Datasheet

9
LTC3440
3440fb
OPERATIO
U
Buck Region (V
IN
> V
OUT
)
Switch D is always on and switch C is always off during this
mode. When the internal control voltage, V
CI
, is above
voltage V1, output A begins to switch. During the off time
of switch A, synchronous switch B turns on for the
remainder of the time. Switches A and B will alternate
similar to a typical synchronous buck regulator. As the
control voltage increases, the duty cycle of switch A
increases until the maximum duty cycle of the converter in
Buck mode reaches D
MAX
_
BUCK
, given by:
D
MAX
_
BUCK
= 100 – D4
SW
%
where D4
SW
= duty cycle % of the four switch range.
D4
SW
= (150ns • f) • 100 %
where f = operating frequency, Hz.
Beyond this point the “four switch,” or Buck/Boost region
is reached.
Buck/Boost or Four Switch (V
IN
~ V
OUT
)
When the internal control voltage, V
CI
, is above voltage V2,
switch pair AD remain on for duty cycle D
MAX_BUCK
, and
the switch pair AC begins to phase in. As switch pair AC
phases in, switch pair BD phases out accordingly. When
the V
CI
voltage reaches the edge of the Buck/Boost range,
at voltage V3, the AC switch pair completely phase out the
BD pair, and the boost phase begins at duty cycle D4
SW
.
Figure 2. Switch Control vs Internal Control Voltage, V
CI
The input voltage, V
IN
, where the four switch region begins
is given by:
V
V
ns f
V
IN
OUT
=
1 150–( • )
The point at which the four switch region ends is given by:
V
IN
= V
OUT
(1 – D) = V
OUT
(1 – 150ns • f) V
Boost Region (V
IN
< V
OUT
)
Switch A is always on and switch B is always off during
this mode. When the internal control voltage, V
CI
, is above
voltage V3, switch pair CD will alternately switch to
provide a boosted output voltage. This operation is typical
to a synchronous boost regulator. The maximum duty
cycle of the converter is limited to 75% typical and is
reached when V
CI
is above V4.
Burst Mode Operation
Burst Mode operation is when the IC delivers energy to the
output until it is regulated and then goes into a sleep mode
where the outputs are off and the IC is consuming only
25μA. In this mode the output ripple has a variable
frequency component that depends upon load current.
During the period where the device is delivering energy to
the output, the peak current will be equal to 400mA typical
and the inductor current will terminate at zero current for
each cycle. In this mode the maximum average output
current is given by:
I
V
VV
A
OUT MAX BURST
IN
OUT IN
()
.•
≈
+
01
Burst Mode operation is user controlled, by driving the
MODE/SYNC pin high to enable and low to disable.
The peak efficiency during Burst Mode operation is less
than the peak efficiency during fixed frequency because
the part enters full-time 4-switch mode (when servicing
the output) with discontinuous inductor current as illus-
trated in Figures 3 and 4. During Burst Mode operation, the
control loop is nonlinear and cannot utilize the control
75%
D
MAX
BOOST
D
MIN
BOOST
D
MAX
BUCK
DUTY
CYCLE
0%
V4 (≈2.05V)
V3 (≈1.65V)
BOOST REGION
BUCK REGION
BUCK/BOOST REGION
V2 (≈1.55V)
V1 (≈0.9V)
3440 F02
A ON, B OFF
PWM CD SWITCHES
D ON, C OFF
PWM AB SWITCHES
FOUR SWITCH PWM
INTERNAL
CONTROL
VOLTAGE, V
CI