Datasheet

LTC3538
9
3538fb
OPERATION
Depending on the V
C
voltage, the LTC3538 will operate in
either buck, buck-boost or boost mode. The four power
switches are properly phased so the transfer between
operating modes is continuous, smooth and transparent to
the user. When V
IN
approaches V
OUT
the buck-boost region
is entered, where the conduction time of the four-switch
region is typically 150ns. Referring to Figures 1 and 2, the
various regions of operation will now be described.
Buck-Boost or Four Switch (V
IN
~ V
OUT
)
When the control voltage, V
C
, is above voltage V2, switch
pair AD remains 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 V
C
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
. The input voltage, V
IN
,
where the four switch region begins is given by:
V
IN
= V
OUT
(1 – D4
SW
) ≈ 0.85 • V
OUT
The point at which the four-switch region ends is given
by:
V
IN
=
V
OUT
1− D4
SW
V ≈ 1.18• V
OUT
Boost Region (V
IN
< V
OUT
)
Switch A is always on and switch B is always off during
this mode. When the control voltage, V
C
, is above volt-
age 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 88% typical and is reached
when V
C
is above V4.
Burst Mode OPERATION
Burst Mode operation reduces quiescent current consump-
tion of the LTC3538 at light loads and improves overall
conversion effi ciency, increasing battery life. During Burst
Mode operation the LTC3538 delivers energy to the out-
put until it is regulated and then goes into a sleep mode
where the outputs are off and the quiescent current drops
to 35μA. In this mode the output ripple has a variable
frequency component that depends upon load current,
and will typically be about 2% peak-to-peak. Burst Mode
operation ripple can be reduced slightly by using more
output capacitance. Another method of reducing Burst
Mode operation ripple is to place a small feed-forward
capacitor across the upper resistor in the V
OUT
feedback
divider network (as in Type III compensation).
3538 F02
A ON, B OFF
PWM C, D SWITCHES
BOOST REGION
FOUR-SWITCH PWM
BUCK-BOOST
REGION
D ON, C OFF
PWM A, B SWITCHES
D
MIN
BOOST
D
MAX
BUCK
D
MAX
BOOST
0%
88%
DUTY
CYCLE
V3 (~1.8V)
V2 (~1.7V)
V4 (~2.2V)
V1 (~1.2V)
CONTROL
VOLTAGE, V
C
BUCK REGION
Figure 2. Switch Control vs Control Voltage, V
C
Buck Region (V
IN
> V
OUT
)
Switch D is always on and switch C is always off during
this mode. When the control voltage, V
C
, is above volt-
age V1, output A begins to switch. During the off time of
switch A, synchronous switch B turns on for the remainder
of the period. Switches A and B will alternate similar to a
typical synchronous buck regulator. As the control volt-
age 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.