Choose a Regulator with an Accurate Input Current Limit to Safely Extract Maximum Power from USB

Choose a Regulator with an Accurate Input Current Limit to
Safely Extract Maximum Power from USB
Albert Lee
Most of us would not use
a paintbrush to sign our
names on our checks. It’s
simply the wrong tool for the
job—wasting paint where a
thin line of ink offers better
results. Likewise, using
a power supply IC with a
broad-brush ±40% output
current limit is wasteful,
requiring a designer to leave
80% of the input current
on the table, or worse,
ignore the tolerance and risk
collapse of the input supply.
To design an input-currentlimited supply that maximizes
input power usage requires
a detailed approach, ideally
incorporating a regulator
with tight tolerances.
The LTC3619 and LTC3619B are 400mA and
800mA dual monolithic synchronous
buck regulators with tight ±5% programmable average input current limits. The
LTC3619 uses Burst Mode operation to
improve efficiency at light loads, while
the LTC3619B uses pulse-skipping to
improve efficiency and also reduce noise.
The accurate input current limit allows
utilization of 90% of the maximum input
current for fast supercap charging, strong
signal and lag-free operation without
risk of collapsing the input supply.
22 | July 2010 : LT Journal of Analog Innovation
VIN
3.4V TO 5.5V
CIN
10µF
RPGD2
499k
PGOOD2
L2
1.5µH
VOUT2
3.4V AT
800mA
+
R4
1210k
COUT2
2.2mF
×2
SUPERCAP
RPGD1
499k
RUN2 VIN RUN1
R3
255k
PGOOD2 PGOOD1
LTC3619B
SW2
SW1
VFB2
VFB1
RLIM GND
CLIM
1000pF
CIN, COUT1: AVX 08056D106KAT2A
COUT2: VISHAY 592D228X96R3X2T20H
RLIM
116k
L1
3.3µH
PGOOD1
CF1, 22pF
VOUT1
1.8V AT
400mA
COUT1
10µF
R1
R2
255k 511k
ILIM = 475mA
L1: COILCRAFT LPS4012-332ML
L2: COILCRAFT LPS4012-152ML
Figure 1. GSM receiver power supply operates from the USB. Although GSM receivers require bursts of power
beyond the USB current limit, this design complies with USB current limits by incorporating a supercap to
provide short bursts of current at VOUT2 . Only VOUT2 is actively input current limited. The current at VOUT1
must operate within the power constraints of the USB input, but it is not actively input current limited, so the
voltage at VOUT1 remains stable for important circuits.
An increasing number of portable electronics are powered from the USB, which is
current limited to 500mA. Surges in current
on the USB commonly occur from plugging
a device into a port. If the surge current is
high, non-current-limited load dumps at
the USB port can glitch the source power
supply, which can affect other systems
that depend on the supply. Plugging in an
improperly current-limited USB device into
a laptop can glitch the laptop CPU, causing
it to lock up or reboot. High peak current
pulses during GSM wireless data transfers can also cause supply glitches. The
accurate current limits of the LTC3619 and
LTC3619B protect the supply while maximizing usage of available input current.
GSM APPLICATION
Users expect a high level of mobile functionality in their electronic devices—they
want their GSM modems to detect a strong
signal at all corners of the city. As more
current is required for better transmission
and reception, the importance of an accurate input current limit cannot be ignored.
Programming the LTC3619B’s accurate
input current limit to maximize available current usage is simple with an
external resistor RLIM and capacitor CLIM,
sized using the following expression:
RLIM =
55kΩ - A
IDC
Determine RLIM for the average input
current being limited (IDC), and choose
design features
Users expect a high level of mobile functionality in their
electronic devices—they want their GSM modems to detect
a strong signal at all corners of the city. As more current is
required for better transmission and reception, the importance
of an accurate input current limit cannot be ignored.
VOUT2
200mV/DIV
VIN
(AC COUPLED)
1V/DIV
IOUT
500mA/DIV
IIN
500mA/DIV
1ms/DIV
VIN = USB 5V, 500mA COMPLIANT
RLIM = 116k, CLIM = 2200pF
ILOAD = 0A TO 2.2A, COUT2 = 4.4mF, VOUT2 = 3.4V
ILIM = 475mA, CHANNEL 1 NOT LOADED
Figure 2. Operation of the GSM modem power
supply showing bursts of current beyond the input
current limit and quick charging of the supercap on
VOUT2 . Note the stability of VIN.
CLIM greater than 100pF for averaging
ILIM current. When the sum of both channels’ current exceeds the input current
limit, only channel 2 is current limited
while channel 1 remains regulated. Input
current limit starts when VRLIM = 1V ±5%.
VIN
3.4V TO 5.5V
CIN
10µF
The LTC3619B’s input current limit is
trimmed to less than 2% at room temperature and deviates no more than a
few percent over temperature. (See the
LTC3619B data sheet for detailed explanation in selecting RLIM and CLIM .) Figure 1
shows the LTC3619B in a solution that converts USB input (VIN = 5V) to VOUT2 = 3.4V to
deliver GSM pulsed current load. Figure 2
shows the GSM output waveforms.
GSM modems demand high bursts of
current, up to 2A, to the RF power amplifier, which well exceeds the maximum
500mA input current available via USB.
In Figure 1, LTC3619B quickly charges a
reservoir cap or supercap, which in turn
can adequately provide bursts of current.
PGOOD2
VOUT2
3.4V AT
800mA
+
R4
1210k
COUT2
2.2mF
×2
SuperCap
R3
255k
PGOOD1
L1
3.3µH
PGOOD2 PGOOD1
LTC3619B
SW2
SW1
CF1, 22pF
VFB1
VFB2
R1
R2
255k 1150k
RLIM GND
VRLIM
ILIM = 475mA
CLIM
1000pF
Figure 3. Overcurrent indicator takes advantage
of input current monitoring at the RLIM pin.
The PGOOD indicator pins are useful
for monitoring the regulation status of
the two outputs. The PGOOD pins are
open-drain outputs—pulled high with
an external pull-up resistor when in
regulation. In Figure 1, if the supercap is
not fully charged or the output is not in
regulation, the PGOOD2 pin is pulled low
by the internal NFET. A red LED indicator
can be used instead of a pull-up resistor.
Of course, PGOOD can be used to handshake with other circuitry, providing a
ready signal for the next load dump.
In applications that require additional
system control, the LTC3619B provides
accurate current sense information for
both channels. This information can be
used for protection schemes, feedback
control and other features. Figure 3 shows
a scheme for an LED overcurrent indicator.
RPGD1
499k
RUN2 VIN RUN1
L2
1.5µH
V OUT POWER GOOD
OVERCURRENT INDICATOR
Because channel 1 is not input current limited, its output voltage does not
collapse even as the GSM modem draws
high current bursts. Thus, it is safe to
use this channel to power up baseband chips, power management ICs or
RPGD2
499k
keep-alive circuitry while maximizing
the available current for channel 2.
+
VOUT1
3.3V AT
400mA
COUT1
10µF
LT1634-1.25
RB1
121k
RB2
75k
RB3
158k
RD1
20Ω
VREF
LED
+
LT1716
OVERCURRENT
–
RLIM
116k
RH
6k
Q1
July 2010 : LT Journal of Analog Innovation | 23
VREF = (400mA/55kΩ-A) × 116kΩ = 0.85V.
The 0.85V reference is created via a resistor
divider off of the LT1634 precision shunt
voltage reference and is compared to
VRLIM using the LT1716 precision comparator to create the overcurrent signal.
In this example, when VRLIM rises above
VREF, the LT1716 pulls OVERCURRENT low
and turns on LED1 while turning off
Q1, allowing RH to provide about 5%
of hysteresis for VRLIM . VRLIM would
have to be reduced by 5% in order to
clear the OVERCURRENT condition.
POWERING AN LED LIGHT AT INPUT
CURRENT LIMIT
If an application requires independent monitoring of only one channel,
the LTC3606B pulse-skipping, single
channel monolithic buck with input
current limit is a good choice. The
LTC3606B can be used to power an
LED driver chip or to directly drive a
large LED light, such as LED1 in Figure 5.
LED1 has a nominal on-voltage of 3.2V and
its current is limited by the input current limit of 400mA set by RLIM . In this
case, the current through LED1 is limited to 625mA, as calculated from
(VIN/VOUT) × ILIM = (5V/3.2V) × 400mA,
assuming VIN = 5V. The circuit is configured so that the LAMPGOOD indicator goes high when LED1 turns off or
burns out. When turned on, the current
through LED1 is at the input current limit
and the voltage at VOUT is 3.2V instead
of the regulated 4V. Because the operating LED forces VOUT more than 11% out
of regulation, PGOOD (a.k.a. LAMPGOOD)
falls low, indicating the lamp is on. If
LED1 turns off or burns out (no current
24 | July 2010 : LT Journal of Analog Innovation
K, the input current is derived. Setting
input current to the input current limit,
we get the following expression.
1.2
1
0.8
VRLIM (V)
Figure 4 shows the relationship between
the VRLIM voltage and the input current.
Set VREF equal to VRLIM voltage at the
overcurrent limit of interest. For example,
if the overcurrent limit were to be set at
85% of max current or 400mA, then set
ILIM = IOUT1 × D1 + IOUT2 × D2,
0.6
Channel 2 is configured to power LED1
at VOUT2 = 3.2V. If channel 1 is loaded at
400mA with VOUT1 = 1.8V, which translates
to 144mA of input current (IOUT1 × D1),
this subsequently leaves 400mA through
LED2 instead of the original 625mA, which
translates to 256mA of input current
(IOUT2 × D2) available for channel 2.
0.4
0.2
0
ILIM = 475mA
RLIM = 116K
0
100
200
400
300
IIN (mA)
500
600
Figure 4. VRLIM vs IIN for circuit in Figure 3
through it), VOUT returns to the regulated
4V and LAMPGOOD is pulled high via RPGD,
indicating that the lamp is not operating.
According to the above ILIM expression, a
higher LED1 turn-on voltage, for instance
due to looser manufacturing tolerance,
would result in reduced current through
LED1 and vice versa. This is an appealing,
self-adjusting feature in this application to
keep the intensity constant over manufacturing differences. Using an LED driver
IC to power multiple LEDs on channel 2 is
preferred although the LTC3606B may be
used to drive an LED light bulb efficiently.
If the LTC3619B were used in the previous example, the available input current to channel 2 would be dependent
on the input current of channel 1. Using
the expression below, the current out of
RLIM pin can be calculated. This is the
summed representation of the inductor
currents from both channels and illustrates how the currents are distributed.
CONCLUSION
The LTC3619 and the LTC3606B are buck
regulators that combine average input
current limit and current sense information in a 10-lead MSOP and DD package.
The LTC3619’s accurate input current limit
is ideal for USB powered applications,
where the USB port’s output current is
limited. At the same time, the current sense
information simplifies designs in applications that require detection and monitoring of current in a single-chip solution
requiring no additional board area. n
IR(LIM) = IOUT1 × D1 × K1 + IOUT2 × D2 × K2,
where D1 = VOUT1 /VIN and
D2 = VOUT2 /VIN are the duty cycles
of channels 1 and 2, respectively.
K1 and K2 are the ratio
RDS(ON) (POWER PFET)
RDS(ON) (SENSE PFET)
of channels 1 and 2, respectively and
are internally trimmed to better than
2% accuracy at 1/55kΩ-A. Assuming
K = K1 = K2, and dividing both sides by
Figure 5. LED driver with lamp status indicator (LAMPGOOD)
L1
1.5µH
VIN
USB INPUT 5V
CIN
10µF
RPGD
499k
VIN
LTC3606B
RUN
PGOOD
VFB
RLIM
LAMPGOOD
GND
CLIM
1000pF
VOUT
4V AT
800mA
SW
RLIM
137k
ILIM = 400mA
R2
1470k
R1
255k
+
COUT
10µF
LED1
RS
0.1Ω
1W
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