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LTC3114MPDHC-1#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P34
LTC3114-1
22
31141fb
For more information www.linear.com/LTC3114-1
applicaTions inForMaTion
applications are generally similar except that voltage ripple
is generally not a concern. Some capacitors exhibit a high
DC leakage current which may preclude their consideration
for applications that require a very low quiescent current
in Burst Mode operation.
Ceramic capacitors are often utilized in switching con
-
verter applications due to their small size, low ESR and
low leakage currents. However, many ceramic capacitors
intended for power applications experience a significant
loss in capacitance from their rated value as the DC bias
voltage on the capacitor increases. It is not uncommon
for a small surface mount capacitor to lose more than
50% of its rated capacitance when operated near its
maximum rated voltage. This effect is generally reduced
as the case size is increased for the same nominal value
capacitor. As a result, it is often necessary to use a larger
value capacitance or a higher voltage rated capacitor than
would ordinarily be required to actually realize the intended
capacitance at the operating voltage of the application. X5R
and X7R dielectric types are recommended as they exhibit
the best performance over the wide operating range and
temperature of the LTC3114-1. To verify that the intended
capacitance is achieved in the application circuit, be sure
to consult the capacitor vendor’s curve of capacitance
versus DC bias voltage.
Programming Custom V
IN
Turn-On and Turn-Off
Thresholds
With the addition of an external resistor divider connected
to the input voltage as shown in Figure 3, the RUN pin
can be used to program the input voltage at which the
LTC3114-1 is enabled and disabled.
For a rising input voltage, the LTC3114-1 is enabled when
V
IN
reaches the threshold given by the following equation,
where R1 and R2 are the values of the resistor divider
resistors specified in kΩ:
V
TH(RISING)
= 1.2
R1
+
R2
R2
⎛
⎝
⎜
⎞
⎠
⎟
Volts
Once the IC is enabled, it will remain so until the input
voltage drops below the comparator threshold by the
hysteresis voltage of approximately 100mV, measured
on the RUN pin. Therefore, the amount of hysteresis is
approximately 8.33% of the programmed turn-on threshold
level given in the previous equation.
Bootstrapping the LDO Regulator
The hi
gh and low side gate drivers are powered through the
PLDO rail, which is generated from the input voltage, V
IN
,
through an internal linear regulator. In some applications,
especially at high input voltages, the power dissipation
in the linear regulator can become a major contributor
to thermal heating of the IC. The Typical Performance
Characteristics section of this data sheet provides data on
the LDO/PLDO current and resulting power loss versus
V
IN
and V
OUT
. A significant performance advantage can
be attained in applications where converter output voltage
(V
OUT
) is programmed to 5V, if V
OUT
is used to power the
LDO/PLDO rails. Powering the LDO/PLDO rails in this
manner is referred to as bootstrapping. This can be done
by connecting a Schottky diode from V
OUT
to LDO/PLDO
as shown in Figure 5. With the bootstrap diode installed,
the gate driver currents are supplied by the buck-boost
converter at high efficiency rather than through the inter
-
nal linear regulator. The internal linear regulator contains
reverse blocking circuitr
y that allows LDO/PLDO pins to
be driven slightly above their nominal regulation level with
only a very slight amount of reverse current. Please note
that the bootstrapping supply (either V
OUT
or a separate
regulator) must be limited to less than 5.7V.
V
OUT
4.7µF
31141 F05
PV
OUT
LTC3114-1
LDO
PLDO
Figure 5. Bootstrapping PLDO and LDO
Average Output Current Limit Programming
The LTC3114-1 includes an average output current pro-
gramming feature that transforms the LT
C3114
-1 into a
wide voltage compliance range, high efficiency, constant
current source. A resistor from PROG to ground programs
the desired level of average output current up to 1A.
Potential uses include high brightness LED driving and
constant current battery or capacitor charging.
LTC3114-1
23
31141fb
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applicaTions inForMaTion
A simplified diagram of the average output current program-
ming circuitry is shown in the Block Diagram. An internal
sense resistor, R
S
, and low offset amplifier directly measure
current in the V
OUT
path and produce a small fraction of
this current out of the PROG pin. Accordingly, a resistor
and filtering capacitor connected from PROG to ground
produce a voltage proportional to average output current
on PROG. An internal transconductance amplifier compares
the PROG voltage to the fixed 1V internal reference. If the
PROG voltage tries to exceed the 1V reference level, this
amplifier will pull down on VC and take command of the
PWM. As described earlier, VC is the current command
voltage, so limiting VC in this manner will also limit output
current. The resulting average output current is given by
the following equation:
I
OUT(AVG)
≅ 25,000•
1V
R
PROG
where: R
PROG
= 24.9k to 100k.
The largest recommended PROG pin resistor is 100k. Val-
ues of R
PROG
larger than 100k may latch-off the LTC3114-1
if V
OUT
is forced to less than 2V by an external load. This
is generally not an issue for battery charging applications,
but may prevent the charging of very large capacitors. In
some general purpose power supply applications, this
latch-off behavior may be desirable and in these cases,
values of R
PROG
> 100k are acceptable to use.
The gain of 25,000 is generated internal to the LTC3114-1
and is factory trimmed to provide the best accuracy at
500mA of output current. The accuracy of the programmed
output current is best at the high end of the range as the
residual internal current sense amplifier offset becomes
a smaller percentage of the total current sense signal
amplitude with increasing current. The provided electrical
specifications define the PROG pin current accuracy over
a range of output currents.
Selecting the capacitor, C
PROG
, to put in parallel with
R
PROG
is a trade-off between response time, output cur-
rent ripple and interaction with the normal output voltage
control loop. In general, if speed is not a concern as is the
case for most current sourcing applications, then C
PROG
should be made at least 3 times higher than the voltage
error amplifier compensation capacitor, C
P1
, described
in the Compensation section of this data sheet. This will
ensure minimal to no interaction when the transition oc-
curs between voltage regulation mode and output current
regulation mode.
In current sourcing applications, the maximum output
compliance voltage of the LTC3114-1 is set by the voltage
error amplifier dividers resistors as it is for standard volt
-
age regulation applications. For LED drivIng applications,
select the V
OUT
divider resistors for a clamping level 1V
to 2V higher than the expected forward voltage drop of
the LED string.
The average output current circuitry can
also be used to monitor, rather than control the output
current. To do this, select an R
PROG
value that will limit
the voltage on the PROG pin to 0.8V or less at the highest
output current expected in the application.
Connect a 20k resistor and 33nF capacitor from PROG to
ground if the function is not going to be used to provide a
higher level of protection against inadvertent short-circuit
conditions on V
OUT
.
Compensation of the Buck-Boost Converter
The LTC3114-1 utilizes average current mode control to
regulate the output voltage. Average current mode control
has two loops that require frequency compensation, the
inner average current loop and the outer voltage loop.
The compensation for the inner average current loop is
fixed within the LTC3114-1 in order to provide the highest
possible bandwidth over the wide operating range of the
LTC3114-1. Therefore, the only control loop that requires
compensation design is the outer voltage loop. As will be
shown, compensation design of the outer loop is similar
to the techniques used in well known peak current mode
control devices.
The LTC3114-1 utilizing average current mode control
can be conceptualized in its simplest form as a voltage-
controlled current source (V
CCS
), driving the output load
formed primarily by R
LOAD
and C
OUT
, as shown in Figure 6.
The error amplifier output (VC), provides the command
input to the V
CCS
. The full-scale range of VC is 0.865V
(135mV to 1V). With a full-scale command on VC,
the LTC3114-1 buck-boost converter will generate an
average 1.7A of inductor current (typical) from the con-
LTC3114-1
24
31141fb
For more information www.linear.com/LTC3114-1
applicaTions inForMaTion
verter for a transconductance gain of 1.97A/V. Similar to
peak current mode control, the inner average current mode
control loop effectively turns the inductor into a current
source over the frequency range of interest, resulting in a
frequency response from the power stage that exhibits a
single pole (–20dB/decade) roll off. The output capacitor
(C
OUT
) and load resistance (R
LOAD
) form the normally
dominant low frequency pole and the effective series
resistance of the output capacitor and its capacitance form
a zero, usually at a high enough frequency to be ignored. A
potentially troublesome right half plane zero (RHPZ) is also
encountered if the
LTC3114-1 is operated in boost mode.
The RHPZ causes an increase in gain, like a zero, but a
decrease in phase, like a pole. This will ultimately limit the
maximum converter bandwidth that can be achieved with
the LTC3114-1. The RHPZ is not present when operating
in buck mode. The overall open loop gain at DC is the
product of the following terms:
Voltage Error Amp Gain:
g
m
• R
O
= 120µs • 3.6M = 432V/V (not adjustable)
Voltage Divider Gain:
V
FB
V
OUT
=
1V
V
OUT
(determined by the application, V
FB
is the reference
voltage for the voltage error amplifier)
Current Loop Transconductance:
G
C
=
1.7A
0.865V
= 1.97A/V
(not adjustable)
Load Resistance (R
LOAD
) (determined by the application)
The frequency dependent terms that affect the loop gain
include:
Output Load Pole(P1):
1
2π •R
LOAD
•C
OUT
(application dependent)
Error Amplifier Compensation (2 Poles and 1 Zero):
These are the design variables available
Right Half Plane Zero (RHPZ): boost mode only (de
-
termined by maximum load, V
IN
, V
OUT
and inductor)
Current Amplifier Compensation Components (Fixed
Internal to the LTC3114-1)
The internal current amplifier and inner current loop
have a much higher bandwidth than the overall loop,
however, unlike an ideal V
CCS
with a flat gain versus
frequency characteristic, the inner loop exhibits gain
peaking in the range of approximately 2kHz to 20kHz
that is an artifact of the fixed current amplifier compen
-
sation. This gain peaking has the effect of pushing out
the overall loop crossover frequency, while
providing some phase margin boost as well. As long as
there is sufficient margin between the loop crossover
frequency and the worst-case RHPZ frequency, then
stable operation over all conditions is relatively easy
to achieve.
The design parameters for compensation design will focus
on the series resistor and capacitors connected from VC to
ground (R
Z
, C
P1
and C
P2
). The general goal is to provide
a phase boost using the compensation network zero in
order to maximize the bandwidth and phase margin of the
converter. Being a buck-boost converter, the target loop
crossover frequency for the compensation design will be
dictated by the highest boost ratio and load current that is
expected as this will result in the lowest RHPZ frequency.
An illustrative example is provided next that will derive
the compensation components for a typical LTC3114-1
application.
Compensation Example
This section will demonstrate how to derive and select
the compensation components for a typical LTC3114-1
application. Designing compensation for other applications
+
–
g
m
1V
1V
g
m
= 1.7A/0.865V
+
–
FB
VOLTAGE
ERROR
AMP
VOLTAGE
CONTROLLED
CURRENT
SOURCE
VC
GND
R
Z
R
TOP
1.7M
R
BOT
100k
R
COSER
0.01Ω
R
LOAD
18Ω
C
P1
C
OUT
22µF
V
OUT
C
P2
31141 F06
Figure 6. Simplified Representation of
Average Current Mode Control Loop
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LTC3114MPDHC-1#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 40V, 1A Sync. Buck-Boost Converter with Programmable Current Limit
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