Datasheet

LTC3533
9
3533fc
For more information www.linear.com/LTC3533
operation
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 voltage
V1, switch 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 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 control voltage, V
C
, is above voltage V2, switch
pair AD remain on for duty cycle D
MAX_BUCK
, and 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 – D) = V
OUT
(1 – 150ns • f) V
The point at which the four switch region ends is given by:
V
IN
=
V
OUT
1− (150ns • f)
V
where f = operating frequency, Hz.
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 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 90% typical and is reached
when V
C
is above V4.
3
SW1
7
SW2
PMOS A
NMOS B
11
PV
IN
PMOS D
NMOS C
3533 F01
9
PV
OUT
L1
85%
D
MAX
BOOST
D
MIN
BOOST
D
MAX
BUCK
DUTY
CYCLE
0%
V4 (≈1.5V)
V3 (≈1.15V)
BOOST REGION
BUCK REGION
BUCK/BOOST REGION
V2 (≈1V)
V1 (≈0.7V)
3533 F02
A ON, B OFF
PWM CD SWITCHES
D ON, C OFF
PWM AB SWITCHES
FOUR SWITCH PWM
CONTROL
VOLTAGE, V
C
Figure 1. Simplified Diagram of Output Switches
Figure 2. Switch Control vs Control Voltage, V
C
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