DEMO MANUAL DC270
NO-DESIGN SWITCHER
LTC1772 Constant Frequency
Current Mode Step-Down
DC/DC Converter
DESCRIPTIO
Demo board DC270 is a step-down (buck) regulator using
the LTC1772. The exclusive use of surface mount compo-
nents results in a highly efficient application in a very small
board space. It is ideal for cell phones and other portable
electronics operating from one or two Li-Ion cells or three
to six NiCd cells. DC270 is capable of providing 1A at an
output voltage of 2.5V with an input supply of 4.2V. This
demo board highlights the capabilities of the LTC1772,
which uses a current mode PWM architecture to drive an
external P-channel power MOSFET. The result is a high
performance power supply that has low output voltage
ripple. Constant operating frequency makes the LTC1772
PERFOR A CE SU
SYMBOL
V
IN
V
OUT
V
FB
I
Q
PARAMETER
Output Voltage
Feedback Voltage
Input Working Voltage Range
Typical Supply Current
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
Efficiency vs Load Current
100
V
IN
= 3.3V
90
80
70
60
50
40
1
V
OUT
= 2.5V
R
SENSE
= 0.03Ω
100
1000
10
LOAD CURRENT (mA)
10000
DC270 TA01
V
IN
= 5V
V
IN
= 8V
EFFICIENCY (%)
U
WW
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U
attractive for noise-sensitive applications. In addition,
high efficiency over a wide load current range makes the
LTC1772 ideal for battery-powered applications. In drop-
out, the external P-channel MOSFET is turned on continu-
ously (100% duty cycle), providing low dropout operation
with V
OUT
≅
V
IN
. To further enhance efficiency at low load
currents, the LTC1772 is configured for Burst Mode
TM
operation. The LTC1772 is capable of operating down to
approximately 2.2V input voltage before the undervoltage
lockout feature is activated.
Gerber files for this circuit
board are available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
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ARY
CONDITIONS
V
OUT
= 2.5V
I
OUT
= 1A
Normal Mode
Shutdown
V
IN
= 4.2V, I
OUT
= 0mA
V
IN
= 4.2V, V
ITH/RUN
= 0V
VALUE
2.5V to 9.8V
2.5V
±
0.0625V
0.8V
±
0.02V
220µA
7µA
Component Side
1
DEMO MANUAL DC270
NO-DESIGN SWITCHER
PERFOR A CE SU
SYMBOL
I
OUT
∆V
OUT
V
RIPPLE
PARAMETER
Maximum Output Current
Typical Load Regulation
Typical Output Ripple in Burst Mode Operation
PACKAGE A D SCHE ATIC DIAGRA SM
SHDN
C
C1
220pF R
C1
10k
1
2
3
R
F2
80.6k
1%
U1
LTC1772
I
TH
/RUN PGATE
GND
V
FB
V
IN
SENSE
–
6
5
4
4
R
F1
174k
1%
5 6
M1
FDC638P
3 2 1
L1
4.7µH
R
CS
0.040Ω
C
IN
10µF
10V
V
IN+
D1
MBRM120T3
Figure 1. LTC1772 Constant Frequency, Current Mode, Step-Down DC/DC Converter Schematic
PARTS LIST
REFERENCE
DESIGNATOR QUANTITY
C
O1
C
O2
(Optional)
C
IN
C
C1
D1
L1
M1
R
CS
R
C1
R
F1
R
F2
U1
1
1
1
1
1
1
1
1
1
1
1
1
PART NUMBER
6TPA47M
JMK212BJ475MG
LMK325BJ106K-T
06035A221KAT
MBRM120T3
DO1608C-472
FDC638P
LR1206-01-R040F
CR16-103JM
CR16-1743FM
CR16-8062FM
LTC1772CS6
DESCRIPTION
47µF 6V POSCAP Capacitor
4.7µF 6.3V Capacitor
10µF 10V Capacitor
220pF 10% NPO Capacitor
Schottky Diode
4.7µH Inductor
MOSFET
0.040Ω 1% 0.25W 1206 Resistor
10k 5% 1/8W 0603 Resistor
174k 1% 0.1W 0603 Resistor
80.6k 1% 0.1W 0603 Resistor
6-Pin SOT-23 IC
VENDOR
Sanyo
Taiyo Yuden
Taiyo Yuden
AVX
ON Semiconductor
Coilcraft
Fairchild
IRC
TAD
TAD
TAD
LTC
TELEPHONE
(619) 661-6835
(408) 573-4150
(408) 573-4150
(843) 946-0362
(602) 244-6600
(847) 639-6400
(408) 822-2126
(361) 992-7900
(800) 508-1521
(800) 508-1521
(800) 508-1521
(408) 432-1900
2
W
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U
ARY
CONDITIONS
V
IN
= 4.2V, V
OUT
= 2.5V
0mA
≤
I
OUT
≤
1A, V
IN
= 8.5V
I
OUT
= 100mA, V
IN
= 4.2V
VALUE
1A (Min)
–1%
120mV
P-P
GND
TOP VIEW
I
TH
/RUN 1
GND 2
V
FB
3
6 PGATE
5 V
IN
4 SENSE
–
+
C
O1
47µF
6V
C
O2
4.7µF
6.3V
OPTIONAL
V
OUT
2.5V
S6 PACKAGE
6-LEAD PLASTIC SOT-23
LTC1772CS6
GND
DC270 F01
DEMO MANUAL DC270
NO-DESIGN SWITCHER
QUICK START GUIDE
This demonstration board is easy to set up to evaluate the
performance of the LTC1772. Please follow the proce-
dure outlined below for proper operation.
1. Connect the input power supply to the V
IN
and GND
terminals.
2. Connect the load between the V
OUT
and GND termi-
nals. Refer to Figure 4 for proper measurement equip-
ment setup.
3. To shut down the circuit, connect the I
TH
/RUN pin to
ground.
OPERATIO
The circuit shown in Figure 1 operates from an input
voltage between 2.5V and 9.8V. The output voltage of 2.5V
is fixed. For other output voltages, resistor R
F1
must be
replaced (see Output Voltage Setup).
This demonstration circuit has been optimized for effi-
ciency and physical footprint. For other requirements,
please contact the factory. This demonstration circuit is
intended for the evaluation of the LTC1772 switching
regulator IC and was not designed for any other purpose.
OPERATION
The LTC1772 uses the constant-frequency, pulse-width-
modulated, current mode architecture shown in Figure 2.
Current mode operation provides the well known advan-
tages of clean start-up and excellent line and load
regulation.
The LTC1772 is designed to operate down to approxi-
mately 2.2V input voltage, making it suitable for applica-
tions that are powered either by a low input supply or a
single lithium-ion battery. The external MOSFET can limit
the minimum input voltage; therefore, be careful when
specifying the MOSFET.
To prevent damage to a lithium-ion battery by deep dis-
charge, an undervoltage lockout circuit is incorporated
into the LTC1772. When the input supply drops to approxi-
mately 2.2V, all circuitry except the undervoltage detector
block is turned off.
U
The LTC1772 operates as follows: the external P-channel
power MOSFET is turned on at the beginning of each cycle
when the oscillator sets the latch (RS1) and is turned off
when the current comparator (ICOMP) resets the latch.
The peak inductor current at which ICOMP resets the RS
latch is controlled by the voltage on the I
TH
/RUN pin, which
is the output of the error amplifier, EAMP. An external
resistive divider connected between V
OUT
and ground
allows the EAMP to receive an output feedback voltage,
V
FB
. When the load current increases, it causes a slight
decrease in V
FB
relative to the 0.8V reference, which, in
turn, causes the I
TH
/RUN voltage to increase until the
average inductor current matches the new load current.
The main control loop is shut down by pulling the I
TH
/RUN
pin low. Releasing I
TH
/RUN allows an internal 0.5µA
current source to charge the external compensation net-
work. When the I
TH
/RUN pin reaches 0.4V, the main
control loop is enabled with the I
TH
/RUN voltage, and then
pulled up to its zero-current level of approximately 0.7V.
As the external compensation network continues to charge,
the corresponding output current trip level follows, allow-
ing normal operation.
Comparator OVP guards against transient overshoots
>7.5% of the target output voltage by turning off the
P-channel power MOSFET and keeping it off until the fault
is removed.
3
DEMO MANUAL DC270
NO-DESIGN SWITCHER
OPERATIO
V
IN
5
SENSE
–
4
FREQ
FOLDBACK
SCD
V
IN
EAMP
0.5µA
+
V
IN
0.3V
–
VOLTAGE
REFERENCE
GND
2
UNDERVOLTAGE
LOCKOUT
V
REF
0.8V
Figure 2. LTC1772 Block Diagram
Burst Mode Operation
The LTC1772 enters Burst Mode operation at low load
currents. In this mode, the peak current of the inductor is
set as if V
ITH
/RUN = 1V (at low duty cycles), even though
the voltage at the I
TH
/RUN pin is at a lower value. If the
inductor’s average current is greater than the load require-
ment, the voltage at the I
TH
/RUN pin will drop. When the
I
TH
/RUN voltage goes below 0.85V, the sleep signal goes
high, turning off the external MOSFET. The sleep signal
goes low when the I
TH
/RUN voltage goes above 0.925V
and the LTC1772 resumes normal operation. The next
oscillator cycle will turn the external MOSFET on and the
switching cycle repeats.
4
–
+
U
+
ICMP
–
RS1
OSC
SLOPE
COMP
R
Q
S
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
V
IN
PGATE
6
0.3V
+
–
BURST
CMP
SLEEP
OVP
+
–
V
REF +
60mV
0.15V
+
–
V
REF
0.8V
V
FB
3
V
IN
1 I
TH
/RUN
0.4V
+
SHDN
CMP
SHDN
UV
–
1.2V
DC270 F02
Undervoltage Lockout
To prevent deep discharge of a lithium-ion battery when
it is near its end of charge, an undervoltage lockout
circuit is incorporated into the LTC1772. When the input
supply voltage drops below approximately 2.2V, the UV
lockout feature turns off the P-channel MOSFET and all
circuitry except the undervoltage block, which draws
only several microamperes.
DEMO MANUAL DC270
NO-DESIGN SWITCHER
OPERATIO
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator is reduced to about 90kHz. This low frequency
allows the inductor current to safely discharge, thereby
preventing current runaway. The oscillator’s frequency
will gradually increase to its designed rate when the
feedback voltage again approaches 0.8V.
Output Voltage Setup
In this demonstration circuit, the output voltage is set for
2.5V. Output voltages other than 2.5V can be obtained by
removing component R
F1
= 174k and replacing it with a
resistor of the value:
V
OUT
R
F 1
=
80.6k
– 1
0.8
Note that output votlages below 0.8V are not possible with
this topology.
Higher output voltages may require a substitute output
capacitor, since the installed output capacitor is rated for
6V.
LTC1772
DC270 F03
Figure 3. Output Voltage Setting
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HOW TO MEASURE VOLTAGE REGULATION
When measuring voltage regulation, all measurements
must be taken at the point of regulation. This point is where
the LTC1772 control loop looks for the information to keep
the output voltage constant. This information appears
between Pin 3 and Pin 2 of the LTC1772. For output
voltages above 0.8V, the voltage at Pin 3 can be adjusted
by the resistor divider network. These points correspond
to the output terminals of the demonstration board. Test
leads should be attached to these terminals and the load
should be attached as close to these terminals as possible.
This applies to line regulation (input-to-output voltage
regulation) as well as load regulation tests. In performing
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 powered by a regulated DC bench supply so
additional variation on the DC input does not add an error
to the regulation measurements.
A
0.8V
V
OUT
LTC1772
+VIN
+VOUT
+
+
V
GND
+
A
+
EAMP
–
V
FB
R
F1
174k
R
F2
80.6k
+
V
LOAD
GND
SD
DC232 F04
Figure 4. Correct Measurement Setup
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