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LT3791MPFE-1#PBF

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LT3791-1
13
37911fa
For more information www.linear.com/LT3791-1
operaTion
Buck Region (V
IN
> V
OUT
)
Switch M4 is always on and switch M3 is always off during
this mode. At the start of every cycle, synchronous switch
M2 is turned on first. Inductor current is sensed when
synchronous switch M2 is turned on. After the sensed
inductor current falls below the reference voltage, which
is proportional to V
C
, synchronous switch M2 is turned off
and switch M1 is turned on for the remainder of the cycle.
Switches M1 and M2 will alternate, behaving like a typical
synchronous buck regulator. The duty cycle of switch M1
increases until the maximum duty cycle of the converter
in buck operation reaches D
MAX(BUCK, TG1)
, given by:
D
MAX(BUCK,TG1)
= 100% – D
(BUCK-BOOST)
where D
(BUCK-BOOST)
is the duty cycle of the buck-boost
switch range:
D
(BUCK-BOOST)
= 8%
Figure 3 shows typical buck operation waveforms. If V
IN
approaches V
OUT
, the buck-boost region is reached.
Buck-Boost Region (V
IN
~ V
OUT
)
When V
IN
is close to V
OUT
, the controller is in buck-boost
operation. Figure 4 and Figure 5 show typical waveforms in
this operation. Every cycle the controller turns on switches
M2 and M4, then M
1 and M4 are turned on until 180° later
when
switches M1 and M3 turn on, and then switches
M1 and M4 are turned on for the remainder of the cycle.
Boost Region (V
IN
< V
OUT
)
Switch M1 is always on and synchronous switch M2 is
always off in boost operation. Every cycle switch M3 is
turned on first. Inductor current is sensed when synchro
-
nous switch M3 is turned on. After the sensed inductor
cur
rent exceeds the reference voltage which is proportional
to V
C
, switch M3 turns off and synchronous switch M4
is turned on for the remainder of the cycle. Switches M3
and M4 alternate, behaving like a typical synchronous
boost regulator.
The duty cycle of switch M3 decreases until the minimum
duty cycle of the converter in boost operation reaches
D
MIN(BOOST,BG2)
, given by:
D
MIN(BOOST,BG2)
= D
(BUCK-BOOST)
where D
(BUCK-BOOST)
is the duty cycle of the buck-boost
switch range:
D
(BUCK-BOOST)
= 8%
Figure 6 shows typical boost operation waveforms. If V
IN
approaches V
OUT
, the buck-boost region is reached.
Low Current Operation
The LT3791-1 is recommended to run in forced continuous
conduction mode at heavy load by pulling
the CCM pin
higher
than 1.5V. In this mode the controller behaves as
a continuous, PWM current mode synchronous switching
regulator. In boost operation, switch M1 is always on,
switch M3 and synchronous switch M4 are alternately
turned on to maintain the output voltage independent
of the direction of inductor current. In buck operation,
synchronous switch M4 is always on, switch M1 and syn
-
chronous switch
M2 are alternately turned on to maintain
the output voltage independent of the direction of inductor
current. In the forced continuous mode, the output can
source or sink current.
However, reverse inductor current from the output to the
input is not desired for certain applications. For these ap
-
plications, the CCM pin must be connected to C/10 (pin 4)
with a pull-up resistor to INTV
CC
(see front page Typical
Application). Therefore, the CCM pin will be pulled lower
than 0.3V for discontinuous conduction mode by the C/10
pin when the output current is low. In this mode, switch
M4 turns off when the inductor current flows negative.
LT3791-1
14
37911fa
For more information www.linear.com/LT3791-1
Figure 3. Buck Operation (V
IN
> V
OUT
)
M2 + M4 M2 + M4 M2 + M4
M1 + M4
37911 F03
M1 + M4M1 + M4
Figure 4. Buck-Boost Operation (V
IN
≤ V
OUT
)
Figure 5. Buck-Boost Operation (V
IN
≥ V
OUT
)
M2 + M4 M2 + M4 M2 + M4
M1 + M4M1 + M4M1 + M4
M1 + M4 M1 + M4 M1 + M4
M1+ M3 M1+ M3 M1+ M3
37911 F04
M2 + M4 M2 + M4 M2 + M4
M1 + M4 M1 + M4 M1 + M4
M1 + M4 M1 + M4 M1 + M4
M1 + M3 M1 + M3 M1 + M3
37911 F05
M1 + M3
M1 + M3
M1 + M4 M1 + M4
M1 + M3
M1 + M4
37911 F06
Figure 6. Boost Operation (V
IN
< V
OUT
)
operaTion
LT3791-1
15
37911fa
For more information www.linear.com/LT3791-1
applicaTions inForMaTion
The Typical Application on the front page is a basic LT3791-1
application circuit. External component selection is driven
by the load requirement, and begins with the selection of
R
SENSE
and the inductor value. Next, the power MOSFETs
are selected. Finally, C
IN
and C
OUT
are selected. This circuit
can operate up to an input voltage of 60V.
Programming The Switching Frequency
The RT frequency adjust pin allows the user to program the
switching frequency from 200kHz to 700kHz to optimize
efficiency/performance or external component size. Higher
frequency operation yields smaller component size but
increases switching losses and gate driving current, and
may not allow sufficiently high or low duty cycle operation.
Lower frequency operation gives better performance at the
cost of larger external component size. For an appropriate
R
T
resistor value see Table 1. An external resistor from
the RT pin to GND is required; do not leave this pin open.
Table 1. Switching Frequency vs R
T
Value
f
OSC
(kHz) R
T
(kΩ)
200 147
300 84.5
400 59.0
500 45.3
600 35.7
700 29.4
Frequency Synchronization
The LT3791-1 switching frequency can be synchronized
to an external clock using the SYNC pin. Driving SYNC
with a 50% duty cycle waveform is always a good choice,
otherwise maintain the duty cycle between 10% and 90%.
The falling edge of CLKOUT corresponds to the rising edge
of SYNC thus allowing 2-phase paralleling converters. The
rising edge of CLKOUT turns on switch M3 and the falling
edge of CLKOUT turns on switch M2.
Inductor Selection
The operating frequency and inductor selection are inter
-
related in
that higher operating frequencies allow the use
of
smaller inductor and capacitor values. The inductor
value has a direct effect on ripple current. The maximum
inductor current ripple ΔI
L
can be seen in Figure 7. This
is the maximum ripple that will prevent subharmonic
oscillation and also regulate with zero load. The ripple
should be less than this to allow proper operation over
all load currents. For a given ripple the inductance terms
in continuous mode are as follows:
L
BUCK
>
V
OUT
• V
IN(MAX)
– V
OUT
( )
• 100
f • I
OUT(MAX)
• %Ripple • V
IN(MAX)
L
BOOST
>
V
IN(MIN)
2
• V
OUT
– V
IN(MIN)
( )
• 10
0
f • I
OUT(MAX)
• %Ripple • V
OUT
2
where:
f is operating frequency
% ripple is allowable inductor current ripple
V
IN(MIN)
is minimum input voltage
V
IN(MAX)
is maximum input voltage
V
OUT
is output voltage
I
OUT(MAX)
is maximum output load current
For high efficiency, choose an inductor with low core
loss. Also, the inductor should have low DC resistance to
reduce the I
2
R losses, and must be able to handle the peak
inductor current without saturating. To minimize radiated
noise, use a shielded inductor.
R
SENSE
Selection and Maximum Output Current
R
SENSE
is chosen based on the required output current. The
current comparator threshold sets the peak of the inductor
BG1, BG2 DUTY CYCLE (%)
50
∆I
L
/I
SENSE(MAX)
(%)
120
160
200
90
37911 F07
80
40
100
140
180
60
20
0
6055
7065
80 85 95
75
100
BOOST ∆I
L
/
I
SENSE(MAX)
LIMIT
BUCK ∆I
L
/
I
SENSE(MAX)
LIMIT
Figure 7. Maximum Peak-to-Peak Ripple vs Duty Cycle
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LT3791MPFE-1#PBF

Mfr. #:
Buy LT3791MPFE-1#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 60V 4-Switch Sync Buck-Boost Cntr
Lifecycle:
New from this manufacturer.
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