
OUT
as shown in this equation:
ΔI
L
=
1
f •L
• V
OUT
• 1–
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
A good starting design point is to use an inductor that
gives ripple equal to 30% output current. Select an induc-
tor with
a DC current rating at least 1.5 times larger than
the
maximum load current to ensure the inductor does
not saturate.
Input and Output Capacitor Selection
Low ESR ceramic capacitors should be used at both the
output and input supply of the switching regulators. Only
X5R or X7R ceramic capacitors should be used since they
have better temperature and voltage stability than other
ceramic types.
Operating Frequency
The switching frequency of each of the LTC3676 switching
regulators may be set using the I
2
C command registers.
The default switching frequency is 2.25MHz and the select-
able frequency
is 1.125MHz. Operation at lower frequency
improves efficiency by reducing internal gate charge and
switching losses at the expense of a larger inductor.
The lowest duty cycle of the step-down converter is de
-
termined by minimum on-time. Minimum on-time is the
shortest
time duration that the converter can turn its top
PMOS on and off again. The time is the sum of gate charge
Figure 3. Step-Down Switching Regulator Application Circuit
DEFAULT
725mV
DAC
5
SW
FB
MODE
EN
C
FB
R1
C
OUT
R2
3676 F03
PV
IN
PWM
CONTROL
2
