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LT3791MPFE-1#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P26
LT3791-1
16
37911fa
For more information www.linear.com/LT3791-1
applicaTions inForMaTion
current in boost operation and the maximum inductor valley
current in buck operation. In boost operation, the maximum
average load current at V
IN(MIN)
is:
I
OUT(MAX _ BOOST)
=
51mV
R
SENSE
–
ΔI
L
2
⎛
⎝
⎜
⎞
⎠
⎟
•
V
IN(MIN)
V
OUT
where ΔI
L
is peak-to-peak inductor ripple current. In buck
operation, the maximum average load current is:
I
OUT(MAX _ BUCK)
=
47.5mV
R
SENSE
+
ΔI
L
2
⎛
⎝
⎜
⎞
⎠
⎟
The maximum current sensing R
SENSE
value for the boost
operation is:
R
SENSE(MAX)
=
2 • 51mV • V
IN(MIN)
2 • I
LED
• V
OUT
+ ΔI
L(BOOST)
• V
IN(MIN)
The maximum current sensing R
SENSE
value for the buck
operation is:
R
SENSE(MAX)
=
2 • 47.5mV
2 • I
LED
– ΔI
L(BUCK)
The final R
SENSE
value should be lower than the calculated
R
SENSE(MAX)
in both the boost and buck operation. A 20%
to 30% margin is usually recommended.
C
IN
and C
OUT
Selection
In boost operation, input current is continuous. In buck
operation, input current is discontinuous. In buck opera
-
tion, the selection of input capacitor, C
IN
, is driven by the
need to filter the input square wave current. Use a low ESR
capacitor sized to handle the maximum RMS current. For
buck operation, the input RMS current is given by:
I
RMS
= I
LED
2
• D+
ΔI
L
2
12
• D
The formula has a maximum at V
IN
= 2V
OUT
. Note that
ripple current ratings from capacitor manufacturers are
often based on only 2000 hours of life which makes it
advisable to derate the capacitor.
In boost operation, the discontinuous current shifts
from the input to the output, so C
OUT
must be capable
of reducing the output voltage ripple. The effects of ESR
(equivalent series resistance) and the bulk capacitance
must be considered when choosing the right capacitor
for a given output ripple voltage. The steady ripple due to
charging and discharging the bulk capacitance is given by:
ΔV
RIPPLE BOOST _CAP
(
)
=
I
LED
• V
OUT
– V
IN(MIN)
( )
C
OUT
• V
OUT
• f
ΔV
RIPPLE BUCK _ CAP
(
)
≈
ΔI
L
8 • f • C
OUT
where C
OUT
is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
ΔV
BOOST(ESR)
= I
LED
• ESR
ΔV
BUCK(ESR)
= I
LED
• ESR
Multiple capacitors placed in parallel may be needed to meet
the ESR and RMS current handling requirements. Output
capacitors are also used for stability for the LT3791-1. A
good starting point for output capacitors is seen in the
Typical Applications circuits. Ceramic capacitors have
excellent low ESR characteristics but can have a high
voltage coefficient and are recommended for applications
less than 100W. Capacitors available with low ESR and
high ripple current ratings, such as OS-CON and POSCAP
may be needed for applications greater than 100W.
Programming V
IN
UVLO and OVLO
The falling UVLO value can be accurately set by the resistor
divider R1 and R2. A small 3µA pull-down current is active
when the EN/UVLO is below the threshold. The purpose
of this current is to allow the user to program the rising
hysteresis. The following equations should be used to
determine the resistor values:
V
IN(UVLO
–
)
= 1.2 •
R1
+
R2
R2
V
IN(UVLO
+
)
= 3µA • R1+1.215 •
R1+R2
R2
LT3791-1
17
37911fa
For more information www.linear.com/LT3791-1
Figure 8. Resistor Connection to Set V
IN
UVLO and
OVLO Thresholds
LT3791-1
V
IN
R1
R3
R4
R2
37911 F08
EN/UVLO
OVLO
The rising OVLO value can be accurately set by the resis-
tor divider R3 and R4. The following equations should be
used to determine the resistor values:
V
IN(OVLO
+
)
= 3 •
R3
+
R4
R4
V
IN(OVLO
–
)
= 2.925 •
R3+R4
R4
applicaTions inForMaTion
Table 2. V
(ISP-ISN)
Threshold vs CTRL
V
CTRL
(V) V
(ISP-ISN)
(mV)
1.1 90
1.15 94.5
1.2 98
1.25 99.5
1.3 100
When V
CTRL
is higher than 1.3V, the output current is
regulated to:
I
OUT
=
100mV
R
OUT
The CTRL pin should not be left open (tie to V
REF
if not
used). The CTRL pin can also be used in conjunction with
a thermistor to provide overtemperature protection for
the output load, or with a resistor divider to V
IN
to reduce
output power and switching current when V
IN
is low.
The presence of a time varying differential voltage signal
(ripple) across ISP and ISN at the switching frequency
is expected. The amplitude of this signal is increased by
high output load current, low switching frequency and/
or a smaller value output filter capacitor. Some level of
ripple signal is acceptable: the compensation capacitor
on the V
C
pin filters the signal so the average difference
between ISP and ISN is regulated to the user-programmed
value. Ripple voltage amplitude (peak-to-peak) in excess
of 20mV should not cause mis-operation, but may lead
to noticeable offset between the average value and the
user-programmed value.
ISMON
The ISMON pin provides a linear indication of the current
flowing through the output. The equation for V
ISMON
is
V
(ISP–ISN)
• 10. This pin is suitable for driving an ADC input,
however, the output impedance of this pin is 12.5kΩ so
care must be taken not to load this pin.
Programming Input Current Limit
The
LT3791-1 has a standalone current sense amplifier.
It can be used to limit the input current. The input current
limit is calculated by the following equation:
Programming Output Current
The output current is programmed by placing an appro
-
priate value current sense resistor, R
OUT
, in series with
the output load. The voltage drop across R
OUT
is (Kelvin)
sensed by the ISP and ISN pins. The CTRL pin should
be tied to a voltage higher than 1.2V to get the full-scale
100mV (typical) threshold across the sense resistor. The
CTRL pin can also be used to adjust the output current,
although relative accuracy decreases with the decreasing
sense threshold. When the CTRL pin voltage is less than
1V, the output current is:
I
OUT
=
V
CTRL
– 200mV
R
OUT
• 10
When the CTRL pin voltage is between 1.1V and 1.3V the
output current varies with V
CTRL
, but departs from the
equation above by an increasing amount as V
CTRL
volt-
age increases.
Ultimately, when V
CTRL
> 1.3V the output
current no longer varies. The typical V
(ISP-ISN)
threshold
vs V
CTRL
is listed in Table 2.
LT3791-1
18
37911fa
For more information www.linear.com/LT3791-1
applicaTions inForMaTion
I
IN
=
50mV
R
IN
For loop stability a lowpass RC filter is needed. For
most applications, a 50Ω resistor and 470nF capacitor
is sufficient.
Table 3
R
IN
(mΩ) I
LIMIT
(A)
20 2.5
15 3.3
12 4.2
10 5.0
6 8.3
5 10.0
4 12.5
3 16.7
2 25
IVINMON
The IVINMON pin provides a linear indication of the current
flowing through the input. The equation for V
IVINMON
is
V
(IVINP-IVINN)
• 20. This pin is suitable for driving an ADC
input, however, the output impedance of this pin is 12.5kΩ
so care must be taken not to load this pin.
Programming Output Voltage (Constant Voltage
Regulation)
For a voltage regulator, the output voltage can be set by
selecting the values of R5 and R6 (see Figure 9) according
to the following equation:
V
OUT
= 1.2 •
R5
+
R6
R6
Dimming Control
There are two methods to control the current source for
dimming using the LT3791-1. One method uses the CTRL
pin to adjust the current regulated in the output. A second
method uses the PWM pin to modulate the current source
between zero and full current to achieve a precisely pro
-
grammed average current. To make PWM dimming more
accurate, the switch demand current is stored on the V
C
node during the quiescent phase when PWM is low. This
feature minimizes recovery time when the PWM signal goes
high. To further improve the recovery time a disconnect
switch may be used in the output current path to prevent
the ISP node from discharging during the PWM signal low
phase. The minimum PWM on- or off-time is affected by
choice of operating frequency and external component
selection. The best overall combination of PWM and
analog dimming capabilities is available if the minimum
PWM pulse is at least six switching cycles and the PWM
pulse is synchronized to the SYNC signal.
SHORT Pin
The LT3791-1 provides an open-drain status pin,
SHORT, which
pulls low when the FB pin is below 400mV.
The only time the FB pin will be below 400
mV is during
start-up or if the output is shorted. During start-up the
LT3791-1 ignores the voltage on the FB pin until the soft-
start capacitor reaches 1.75V. To prevent false tripping
after startup, a large enough soft-start capacitor must
be used to allow the output to get up to approximately
40% to 50% of the final value.
C/10 Pin
The LT3791-1 provides an open-drain status pin, C/10,
which pulls low when the FB pin is above 1.15V and the
voltage across V
(ISP-ISN)
is less than 10mV. For voltage
regulator applications with both ISP and ISN pins tied
together to the output (i.e., no output current sense and
limit), the C/10 pin provides a power good flag. For battery
charger applications with output current sense and limit,
the C/10 provides a C/10 charge termination flag.
Figure 9. Resistor Connection for Constant Output
Voltage Regulation
LT3791-1
V
OUT
R5
R6
37911 F09
FB
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LT3791MPFE-1#PBF

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Buy LT3791MPFE-1#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 60V 4-Switch Sync Buck-Boost Cntr
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