DEMO MANUAL DC309
NO-DESIGN SWITCHER
DESCRIPTIO
LTC1773 Low Input Voltage
Synchronous Current Mode
Step-Down DC/DC Converter
make the LTC1773 ideal for battery-powered applications.
In dropout, the external P-channel MOSFET is turned on
continuously (100% duty cycle), providing low dropout
operation when V
IN
approaches V
OUT
. Under light load
conditions, efficiency can be improved by activating Burst
Mode
TM
operation, which is done by connecting the SYNC/
FCB pin to V
IN
or leaving it floating. Tying this pin to ground
will force continuous operation, regardless of the load.
The SYNC/FCB pin can also be used to synchronize the
LTC1773 to frequencies of up to 750kHz. At input voltages
below 2.5V, the LTC1773’s undervoltage lockout feature is
activated, shutting down the output. Soft-start is provided
by connecting an external capacitor to the RUN/SS pin.
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.
Demo board DC309 is a step-down (buck) regulator using
the LTC
®
1773. The exclusive use of surface mount com-
ponents 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. DC309 is capable of providing
2.5A at an output voltage of 1.8V with an input supply of
3.3V. This demo board highlights the capabilities of the
LTC1773, which uses a current mode PWM architecture to
synchronously drive an external pair of P- and N-channel
power MOSFETs. The result is a high performance power
supply that has low output voltage ripple. A constant
operating frequency of 550kHz makes the LTC1773 attrac-
tive for noise-sensitive applications while allowing for
smaller external components such as the inductor and the
output capacitor. In addition, its high efficiency over a
wide load current range and its low quiescent current
PERFOR A CE SU
SYMBOL
V
IN
V
OUT
PARAMETER
Output Voltage
Input Supply Voltage Range
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
Efficiency
100
90
EFFICIENCY (%)
V
OUT
= 2.5V
SEE FIGURE 1
V
IN
= 3.3V
V
IN
= 5V
80
70
60
50
1
V
IN
= 8V
10
100
1000
OUTPUT CURRENT (mA)
10,000
DC309 TA01
U
WW
U W
U
ARY
U W
CONDITIONS
V
OUT
= 1.8V
I
OUT
= 1A
I
OUT
= 1A
I
OUT
= 1A
I
OUT
= 1A
VALUE
2.8V to 8.5V
3.3V
±0.066V
2.5V
±0.050V
1.8V
±0.036V
1.5V
±0.030V
dc309fa
1
DEMO MANUAL DC309
NO-DESIGN SWITCHER
PERFOR A CE SU
SYMBOL
V
FB
I
Q
PARAMETER
Feedback Voltage
Supply Current
Burst Mode Operation
Shutdown
Maximum Output Current
Typical Load Regulation
Typical Output Ripple
Maximum Synchronizable Frequency
I
OUT
∆V
OUT
V
RIPPLE
f
SYNC
PACKAGE A D SCHE ATIC DIAGRA S
TOP VIEW
I
TH
RUN/SS
SYNC/FCB
V
FB
GND
1
2
3
4
5
10
9
8
7
6
SW
SENSE
–
V
IN
TG
BG
MS10 PACKAGE
10-LEAD PLASTIC MSOP
LTC1773EMS
E2
SYNC/FCB
C2
47pF
C3
220pF
U1
LTC1773EMS
1
2
3
1.5V
4
5
I
TH
RUN/SS
SYNC/FCB
V
FB
GND
SW
SENSE
–
V
IN
TG
BG
10
9
8
7
6
2
7
8
Q1A
Si9801
D2*
MBRM120T3
POWERMITE
C6
4.7µF
6.3V
0805
E3
RUN/SS
R1
100k
R2
0.025Ω
1206
R3
39k
C4 0.1µF
R4
80.6k 1%
JP1
HEADER 3
×
2
1
3
5
2
4
6
1.8V
4
3
Q1B
Si9801
L1
3µH
CDRH6D28-3R0
R5
196k 1%
R6
2.5V 57.6k 1%
3.3V
R7
31.6k 1%
C8
10pF
R8
71.5k 1%
C9
1µF
6.3V
X5R
Figure 1. LTC1773 Low Input Voltage Synchronous Current Mode Step-Down DC/DC Converter
dc309fa
2
W
WW
W
U
U W
ARY
CONDITIONS
VALUE
0.8V
±0.012V
V
IN
= 5V, I
OUT
= 0
V
IN
= 5V, V
RUN/SS
= 0V
V
IN
= 5V, V
OUT
= 2.5V (Q1 = Si9801DY)
0mA < I
OUT
< 3A, V
IN
= 8.5V
I
OUT
= 3A, V
IN
= 5V
I
OUT
= 100mA, Burst Mode Operation
V
OUT
= 2.5V, V
IN
≤
8.5V
V
OUT
= 1.8V, V
IN
≤
7V
V
OUT
= 1.5V, V
IN
≤
6V
130µA
10µA
3A
– 1%
40mV
P-P
60mV
P-P
750kHz
750kHz
750kHz
+
E4
V
IN+
C1
2.8V TO 8.5V
47µF
10V
10TPB47MC
E5
GND
5 6
E6
V
OUT +
1.5V/1.8V/2.5V/3.3V
2.5A AVE/3A PK
1
C5
150µF
4V
4TPB150MC
+
E7
GND
DC309 SD
*D2 OPTIONAL
DEMO MANUAL DC309
NO-DESIGN SWITCHER
PARTS LIST
REFERENCE
DESIGNATOR
C1
C2
C3
C4
C5
C6
C7
C8
C9
D1
D2
E1 to E6
JP1
JP1
L1
Q1
R1
R2
R3
R4
R5
R6
R7
R8
U1
QUANTITY
1
1
1
1
1
1
0
1
1
0
1
6
1
1
1
1
1
1
1
1
1
1
1
1
1
06035A100JAT2A
JMK107BJ105MA
MMSD914T1
MBRM120T3
2501-2
2202S-06-G1
CCIJ2MM-138G
CDRH6D28-3R0
Si9801DY
CR16-104JM
LRF1206-01-R025-J
CR16-393JM
CR16-8062FM
CR16-1963FM
CR16-5762FM
CR16-3162FM
CR16-7152FM
LTC1773EMS
PART NUMBER
10TPB47MC
06035A470JAT1A
06035A221JAT1A
EMK107BJ104MA
4TPB150MC
JMK212BJ475MG
DESCRIPTION
47µF 10V POSCAP Capacitor
47pF 50V NPO Ceramic Capacitor
220pF 50V NPO Ceramic Capacitor
0.1µF 16V X5R Ceramic Capacitor
150µF 4V POSCAP Capacitor
4.7µF 6.3V X5R Ceramic Capacitor
Don’t Stuff
10pF 50V 5% NPO
1µF 6.3V Y5V Ceramic Capacitor
Switching Diode (Don’t Stuff)
20V 1A Schottky Diode (Optional)
0.094" Turret Testpoint
3-Pin 2-Row 0.079cc Header
0.079" Center Shunt
3µH Inductor
Dual N- and P-Channel MOSFET
100k 5% Chip Resistor
0.025Ω Chip Resistor
39k 5% Chip Resistor
80.6k 1% Chip Resistor
196k 1% Chip Resistor
57.6k 1% Chip Resistor
31.6k 1% Chip Resistor
71.5k 1% Chip Resistor
IC
AVX
Taiyo Yuden
ON Semiconductor
ON Semiconductor
Mill-Max
Comm-Con
Comm-Con
Sumida
Siliconix
AAC
IRC
AAC
AAC
AAC
AAC
AAC
AAC
LTC
(843) 946-0362
(408) 573-4150
(602) 244-6600
(602) 244-6600
(516) 922-6000
(626) 301-4200
(626) 301-4200
(847) 956-0667
(800) 554-5565
(800) 508-1521
(361) 992-7900
(800) 508-1521
(800) 508-1521
(800) 508-1521
(714) 255-9186
(714) 255-9186
(714) 255-9186
(408) 432-1900
VENDOR
Sanyo
AVX
AVX
Taiyo Yuden
Sanyo
Taiyo Yuden
TELEPHONE
(619) 661-6835
(843) 946-0362
(843) 946-0362
(408) 573-4150
(619) 661-6835
(408) 573-4150
dc309fa
3
DEMO MANUAL DC309
NO-DESIGN SWITCHER
QUICK START GUIDE
The DC309 demonstration board is easy to set up to
evaluate the performance of the LTC1773. Please follow
the procedure outlined below for proper operation.
1. Move jumper JP1 to the appropriate position for the
required output voltage. The board is set up for a
default output voltage of 1.5V if no jumper is used. To
avoid possible damage to the LTC1773 device, make
sure V
IN
is off when moving the jumper JP1.
2. To shut down the circuit, connect the RUN/SS termi-
nal to ground.
3. For synchronized operation, connect the clock signal
between the SYNC/FCB and GND terminals. Do not
apply more than the input voltage (V
IN
) on the SYNC/
FCB terminal.
4. For Burst Mode operation at low load currents, float
the SYNC/FCB terminal or connect it to V
IN
. Grounding
this terminal will force continuous operation regard-
less of load current.
5. Connect the input power supply to the V
IN
and GND
terminals.
6. Connect the load between the V
OUT
and GND terminals.
Refer to Figure 2 for proper measurement equipment
setup.
+
A
V
OUT
V
IN
A
+
+
+
+
LOAD
+
V
GND
LTC1773
GND
V
SYNC/FCB
RUN/SS
DC309 F01
Figure 2. Proper Measurement Setup
CO PO E T SELECTIO
Quick Components Selection Guide
COMPONENTS
R2
Q1
L1
I
OUT
= 1A
0.05Ω
Si6803 or
Si6801
6µH
I
OUT
= 2.5A
0.025Ω
Si9801
3µH
I
OUT
= 3A
0.02Ω
Si9803 and
Si9804
3µH
The circuit shown in Figure 1 operates from an input
voltage between 2.8V and 8.5V. Output voltages of 1.5V,
1.8V, 2.5V and 3.3V can be easily set by moving jumper
JP1 to the appropriate position; make sure V
IN
is off when
doing this.
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 LTC1773 switching
regulator IC and was not designed for any other purpose.
External component selection is driven by the load
requirement and begins with the selection of R
SENSE
. Once
R
SENSE
is known, L can be chosen, followed by the exter-
nal power MOSFETs. Finally, C
IN
and C
OUT
are selected.
4
U
R
SENSE
Selection for Output Current
I
OUT
= 5A
0.015Ω
2x Si9803
and Si9804
1µH
U U
W
R
SENSE
is chosen based on the required output current.
The LTC1773 current comparator has a maximum thresh-
old of 100mV/R
SENSE
. The current comparator threshold
sets the peak inductor current, yielding a maximum aver-
age output current, I
MAX
, equal to the peak value less half
the peak-to-peak ripple current,
∆I
L
.
Allowing a margin for variations in the LTC1773 and
external component values yields:
R
SENSE
= 70mV/I
MAX
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC1773. The internal preset frequency is
550kHz, but can be externally synchronized to frequencies
of up to 750kHz.
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. However,
dc309fa
DEMO MANUAL DC309
NO-DESIGN SWITCHER
CO PO E T SELECTIO
operating at a higher frequency generally results in lower
efficiency because of external gate charge losses.
The inductor value has a direct effect on ripple current. The
ripple current,
∆I
L
, decreases with higher inductance or
frequency and increases with higher V
IN
or V
OUT
.
∆I
L
=
V
1
V
OUT
1–
OUT
(
f
)(
L
)
V
IN
Accepting larger values of
∆I
L
allows the use of lower
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is
∆I
L
= 0.4(I
MAX
).
Power MOSFET and Schottky Diode Selection
Two external power MOSFETs must be selected for use
with the LTC1773: a P-channel MOSFET for the top (main)
switch, and an N-channel MOSFET for the bottom (syn-
chronous) switch.
This board is laid out with MOSFET footprints for three
different output load requirements. For 1A or less applica-
tions, use the 8-pin TSSOP footprint on the back for
complementary 1A MOSFETs in one package. For 2.5A
applications, use the SO-8 footprint on the front for
complementary 3A MOSFETs in one package. This is the
default MOSFET footprint used on the board. For applica-
tions requiring more output current, use single packaged
SO-8 MOSFETs (P-channel on the front and N-channel on
the back).
The peak-to-peak gate drive levels are set by the V
IN
voltage. Therefore, for V
IN
> 5V, logic-level threshold
MOSFETs should be used. But for V
IN
< 5V, sublogic-level
threshold MOSFETs (V
GS(TH)
< 3V) should be used. In
these applications, make sure that the V
IN
to the LTC1773
is less than 8V because the absolute maximum VGS rating
of a lot of these sublogic threshold MOSFETs is 8V.
Selection criteria for the power MOSFETs include the “ON”
resistance, R
DS(ON)
, reverse transfer capacitance, CRSS,
input voltage, maximum output current and total gate
charge.
A Schottky diode can be placed in parallel with the syn-
chronous MOSFET to improve efficiency. It conducts
U
during the dead-time between the conduction of the two
power MOSFETs. This prevents the body diode of the
bottom MOSFET from turning on and storing charge
during the dead-time, which could cost as much as 1% in
efficiency. A 1.5A Schottky is generally a good size for 5A
to 8A regulators due to the relatively small average cur-
rent. Larger diodes result in additional transition losses
due to their larger junction capacitance. The diode may be
omitted if the efficiency loss can be tolerated.
C
IN
Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
OUT
/V
IN
. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
U U
W
[
V
OUT
(
V
IN
– V
OUT
)
]
1 2
C
IN
required I
RMS
≅
I
MAX
V
IN
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is
commonly used for design because even significant de-
viations do not offer much relief. Note that capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor or choose a capacitor rated at a higher
temperature than required. Several capacitors may also be
paralleled to meet size or height requirements in the
design. Always consult the manufacturer if there is any
question.
C
OUT
Selection
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The output ripple (∆V
OUT
) is determined by:
1
∆
V
OUT
≅ ∆
I
L
ESR
+
8fC
OUT
where f = operating frequency, C
OUT
= output capacitance
and
∆I
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since
∆I
L
increases
dc309fa
5
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