1
DEMO MANUAL DC293
NO-DESIGN SWITCHER
SYMBOL PARAMETER CONDITIONS (SEE NOTE) BOARD SUFFIX VALUE
V
IN
Input Voltage Range A 2.8V to 12V
B 4.5V to 18V
V
OUT
Output Voltage I
OUT
= 1A A 1.8V ±0.036V
A, B 2.5V ±0.050V
A, B 3.3V ±0.066V
B5V ±0.10V
I
OUT
Maximum Output Current R
SENSE
= 0.05ΩA, B 2A
LTC1771 Ultralow
Supply Current, High Efficiency
Step-Down Regulator
, LTC and LT are registered trademarks of Linear Technology Corporation.
DESCRIPTIO
U
PERFOR A CE SU ARY
UWWW
Demo Board DC293 is a step-down (buck) regulator using
the LTC
®
1771. Exclusive use of surface mount compo-
nents and the LTC1771’s tiny MS8 package results in a
very efficient application in a small board space. Featuring
outstanding light load efficiency and requiring as little as
10µA supply current to regulate the output at no load, it is
ideal for cell phones and other portable electronics that
have long standby times and need ultralow supply current
to maximize battery life. DC293 is capable of providing 2A
at various output voltages programmable from 1.8V to 5V
via a jumper.
T
his demo board highlights the capabilities of the LTC1771,
which uses a current mode, constant off-time architec-
ture to control an external P-channel power MOSFET. This
results in a high performance power supply that has low
output voltage ripple and fast transient response. At low
output currents, the LTC1771 automatically switches to
Burst Mode
TM
operation to maintain high operating effi-
ciencies and to minimize supply current. The part can be
shut down to further reduce the supply current to 2µA. Its
wide supply range allows operation from 2.8V to 18V. A
MODE pin is provided to disable Burst Mode operation for
noise-sensitive applications and soft-start is provided by
an external capacitor that can also be used to properly
sequence supplies. In dropout, the P-channel MOSFET is
turned on continuously (100% duty cycle), providing low
dropout operation with VOUT ≅ VIN. Gerber files for this
circuit board are available. Call the LTC factory.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
UU
W
LOAD CURRENT (mA)
EFFICIENCY (%)
100
90
80
70
60
50
400.1 1 100 1000 10000
DC293 TPC01
10
V
IN
= 5V
V
IN
= 10V
V
IN
= 15V
V
OUT
= 3.3V
R
SENSE
= 0.05Ω
LTC1771 Efficiency
2
DEMO MANUAL DC293
NO-DESIGN SWITCHER
SYMBOL PARAMETER CONDITIONS BOARD SUFFIX VALUE
I
Q
Typical Supply Current V
IN
= 10V, I
LOAD
= 0, V
OUT
= 1.8V A 10µA
V
IN
= 10V, V
RUN
= 0 A, B 2µA
V
RIPPLE
Typical Output Ripple I
OUT
= 1A A, B 40mV
I
OUT
= 100mA, Burst Mode Operation Enabled A, B 50mV
I
OUT
= 100mA, Burst Mode Operation Disabled A, B 20mV
∆V
OUT
Typical Line Regulation 2.8V < V
IN
< 12V, I
LOAD
= 1A A 3mV
4.5V < V
IN
< 18V, I
LOAD
= 1A B 4mV
Typical Load Regulation 0 < I
LOAD
< 2A, V
IN
= 10V A 5mV
0 < I
LOAD
< 2A, V
IN
= 10V B 7mV
Note: V
OUT
is voltage associated with the center position of JP3, unless otherwise specified.
PACKAGE A D SCHE ATIC DIAGRA S
UWW
+
U1
LTC1771EMS8
JP2
C6
0.01µF
C4
10pF
C2
330pF
C10
(OPT)
C1 1000pF
DRAIN
R1
10k
R4
1M R5
1.69M
1.8V
3.3V
R6
634k
2
3
1SHDN
RUN
RUN/SS
ITH
VFB
GND
MODE
SENSE
VIN
PGATE
1
2
3
4
8
7
6
5
JP1
2
3
1DISABLE
ENABLE
R7
2.67M
R8 (OPT)
JP3
2.5V
5216
Q1
Si3443DV
DC293 F01
D1
UPS5817
C5
150µF
6.3V
+C9
33µF
16V (OPT)
C8
1µF
VOUT
2A
VIN
2.8V TO 12V
GND
L1
10µH
R2
0.050Ω
C3
10µF
25V
+
U1
LTC1771EMS8
JP2
C6
0.01µF
C4
10pF
C2
330pF
C10
(OPT)
C1 1000pF
R1
10k
R4
1M R5
3.09M
2.5V
5V
R6
1.54M
2
3
1SHDN
RUN
RUN/SS
I
TH
V
FB
GND
MODE
SENSE
V
IN
PGATE
1
2
3
4
8
7
6
5
JP1
2
3
1DISABLE
ENABLE
R7
3.74M
R8 (OPT)
JP3
3.3V
5216
Q1
Si6447DQ
DC293 F02
D1
UPS5817
C5
150µF
6.3V
+
C9
15µF
35V (OPT)
C8
1µF
V
OUT
2A
V
IN
4.5V TO 18V
GND
L1
15µH
DRAIN
R2
0.050Ω
C3
10µF
25V
Figure 1. LTC1771 Demo Board Version A Schematic
LTC1771EMS8
Figure 2. LTC1771 Demo Board Version B Schematic
RUN/SS
ITH
VFB
GND
1
2
3
4
8
7
6
5
MODE
SENSE
VIN
PGATE
TOP VIEW
MS8 PACKAGE
8-LEAD PLASTIC MSOP
PERFOR A CE SU ARY
UWWW
3
DEMO MANUAL DC293
NO-DESIGN SWITCHER
Demonstration Board DC293 is easy to set up for evalu-
ation of the LTC1771. Please follow the procedure below
for proper operation.
• Move jumper JP3 to the appropriate position for the
required output voltage. For voltages other than the
preset value, make sure you install the calculated resis-
tor at the pads (see Output Voltage Setup).
• For Burst Mode operation at low load currents, move
jumper JP1 to the Enable position. To disable Burst Mode
operation, move the jumper to the Disable position.
REFERENCE
DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE
C1 1 06035C102KAT1A 1000pF 50V 10% X7R Capacitor AVX (843) 946-0362
C2 1 06035C331KAT1A 330pF 50V 10% X7R Capacitor AVX (843) 946-0362
C3 1 TMK432BJ106M 10µF 25V X5R Ceramic Capacitor Taiyo Yuden (800) 348-2496
C4 1 06033A100KAT2A 10pF 25V 10% NPO Capacitor AVX (843) 946-0362
C5 1 6TPB150M 150µF 6.3V 20% POSCAP Capacitor Sanyo (619) 661-6835
C6 1 06035C103KAT1A 0.01µF 50V 10% X7R Capacitor AVX (843) 946-0362
C8 1 0603ZG105KAT1A 1µF 10V 80% Y5V Capacitor AVX (843) 946-0362
C9 1 TPSC336M016R0300 33µF 16V 20% Tantalum Capacitor, Board A AVX (207) 282-5111
TPSC156M035R0450 15µF 35V 20% Tantalum Capacitor, Board B
D1 1 UPS5817 2A Schottky Diode Microsemi (617) 926-0404
TP1 to TP4 4 2502-02 Terminal Turret Mill Max (516) 922-6000
JP1, JP2 2 2802S-03-G2 2mm Pin Header Comm Con (626) 301-4200
JP3 1 2802S-02-G2 2mm Pin Jumper Comm Con (626) 301-4200
L1 1 CR75-100MC 10µH Inductor, Board A Sumida (847) 956-0667
CR75-150MC 15µH Inductor, Board B
Q1 1 Si3443DV Sublogic Threshold 12V P-Ch MOSFET, Board A Siliconix (800) 554-5565
Si6447DQ Logic Threshold 20V P-Ch MOSFET, Board B
R1 1 CR16-103JM 10k 5% 0.1W 0603 Resistor AAC (800) 508-1521
R2 1 LR2010-01-050-F 0.05Ω 1% 0.5W 2010 Resistor IRC (361) 992-7900
R4 1 CR16-1004FM 1M 1% 0.1W 0603 Resistor AAC (800) 508-1521
R5 1 WCR0805-1694-F 1.69M 1% 1/16W 0805 Resistor, Board A AAC (800) 508-1521
WCR0805-3094-F 3.09M 1% 1/16W 0805 Resistor, Board B
R6 1 WCR0805-6343-F 634k 1% 1/16W 0805 Resistor, Board A AAC (714) 255-9186
WCR0805-1544-F 1.54M 1% 1/16W 0805 Resistor, Board B
R7 1 WCR0805-2674-F 2.67M 1% 1/16W 0805 Resistor, Board A AAC (714) 255-9186
WCR0805-3744-F 3.74M 1% 1/16W 0805 Resistor, Board B
U1 1 LTC1771EMS8 Switching Regulator Controller IC LTC (408) 432-1900
PARTS LIST
QUICK START GUIDE
• To shut down the circuit, move the jumper JP2 to the
SHDN position. For normal operation, JP2 should be in
the RUN position.
• Connect the input power supply to the V
IN
and GND
terminals.
• Connect the load between the V
OUT
and GND terminals.
Refer to Figure 4 for proper measurement equipment
setup.
4
DEMO MANUAL DC293
NO-DESIGN SWITCHER
OPERATIO
U
INTRODUCTION
The circuits shown in Figures 1 and 2 highlight the
capabilities of the LTC1771. Two versions are available for
two different input supply ranges due to the limited voltage
ranges of the power MOSFETs. Version A is optimized for
lower voltage operation (2.8V to 12V) and provides output
voltages of 1.8V, 2.5V or 3.3V, selectable by the appropri-
ate jumper position. Version B is optimized for higher
voltage operation (4.5V to 18V) and provides output
voltages of 2.5V, 3.3V or 5V.
The LTC1771 is a current mode switching regulator con-
troller that drives an external P-channel power MOSFET
using a constant off-time architecture. Burst Mode opera-
tion and ultralow quiescent current provide outstanding
light-load efficiency, no-load supply current and enable
high efficiencies for over four decades of load current
range. 100% duty cycle provides low dropout operation,
extending operating time in battery-operated systems.
The demonstration circuit is intended for the evaluation of
the LTC1771 switching regulator IC and is not designed
for any other purpose.
The LTC1771 uses the current mode, constant off-time
architecture shown in Figure 3. Current mode operation
provides the well known advantages of clean start-up and
excellent line and load regulation. Constant off-time adds
to this list simplicity (neither an oscillator nor ramp
compensation are required) and inherent 100% duty cycle
in dropout.
MAIN CONTROL LOOP
During normal operation, the P-channel MOSFET is turned
on at the beginning of each cycle and turned off when the
current comparator, C, triggers the one-shot timer. The
external MOSFET switch stays off for the 3.5µs one-shot
duration and then turns back on again to begin a new cycle.
–
+
EA
–
+
C
ON
ON
+
C
SS
RUN/SS
V
IN
V
OUT
V
IN
1
MODE
MODE
READY
SLEEP
READY
250k
22k R
SENSE
1.23V
1V
1V
2V
1µA
(BURST ENABLE)
10% CURRENT
10% CURRENT
SOFT-START
8
V
IN
V
IN
V
OUT
L
C
IN
+
C
OUT
6
SENSE
7
PGATE
3.5µs
DC293 F03
5
V
FB
3
I
TH
R
C
C
C
*
*
OPTIONAL FOR FOLDBACK
CURRENT LIMITING
2
GND
4+
–B
ON TRIGGER
1.23V
REFERENCE
BLANKING
1-SHOT
STRETCH
Figure 3. LTC1771 Block Diagram
5
DEMO MANUAL DC293
NO-DESIGN SWITCHER
The peak inductor current at which C triggers the one-shot
is controlled by the voltage on Pin 2 (I
TH
), the output of the
error amplifier, EA. An external resistive divider connected
between V
OUT
and ground allows EA to receive an output
feedback voltage, V
FB
. When the load current increases, it
causes a slight decrease in V
FB
relative to the 1.23V
reference, which, in turn, causes the I
TH
voltage to increase
until the average inductor current matches the new load
current.
The main control loop is shut down by pulling Pin 1 (RUN/
SS) low. Releasing RUN/SS allows an internal 1µA current
source to charge the soft-start capacitor, C
SS
. When C
SS
reaches 1V, the main control loop is enabled with the I
TH
voltage clamped at approximately 40% of its maximum
value. As C
SS
continues to charge, I
TH
is gradually re-
leased, allowing normal operation to resume. C
SS
can also
be used for power supply sequencing by setting a turn-on
delay equal to approximately C
SS
/I
RUN/SS
seconds.
Burst Mode OPERATION
The LTC1771 provides outstanding low current efficiency
and ultralow no-load supply current by using Burst Mode
operation when Pin 8 (MODE) is pulled above 2V. Burst
Mode operation commences when the load, detected by a
comparator monitoring the I
TH
voltage, falls below about
20% to 30% of the maximum load. During Burst Mode
operation, short burst cycles of normal switching to charge
the output capacitor are followed by a longer sleep period
with the switch off and the load current supplied by the
output capacitor. During this sleep period, only the mini-
mum required circuitry—the reference voltage and the error
amplifier—are left on. Supply current is further reduced
with innovative new circuitry that allows the error amplifier
to run on 10% of its normal operating current during sleep
mode with no degradation in the transient response,
reducing the total supply current to only 9µA. At light loads,
the regulator spends most of the time in this low quiescent
current sleep mode, thus minimizing the losses that would
normally dominate (DC supply current losses and switch-
ing losses due to the MOSFET switch gate charge).
Burst Mode operation can be disabled by pulling the MODE
pin to ground. Disabling Burst Mode operation allows the
loads to decrease by another decade, to about 1% to 2%
OPERATIO
U
of the maximum load, before the regulator must skip cycles
to maintain regulation. Although less efficient, disabling
Burst Mode operation is useful for reducing both audio
and RF interference by reducing voltage and current ripple
and keeping frequency constant to lower output currents.
SHORT-CIRCUIT PROTECTION
When the output is shorted to ground, the off-time is
increased in inverse proportion to V
OUT
, to a maximum of
70µs at V
OUT
= 0V. This increased off-time allows the
inductor current to discharge, preventing runaway. Fold-
back current limiting can be implemented by adding two
diodes in series between the output and the I
TH
pin, as
shown in Figure 3, to minimize heat dissipation in the catch
diode during the short-circuit condition.
OUTPUT VOLTAGE SETUP
In this demonstration circuit, output voltages of 1.8V
(version A only), 2.5V, 3.3V and 5V (version B only) can be
obtained by moving the jumper JP3 to the appropriate
position, as indicated on the demo board. If an output
voltage other than those provided is desired, one of the
feedback resistors R4, R5, R6, R7 or R8 can be removed
and replaced with a new value to set the desired voltage
according to the following equation:
V
OUT
= 1.23(R4 + R5RX)/R4
where RX is the resistor R6, R7 or R8 associated with the
position of jumper JP3.
Note also that the output capacitor is rated at 6.3V; if the
output voltage approaches this limit, the capacitor must
be replaced with a capacitor with the proper rating (pref-
erably twice the output voltage).
CHECKING TRANSIENT RESPONSE
Switching regulators take several cycles to respond to a
step in DC (resistive) load current. When a load step
occurs, V
OUT
shifts by an amount equal to (∆I
LOAD
)(ESR),
where ESR is the effective series resistance of C
OUT
.
∆I
LOAD
also begins to charge or discharge C
OUT
until the
regulator loop adapts to the current change and returns
V
OUT
to its steady-state value. During this recovery time,
V
OUT
can be monitored for overshoot or ringing, which
6
DEMO MANUAL DC293
NO-DESIGN SWITCHER
OPERATIO
U
would indicate a stability problem. The external compo-
nents shown in Figure 1’s circuit will prove adequate for
most applications.
HOW TO MEASURE VOLTAGE REGULATION
When measuring voltage regulation, remember that all
measurements must be taken at the point of regulation.
This point is where the LTC1771’s control loop looks for
the information to keep the output voltage constant. On
this demonstration board, it is located between Pin 5 of the
LTC1771, the signal ground and the output side of R5.
These points correspond to the output terminals of the
demonstration board. Test leads should be attached to
these terminals. Measurements should not be taken at the
end of test leads at the load. This applies to line regulation
(input-to-output voltage regulation) as well as load regu-
lation tests. In doing line regulation tests, always look at
the input voltage across the input terminals. Refer to
Figure 4 for proper monitoring equipment configuration.
For the purposes of these tests, the demonstration circuit
should be fed from a regulated DC bench supply so
additional variation on the DC input does not add an error
to the regulation measurements.
For measurement of no-load supply current and measure-
ment of efficiency at loads below a milliamp,
the input
impedance of the voltmeters may have a significant impact
on these measurements
. For example, for voltmeters with
10MΩ input impedance connected to the input and output,
the no-load supply current at V
IN
= 15V will increase from
10µA with no voltmeters connected, to 11.5µA with them
connected. Likewise, with V
IN
= 15V and I
LOAD
= 100µA,
the efficiency decreases from 55% to 53% when the volt-
meters are connected. Therefore, for the most accurate
measurements at light loads, first record the voltmeter read-
ings, then disconnect the voltmeters before making the
input supply current measurement.
COMPONENT CONSIDERATIONS
This demo board is designed for easy modification. It can
accommodate a variety of different MOSFET footprints:
TSOP-6, TSSOP-8, SO-8 (on bottom) and SOT-23, a larger
catch diode and extra input/output capacitors. Component
selection can be very critical in power supply applications.
Be sure to refer to the LTC1771 data sheet for guidelines
in selecting the external components surrounding the IC.
This section highlights a few of the effects to consider
when changing components to optimize or change the
specifications of the demo board.
Optimizing the Inductor
When the optimal inductance value for L1 is used, the
regulator has the highest efficiency and the smoothest
transition between Burst Mode operation and continuous
mode. The optimal inductor value is, however, dependent
upon output voltage. Since the demo boards provide a
selection of three output voltages, the inductor provided
can only be optimized for one of the three, which is the
output voltage with the jumper in the center position, i.e.,
2.5V for version A and 3.3V for version B. The optimal
inductance for the other output voltages can be calculated
with the following equation:
L
OPT
= 75µH(V
OUT
+ V
D
)R
SENSE
Setting the Maximum Load
The demo board is equipped with a 0.05Ω resistor to set
the maximum current to 2A according to the equation:
I
MAX
= 0.1/R
SENSE
This resistor can be increased or decreased as necessary
to program the regulator for the desired current. If the
current is increased, make sure that the increased current
does not exceed the ratings of the input capacitor (ripple
current), the power MOSFET, Schottky diode or the
inductor.
GND
VOUT
DC293 F04
VIN
LTC1771
GND V
+
A
+
A
+
LOAD
V
+
+
Figure 4. Correct Measurement Setup
7
DEMO MANUAL DC293
NO-DESIGN SWITCHER
Minimizing No-Load Supply Current
The no-load supply current of the regulator originates
from three sources: the LTC1771’s 9µA sleep mode quies-
cent current, Schottky diode reverse leakage and feedback
resistor leakage. The LTC1771’s I
Q
is drawn directly from
the supply, whereas the Schottky and feedback resistor
leakage are drawn from the output; thus their effect on the
supply varies with duty ratio: from about 10µA at low duty
ratios to about 15µA at higher duty ratios.
The feedback resistor leakage can be minimized by simply
using large valued resistors in the megaohm range. Unfor-
tunately, 1% resistors above 1M are currently not avail-
able in sizes smaller than 0805.
Selecting Schottky diodes with low reverse leakage cur-
rent is critical, since the leakage can often approach the
magnitude of the LTC1771 supply current. Selecting a low
leakage Schottky diode, however, is complicated by the
fact that diodes with lower reverse leakage tend to have
higher forward drops. Low forward drop is critical for high
current efficiency, since loss is proportional to forward
drop. Thus a trade-off must be made between low no-load
supply current and high efficiency. The UPS5817 used on
the demo board provides a good trade-off for a 2A
application.
OPERATIO
U
Optional Input Capacitor
The demo board is equipped with an extra input capacitor,
C9, that may not be needed in the final application but is
provided for evaluating the demo board over the full input
supply range. The 10µF ceramic capacitor C3 is usually
sufficient for duty ratios less than about 80% but above
this ratio the optional capacitor C9 is recommended. Also,
when evaluating the demo board connected to a lab bench
supply with typical long leads, disconnecting and recon-
necting the supply may cause transients, due to the
resonance of the high lead inductance with the high Q
ceramic input capacitor, which may exceed the absolute
maximum supply voltage of the LTC1771. The lower Q
tantalum capacitor in parallel with the ceramic greatly
reduces the amplitude of this resonance, eliminating this
potential problem.
Component Manufacturers
Besides those components that are used on the demon-
stration board, other components may also be used.
Table 1 is a partial list of the manufacturers whose
components you can use for the switching regulator.
Using components other than the ones on the demo
board requires recharacterizing the circuit for efficiency.
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
Table 1
MANUFACTURER DEVICE TELEPHONE FAX
AVX Capacitors (843) 448-9411 (843) 448-1943
AVX Resistors (843) 946-0524 (843) 448-6042
Central Semiconductor Diodes (631) 435-1110 (631) 453-1824
Coilcraft Inductors (847) 639-6400 (847) 639-1469
Cooper Electronic Technology Inductors (561) 752-5000 (561) 742-0134
International Rectifier MOSFETs, Diodes (310) 322-3331 (310) 322-3332
Microsemi Diodes (617) 926-0404 (617) 924-1235
ON Semiconductor MOSFETs, Diodes (602) 244-6600 (602) 244-3345
Murata-Erie Capacitors (770) 436-1300 (814) 238-0490
Sanyo Capacitors (619) 661-6835 (619) 661-1055
Vishay Inductors (605) 665-9301 (605) 665-0817
Vishay Siliconix MOSFETs (408) 988-8000 (408) 567-8977
Sprague Capacitors (207) 324-4140 (207) 324-7223
Sumida Inductors (847) 956-0667 (847) 956-0702
TDK Inductors (847) 803-6100 (847) 803-6294
Zetex Diodes (631) 543-7100 (631) 864-7630
8
DEMO MANUAL DC293
NO-DESIGN SWITCHER
dc293 LT/TP 0900 500 • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2000
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
NOTES: UNLESS OTHERWISE SPECIFIED
1. MATERIAL: 2 LAYERS, 0.062" THK. FR-4 GLASS EPOXY 2 0Z
COPPER CLAD
2. ALL HOLES SHALL BE PLATED THRU
3. PLATE THRU HOLES WITH COPPER 0.0014 MIN. THICKNESS
ALL HOLE SIZES IN HOLE TABLE ARE AFTER PLATING
4. SILKSCREEN: WITH WHITE NONCONDUCTIVE EPOXY INK
5. NO SILKSCREEN ALLOWED ON PAD LANDS
6. SOLDERMASK: LPI, GREEN
7. NO BLOCK SOLDERMASKING OF PAD ROWS
8. DO NOT MAKE CHANGES ON SILKSCREEN, SUCH AS
COMPANY LOGO, QC STAMPS
9. DO NOT PLATE TOOLING (3 PLCS) AND SCORING (26 PLCS) HOLES
10. SCORING:
0.017
0.02
2.000
2.000
B
B
A
A
D
DE
E
AC
SYMBOL
A
B
C
D
E
PLTD
PLTD
NPLTD
PLTD
PLTD
PLTD
DIAMETER
0.094
0.070
0.035
0.026
0.010
TOTAL HOLES
NUMBER
OF HOLES
4
2
6
6
47
65
Component Side Silkscreen Component Side Component Side Solder Mask
Component Side Paste Mask Solder Side Solder Side Solder Mask
PCB LAYOUT A D FIL
UW
PC FAB DRAWI G
U