Mar 2003 Micropower SOT-23 Buck Regulator Accepts Inputs to 34V

DESIGN FEATURES
Micropower SOT-23 Buck Regulator
by Jeff Witt
Accepts Inputs to 34V
Introduction
D2
The LT1934 is a micropower buck
regulator with an internal 400mA
power switch. It provides up to 300mA
output current, while consuming
just 12µA of quiescent current, an
important feature for always-on battery-powered applications such as
laptop computer standby supplies
or remote instrumentation that must
operate for years from primary batteries. Its wide input voltage range of
3.4V to 34V makes it applicable to
a variety of power sources including
24V industrial supplies, automotive
batteries and unregulated wall transformers.
3.3V 250mA Supply from 24V
Input Consumes Just 15µA
Figure 1 shows a 3.3V/250mA supply
that accepts inputs from 4.5V to 34V.
With the output in regulation and with
no load, the input current with VIN =
24V is less than 15µA. Input current
falls to less than 1µA when the LT1934
is placed in shutdown mode by pulling
SHDN to ground. The SHDN pin can be
tied to VIN if the shutdown mode is not
used. The capacitor and diode tied to
the BOOST pin provide a bias voltage
above VIN in order to fully saturate the
internal NPN power switch, thereby
maintaining high efficiency over the
entire input voltage range.
100
90
LT1934
VOUT = 3.3V
L = 47µH
EFFICIENCY (%)
VIN = 5V
80
70
VIN = 24V
VIN = 12V
60
50
0.1
1
10
100
LOAD CURRENT (mA)
Figure 1b. Efficiency of Figure 1’s ciruit
6
0.22µF
BOOST
VIN
4.5V TO 34V
ON OFF
VOUT
3.3V
250mA
SW
VIN
C2
2.2µF
L1
47µH
D1
LT1934
SHDN
10pF
FB
1M
+
C1
100µF
604k
GND
C1: SANYO 4TPB100M
C2: TAIYO YUDEN GMK325BJ225MN
D1: ON SEMI MBR0540
D2: CENTRAL CMDSH-3
L1: SUMIDA CDRH4D28-470
(619) 661-6835
(408)573-4150
(602) 244-6600
(516) 435-1110
(847) 956-0667
Figure 1a. This buck regulator supplies 250mA at 3.3V from an input voltage as high as 34V.
Input current with no load applied is less than 15µA.
The LT1934 uses Burst Mode® operation, with low quiescent current, to
achieve high efficiency across a wide
range of load currents, as shown in
Figure 1b. An internal comparator
monitors the voltage at the FB pin.
When the voltage at this pin is above
1.25V, the LT1934 is in its sleep mode
with only its reference and feedback
comparator biased. In this state the VIN
pin current is 12µA. As the load current discharges the output capacitor,
the FB pin voltage falls below 1.25V
and the comparator enables the internal oscillator and drive circuits. The
LT1934 switches with a fixed off time
of 1.8µs, limiting the peak inductor
current to 400mA as it delivers power
to the output. Figure 2 shows the operating waveforms of the circuit in Figure
1a. Output voltage ripple is less than
50mVP–P. Figure 3a shows a circuit
that generates 5V from a 6.5V–34V
input. Figure 3b shows the efficiency
of this circuit.
The accurate, fast, cycle-by-cycle
current limit of the LT1934 keeps the
switch and inductor currents under
control at all times. In addition, the
1.8µs switch off time is extended
during fault conditions when the
output voltage is pulled below the
programmed output voltage. This
ensures that the LT1934 can handle
a shorted output.
The LT1934-1 for Low Current
Supplies
The LT1934-1 is identical to the
LT1934 except that the current limit
is 120mA. The LT1934-1 can be used
in applications with a maximum load
current of 60mA. This lower current
limit allows a better match between the
power components (the inductor and
the input and output capacitors) and
the application, resulting in a smaller
circuit size. Input ripple is also lower,
reducing noise and simplifying input
filtering. The LT1934-1 may be a better choice when the power source
has a high output impedance, such
as 4mA-20mA loops, long-life primary
batteries with high internal resistance
VOUT
50mV/
DIV
VSW
10V/DIV
ISW
0.5A/
DIV
IL1
0.5A/
DIV
5µs/DIV
Figure 2. The operating waveforms of the
circuit in Figure 1 as it delivers 180mA from
a 10V input. The fast, accurate, cycle-by-cycle
current limit results in a robust circuit that
can withstand a shorted output.
Linear Technology Magazine • March 2003
DESIGN FEATURES
D2
100
BOOST
VIN
6.5V TO 34V
VOUT
5V
250mA
SW
VIN
C2
2.2µF
D1
LT1934
ON OFF
L1
68µH
10pF
1M
FB
SHDN
+
VIN = 12V
C1
68µF
332k
GND
C1: SANYO 6TPB68M
C2: TAIYO YUDEN GMK325BJ225MN
D1: ON SEMI MBR0540
D2: CENTRAL CMPD914
L1: SUMIDA CDRH5D28-680
LT1934
VOUT = 5V
90
EFFICIENCY (%)
0.1µF
80
VIN = 24V
70
60
(619) 661-6835
(408) 573-4150
(602) 244-6600
(516) 435-1110
(847) 956-0667
50
0.1
Figure 3a. The LT1934 provides 250mA at 5V.
1
10
100
LOAD CURRENT (mA)
Figure 3b. Efficiency of Figure 3’s circuit
D2
0.1µF
BOOST
VIN
4.5V TO 34V
VIN
C2
1µF
D1
SHDN
10pF
Li-Ion Battery Charger with
Industrial Input Range
VOUT
3.3V
45mA
SW
LT1934-1
ON OFF
L1
100µH
1M
FB
+
The circuit in Figure 5 is a complete,
single-cell Li-Ion battery charger that
operates from an input of 7V to 28V
and provides a charge current of
350mA. This charger requires no microcontroller. A charge cycle is initiated
when power is applied to the circuit,
and end-of-charge is indicated when
the battery voltage reaches 4.2V and
charge current has fallen to one tenth
of the nominal charge current.
The LT1934 acts as a currentlimited 5V pre-regulator for the
LTC4052-4.2 pulse charger. The
LTC4052 monitors the battery voltage
and pulls the CHRG pin low to indicate
that the battery is being charged.
C1
22µF
604k
GND
C1: TAIYO YUDEN JMK316BJ226ML
C2: TAIYO YUDEN GMK316BJ105ML
D1: ON SEMI MBR0540
D2: CENTRAL CMDSH-3
L1: COILCRAFT DO1608C-104 OR
WURTH ELECTRONICS WE-PD4 TYPE S
(408)573-4150
(602) 244-6600
(516) 435-1110
(847) 639-6400
Figure 4. Smaller components can be used with the lower current LT1934-1.
or remote supplies with long cables.
Low maximum switch current is also
helpful in designing intrinsically safe
systems, which have limits on stored
energy and require current limiting
resistors in series with external connections. Figure 4 shows a 3.3V supply
implemented with the LT1934-1.
D2
VIN
7V TO 28V
0.1µF
L1
47µH
BOOST
D3
VIN
SW
SHDN
GATE
0.022µF
CHRG
FB
332k
+
C1
47µF
ACPR
SENSE
BAT
TIMER
C5
10µF
C1: SANYO 6TPB47M
C2: TAIYO YUDEN GMK316BJ105ML
D1, D3: ON SEMICONDUCTOR MBR0540
D2: CENTRAL CMDSH-3
L1: SUMIDA CR43-470
0.047µF
LTC4052
1.00M
GND
+
1k
D1
LT1934
C2
1µF
10k
VIN
1k
GND
CTIMER
0.1µF
(619) 661-6835
(408) 573-4150
(602) 244-6600
(516) 435-1110
(847) 956-0667
350mA
1-CELL 4.2V
Li-Ion
BATTERY
CHARGE STATUS
AC PRESENT
Figure 5. This standalone 350mA Li-Ion battery charger accepts inputs to 28V and requires no heat sinks.
Linear Technology Magazine • March 2003
7
DESIGN FEATURES
500
VIN = 24V
CHARGE CURRENT (mA)
400
VIN = 8V
300
VIN = 12V
200
100
0
2.5
3.0
3.5
4.0
BATTERY VOLTAGE (V)
4.5
Figure 6. Charging current as a function of
battery voltage and input voltage
If this voltage is less than 2.5V,
the LTC4052 applies a 24mA trickle
charge, with the LT1934 maintaining
5V across C1.
When the battery voltage rises
above 2.5V, the LTC4052 enters its
fast charge mode, turning on an internal N-channel FET switch between
its SENSE and BAT pins. This connects
the LT1934’s output to the battery.
As the voltage across C1 falls to the
battery voltage, the LT1934 limits its
output current, charging the battery
at 350mA. Because the FET is fully
on, very little power is dissipated, and
the charging circuit maintains high
efficiency during the entire charge
cycle.
When the battery voltage reaches
4.2V, the LTC4052 turns off the switch
for 100ms. If the battery voltage falls
below 4.2V during this period, the
switch is turned back on for at least
400ms. In this manner, the LTC4052
modulates the duty cycle of the 350mA
current source. When this duty cycle
falls to 10%, the LTC4052 indicates
that the battery is nearly charged by
switching the CHRG pin to a weak
40µA pull down. Pulse charging continues until a timer in the LTC4052
stops the charge cycle after three
hours, when the CHRG pin goes to a
high impedance state. If the battery
voltage is pulled below 4.05V due to
a load current, the LTC4052 reenters
its fast charge mode.
If input power is removed, the
LTC4052 isolates the LT1934’s circuitry and reduces battery load to
less than 1µA. Diode D3 provides
reverse input protection.
8
The LT1934 regulates its output
current by limiting the peak current
in L1 to 400mA. The charge current
equals the average current in L1,
and depends on the input and output voltages, as shown in Figure 6.
In addition to these variations, expect
approximately ±15% variation in the
nominal charge current due to variations in the current limit and switch
off time of the LT1934 and the value
of L1.
In addition to indicating charge
status, the LTC4052 also provides
an AC-present flag that indicates that
power has been applied to the charger.
CLOSING SWITCH
SIMULATES HOT PLUG
IIN
+
LOW
IMPEDANCE
ENERGIZED
24V
SUPPLY
VIN
A similar device, the LTC1730, can be
used to provide additional features,
including a thermistor interface for
monitoring the temperature of the
battery and a shutdown pin that can
be used to reinitialize the charge cycle
timer.
This Li-Ion charger uses only
surface mount devices, requires no
heat sinks, and its wide input voltage range accepts a variety of power
sources including unregulated wall
transformers, car batteries and 24V
industrial supplies. The charger is
completely standalone, requiring no
continued on page 27
VIN
10V/DIV
LT1934
2.2µF
IIN
10A/DIV
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
10µF
35V
AI.EI.
a
10µs/DIV
LT1934
+
2.2µF
b
1Ω
LT1934
0.1µF
2.2µF
c
LT1934-1
uh oh…
1µF
d
4.7Ω
LT1934-1
0.1µF
ahh…
1µF
e
Figure 7. A well chosen input network prevents input voltage overshoot and ensures reliable
operation when the LT1934 is connected to a live supply.
Linear Technology Magazine • March 2003
DESIGN FEATURES
uses a P-channel MOSFET located in
series with the wall adapter power
path. This MOSFET functions like
a lossless ideal diode—replacing the
large series diode that is used to prevent battery current from going back
to the power adapter. The MOSFET
is actively driven in a linear mode to
maintain a constant 25mV forward
voltage, which makes the MOSFET act
as an ideal diode when forward current
flows yet allows fast (10µs) cutoff when
a current reversal is sensed.
Small PCB Footprint
Traditionally, high current chargers
require a large number of external support components, but the LTC4006,
LTC4007 and LTC4008 offer features
to push solution size down.
For instance, N-channel MOSFETs
are traditionally used in high current
applications because of their lower
RDS(ON). Today, P-channel MOSFETs
offer the performance that N-channel
LT1934, continued from page 8
microcontroller coding. On the other
hand, its charge status and AC-present
pins provide useful information to the
user through LEDs or to a microcontroller through a few digital I/O pins,
making it simple to integrate into a
battery powered system.
Hot Plugging Safely
The small size, robustness and low
impedance of ceramic capacitors make
them an attractive option for the input bypass capacitor of LT1934 and
LT1934-1 circuits. However, these
capacitors can cause problems if the
LT1934 is plugged into a live supply.1
The low-loss ceramic capacitor combined with stray inductance in series
with the power source forms an underdamped tank circuit, and the voltage at
the VIN pin of the LT1934 can ring to
twice the nominal input voltage, possibly exceeding the LT1934’s rating
and damaging the part. If the input
supply is poorly controlled or the user
will be plugging the LT1934 into an
energized supply, the input network
should be designed to prevent this
overshoot.
Linear Technology Magazine • March 2003
MOSFETs were offering only a couple
of years ago. Moving from an N-channel to a P-channel MOSFET drastically
simplifies the design of the charger
circuit. There are no boosted topside
gate drive supplies to deal with, saving
components and IC pins.
Increasing the switching frequency
reduces the inductor size and output
capacitance requirements. Loop
response is also improved, further
reducing the output capacitance such
that small ceramic capacitors can be
used. Using a 100mV regulation point
for current sensing, small 1206 sized
sense resistors can be used.
The INFET circuit is also used to
generate the AC present flag without
requiring an extra comparator to detect
the presence of a power adapter. Finally, improved internal circuit design
leads to further reduction in both pin
count, part count and the size of each
external component needed to make
the IC work.
These features add up to an overall
system solution that meets the needs
of today’s smaller products.
Figure 7 shows the waveforms
that result when an LT1934 circuit
is connected to a 24V supply through
six feet of 24-gauge twisted pair. The
first plot is the response with a 2.2µF
ceramic capacitor at the input. The
input voltage rings as high as 35V, and
the input current peaks at 20A. One
method of damping the tank circuit is
to add another capacitor with a series
resistor to the circuit. In Figure 7b an
aluminum electrolytic capacitor has
been added. This capacitor’s high
equivalent series resistance damps
the circuit and eliminates the voltage
overshoot. The extra capacitor improves low frequency ripple filtering
and can slightly improve the efficiency
of the circuit, though it is likely to be
the largest component in the circuit.
An alternative solution is shown in
Figure 7c. A 1Ω resistor is added in
series with the input to eliminate the
voltage overshoot (it also reduces the
peak input current). A 0.1µF capacitor
improves high frequency filtering. This
solution is smaller and less expensive
than the electrolytic capacitor. For high
input voltages its impact on efficiency
is minor, reducing efficiency less than
one half percent for a 5V output at full
load operating from 24V.
Voltage overshoot gets worse with
reduced input capacitance. Figure 7d
shows the hot plug response with a 1µF
ceramic input capacitor, with the input
ringing above 40V. The LT1934-1 can
tolerate a larger input resistance, such
as shown in Figure 7e where a 4.7Ω
resistor damps the voltage transient
and greatly reduces the input current
glitch on the 24V supply.
Conclusion
The LTC4006, LTC4007 and LTC4008
integrate more functions in smaller
circuit footprints than any high power
battery charger IC available today.
The LTC4006 and LTC4007 provide simple standalone solutions for
complete Li-Ion battery care without
the need to write software programs
and/or design a complex battery charger control system. The LTC4006 is
targeted for applications where small
circuit size is most important. If
complete monitoring or limited host
control of the charge process is desired, the LTC4007 offers a complete
set of feedback, status and control
signals. Finally, the LTC4008 is a
general purpose charger that works
with multiple battery chemistries by
offering direct control over the entire
charge process.
Conclusion
The LT1934’s 34V input range and
short circuit robustness make it a great
choice for small industrial systems. It
delivers up to 300mA output with an
all-surface-mount circuit that requires
no heat sinks. The integrated power
switch, SOT-23 package and high
frequency operation result in a very
small regulator, requiring less than
one square centimeter of PCB area.
Efficiency is high over the entire load
range, with the 12µA quiescent current
extending battery life.
Notes
1.See Linear Technology Application Note 88 for a
complete discussion.
27