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LTC3619IDD#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P22
LTC3619
10
3619fa
operaTion
When the output is loaded heavily, for example, with
millifarad of capacitance, it may take longer than 950µs
to charge the output to regulation. If the output is still low
after the soft-start time, the LTC3619 will try to quickly
charge the output capacitor. In this case, the input current
limit (after it engages) can prevent excessive amount of
instantaneous current that is required to quickly charge
the output. See the Channel 2 Start-Up from Shutdown
curve in the Typical Performance Characteristics section.
After input current limit is engaged, the output slowly
ramps up to regulation while limited by its 500mA of
input current.
Short-Circuit Protection
When either regulator output is shorted to ground, the
corresponding internal N-channel switch is forced on for
a longer time period for each cycle in order to allow the
inductor to discharge, thus preventing inductor current
runaway. This technique has the effect of decreasing
switching frequency. Once the short is removed, normal
operation resumes and the regulator output will return to
its nominal voltage.
Input Current Limit
Internal current sense circuitry in each channel measures
the inductor current through the voltage drop across the
power PFET switch and forces the
same voltage across
the small sense PFET. The voltage across the small sense
PFET generates a current representing 1/55,000th of the
inductor current during the on-cycle. The current out of
RLIM pin is the summed representation of the inductor
currents from both channels, which can be expressed in
the following equation.
I
RLIM
= I
OUT1
• D1 • K1 + I
OUT2
• D2 • K2,
where D1 = V
OUT1
/V
IN
and D2 = V
OUT2
/V
IN
are the duty
cycle of channel 1 and 2, respectively.
K1 is the ratio R
DS(ON)
(power PFET)/R
DS(ON)
(sense PFET)
of channel 1, and K2 is the ratio R
DS(ON)
(power PFET)/
R
DS(ON)
(sense PFET) of channel 2. The ratio of the power
PFET to the sense PFET is trimmed to within 2%.
Given that both PFETs are carefully laid out and matched,
their temperature and voltage coefficient effects will be
similar and their terms be canceled out in the equation. In
that case, the constants K1 and K2 will only be dependent
on area scaling, which is trimmed to within 2%. Thus, the
I
RLIM
current will track the input current very well over
varying temperature and V
IN
.
The RLIM pin can be grounded to disable input
current
limit function.
Programming Input Current Limit
Selection of one external R
LIM
resistor will program the
input current limit. The current limit can be programmed
from 200mA up to I
PEAK
current. As the input current
increases, R
LIM
voltage will follow. When R
LIM
reaches
the internal comparator threshold of 1V, channel 2’s
power PFET on-time will be shortened, thereby, limiting
the input current.
Use the following equation to select the R
LIM
resistance
that corresponds to the input current limit.
R
LIM
= 55k / I
DC
I
DC
is the input current (at V
IN
) to be limited. The following
are some R
LIM
values with the corresponding current limit.
R
LIM
I
DC
91.6k 600mA
110k 500mA
137.5k 400mA
Selection of C
LIM
Capacitance
Since I
RLIM
current is a function of the inductor current,
its dependency on the duty cycle cannot be ignored. Thus,
a C
LIM
capacitor is needed to integrate the I
RLIM
current
and smooth out transient currents. The LTC3619 is stable
with any size capacitance >100pF at the RLIM pin.
Each application input current limit will call for different
C
LIM
value to optimize its response time. Using a large C
LIM
capacitor requires longer time for the RLIM pin voltage to
charge. For example, consider the application 500mA input
current limit, 5V input and 1A, 2.5V output with a 50% duty
LTC3619
11
3619fa
cycle. When an instantaneous 1A output pulse is applied,
the current out of the RLIM pin becomes 1A/55k = 18.2µA
during the 50% on-time or 9.1µA full duty cycle. With a
C
LIM
capacitor of 1µF, R
LIM
of 116k, and using I = CdV/dt,
it will take 110ms for C
LIM
to charge from 0V to 1V. This
is the time after which the LTC3619 will start input current
limiting. Any current within this time must be considered
in each application to determine if it is tolerable.
Figure 1a shows V
IN
(I
IN
) current below input current limit
with a C
LIM
capacitor of 0.1µF. Channel 1 is unloaded to
simplify calculations. When the load pulse is applied, under
the specified condition, I
LIM
current is 1.1A/55k•0.66 =
13.2µA, where 0.66 is the duty cycle. It will take a little
more than 7.5ms to charge the C
LIM
capacitor from 0V to
1V, after which the LTC3619 begins to limit input current.
The I
IN
current is not limited during this 7.5ms time and is
more than 725mA. This current transient may cause the
input supply to temporarily droop if the supply current
compliance is exceeded, but recovers after the input
cur-
rent limit engages. The output will continue to deliver the
required current load while the output voltage droops to
allow the input voltage to remain regulated during input
current limit.
For applications with short load pulse duration, a smaller
C
LIM
capacitor may be the better choice as in the example
shown in Figure 1b. Channel 1 is unloaded for simplifi-
cation. In this example, a 577µs, 0A to 2A output pulse
is applied once every 4.7ms. A C
LIM
capacitor of 2.2nF
requires 92µs for V
RLIM
to charge from 0 to 1V. During
this 92µs, the input current limit is not yet engaged and
the output must deliver the required current load. This
may cause the input voltage to droop if the current com-
pliance is exceeded. Depending on how long this time is,
the V
IN
supply decoupling capacitor can provide some of
this current before V
IN
droops too much. In applications
with a bigger V
IN
supply decoupling capacitor and where
V
IN
supply is allowed to droop closer to dropout, the C
LIM
capacitor can be increased slightly. This will delay the start
of input current limit and artificially regulated V
OUT
before
input current limit is engaged. In this case, within the
577µs
load pulse, V
OUT
voltage will stay artificially regulated for
92µs out of the total 577µs before the input current limit
activates. This approach may be used if a faster recovery
on the output is desired.
Selecting a very small C
LIM
will speed up response time
but it can put the device within threshold of interfering
with normal operation and input current limit in every
few switching cycles. This may be undesirable in terms
of noise. Use 2πRC >> 100/clock frequency (2.25MHz) as
a starting point, R being R
LIM
, C being C
LIM
.
operaTion
Figure 1a. Input Current Limit Within 100ms Load Pulses Figure 1b. Input Current Limit Within 577µs,
2A Repeating Load Pulses
50ms/DIV
V
OUT
2V/DIV
I
L
1A/DIV
V
RLIM
1V/DIV
I
VIN
500mA/DIV
3619 F01a
V
IN
= 5V, 500mA COMPLIANT,
R
LIM
= 116k, C
LIM
= 0.1µF
I
LOAD
= 0A TO 1.1A, C
OUT
= 2.2mF, V
OUT
= 3.3V
I
LIM
= 475mA, CHANNEL 1 UNLOADED
1ms/DIV
V
OUT
200mV/DIV
I
IN
500mA/DIV
I
OUT
500mA/DIV
V
IN
1V/DIV
AC-COUPLED
3619 F01b
V
IN
= 5V, 500mA COMPLIANT,
R
LIM
= 116k, C
LIM
= 2200pF
I
LOAD
= 0A TO 2A, C
OUT
= 2.2mF, V
OUT
= 3.3V
I
LIM
= 475mA, CHANNEL 1 NOT LOADED
LTC3619
12
3619fa
A general LTC3619 application circuit is shown in Figure 2.
External component selection is driven by the load require-
ment, and begins with the selection of the inductor L. Once
the inductor is chosen, C
IN
and C
OUT
can be selected.
Inductor Selection
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current DI
L
decreases with
higher inductance and increases with higher V
IN
or V
OUT
:
∆I
L
=
V
OUT
f
O
• L
• 1−
V
OUT
V
IN
(1)
Accepting larger values of DI
L
allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
40% of the maximum output load current. So, for a 800mA
regulator, DI
L
= 320mA (40% of 800mA).
The inductor value will also have an effect on Burst Mode
operation. The transition to low current operation begins
when the peak inductor current falls below a level set by
the internal burst clamp. Lower inductor values result in
higher ripple current which causes the transition to occur
at lower load currents. This causes a dip in efficiency in
the upper range of low current operation. Furthermore,
lower inductance values will cause the bursts to occur
with increased frequency.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. To-
roid or shielded pot cores in ferrite or permalloy materials
are small and do not radiate much energy, but generally
cost more than powdered iron core inductors with similar
electrical characteristics. The choice of which style induc-
tor to use often depends more
on the price versus size
requirements,
and any radiated field/EMI requirements,
than on what the LTC3619 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3619 applications.
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V
OUT
/ V
IN
.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
I
RMS
≈I
MAX
V
OUT
(V
IN
− V
OUT
)
V
IN
Where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– DI
L
/2. This formula has a maximum at
V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case
is commonly used to design because even significant
deviations do not offer much relief. Note that capacitor
manufacturer’s ripple current ratings are often based on
only 2000 hours lifetime. This makes it advisable to further
V
IN
RUN2 RUN1
LTC3619
V
FB2
SW2
SW1
PGOOD1PGOOD2
V
FB1
C
F2
C
F1
GND
V
IN
2.5V TO 5.5V
V
OUT2
V
OUT1
3619 F02
R4 R2
R3 R1
L2 L1
C
OUT2
C
OUT1
C
IN
R
LIM
C
LIM
RLIM
Figure 2. LTC3619 General Schematic
applicaTions inForMaTion
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LTC3619IDD#PBF

Mfr. #:
Buy LTC3619IDD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 400mA/800mA Synchronous Step-Down DC/DC Converter with Average Input Current Limit
Lifecycle:
New from this manufacturer.
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