DEMO MANUAL DC257
NO-DESIGN SWITCHER
LTC1707 Monolithic
Synchronous Step-Down Regulator
DESCRIPTIO
Demonstration circuit DC257 is a constant-frequency
step-down converter using an LTC
®
1707 monolithic syn-
chronous regulator. It provides low input voltage, high
conversion efficiency for cell phones and other portable
electronics operating from one or two Li-Ion cells or three
to six NiCd cells. Constant 350kHz operation and up to
600mA output capability in an SO-8 package provide a low
noise, space-efficient solution for wireless applications.
The circuit highlights the capability of the LTC1707. De-
signed to work at low voltages, the input voltage (V
IN
) can
range from 2.85V to 8.5V. At V
IN
< 2.7V, the LTC1707
shuts down and draws just a few microamperes, making
PERFOR A CE SU
SYMBOL
V
IN
V
OUT
PARAMETER
Input Voltage Range
Output Voltage
I
Q
Pulse Skipping Mode Supply Current
Burst Mode Enabled Supply Current
Shutdown Current
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
LTC1707 Efficiency Curve
100
V
OUT
= 3.3V
95
V
IN
= 6V
EFFICIENCY (%)
90
85
80
75
70
1
10
100
OUTPUT CURRENT (mA)
1000
DC257 TA01
V
IN
= 3.6V
V
IN
= 8.4V
U
WW
U W
U
it ideal for single lithium-ion battery applications. DC257’s
output voltage is programmable from 1.5V to 3.3V via a
jumper.
At low output currents, the LTC1707 automatically switches
to Burst Mode
TM
operation to reduce switching losses and
maintain high operating efficiencies. In switching-noise
sensitive applications, Burst Mode operation can be inhib-
ited by grounding the SYNC/MODE pin or synchronizing it
with an external clock.
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.
U W
ARY
CONDITIONS
See Figure 2
JUMPER POSITION
JP1 = “1.5V”
JP1 = “1.8V”
JP1 = “2.5V”
JP1 = “2.9V”
JP1 = “3.3V”
JP1 = “OPEN”
JP1 = “3.3V”
JP1 = “3.3V”
JP1 = “3.3V”
VALUE
2.85V to 8.5V
1.51V
±0.036V
1.82V
±
0.043V
2.52V
±
0.06V
2.94V
±
0.07V
3.33V
±
0.079V
Note 1
300µA
200µA
15µA
V
IN
= 5V, SYNC/MODE = 0V, RUN/SS = 2V, I
OUT
= 0mA
V
IN
= 5V, SYNC/MODE = 2V, RUN/SS = 2V, I
OUT
= 0mA
V
IN
= 5V, RUN/SS = 0V, I
OUT
= 0mA
Demo Board
1
DEMO MANUAL DC257
NO-DESIGN SWITCHER
PERFOR A CE SU
SYMBOL
I
OUT
PARAMETER
Minimum Output Current
f
OSC
V
RIPPLE
V
OUT
V
SYNC
V
RUN/SS
V
REF
I
REF
Operating Frequency
Typical Output Ripple
Typical Load Regulation
Synchronize Threshold Voltage
Minimum Shutdown Threshold Voltage
Reference Output Voltage
Maximum V
REF
Output Currrent
Note 1:
Programmable via optional R6. V
OUT
= 0.8V(1 + 210k/R6)
PACKAGE A D SCHE ATIC DIAGRA S
TOP VIEW
I
TH
1
RUN/SS 2
8
7
6
5
V
REF
SYNC/MODE
V
IN
SW
C
C2
47pF
OPTIONAL
R
C
C
C1
1
I
TH
V
REF
SYNC/
MODE
V
IN
SW
8
E1
V
REF
E8
SYNC/MODE
2
C
SS
0.1µF
RUN/SS
LTC1707
V
FB
GND
7
3
4
6
5
L1 15µH
R7
210k
R8
10Ω
E3
RUN/SS
C
REF
1000pF
+
C
IN
22µF
16V
+
C
OUT
100µF
6.3V
*
SPACE IS PROVIDED FOR AN OPTIONAL RESISTOR TO PROGRAM A CUSTOM OUTPUT VOLTAGE. THE OUTPUT VOLTAGE MUST NOT EXCEED 3.3V
Figure 1. LTC1707 Constant Frequency, High Efficiency Converter
2
W
WW
W
U W
U
ARY
CONDITIONS
V
IN
= 5V, V
OUT
= 2.5V
V
IN
= 5V, V
OUT
= 3.3V
V
IN
= 3V, V
OUT
= 2.5V
Unsynchronized
Synchronized
I
OUT
= 500mA, V
IN
= 5V, V
OUT
= 3.3V
0mA < I
OUT
< 0.5A, V
IN
= 5V, V
OUT
= 3.3V
All
All
I
REF
= 0
All
All
All
All
JUMPER POSITION
VALUE
600mA
500mA
300mA
350kHz
385kHz to 550kHz
30mV
P-P
0.5%
1.2V
0.4V
1.19V
100µA
V
FB
3
GND 4
S8 PACKAGE
8-LEAD PLASTIC SO
LTC1707CS8
E2
V
IN
≤
8.5V
E7
V
OUT
E6
V
OSENSE
R5
66.5k
3.3V
DC257 F01
R1
237k
1.5V
R2
165k
1.8V
R3
97.6k
2.5V
R4
78.7k
JP2
2.9V
R6*
OPTIONAL
OPEN
E4/E5
GND
DEMO MANUAL DC257
NO-DESIGN SWITCHER
PARTS LIST
REFERENCE
DESIGNATOR
C
IN
C
OUT
C
SS
C
REF
C
C1
C
C2
E1 to E8
JP1
JP1
L1
R1
R2
R3
R4
R5
R7
R8
R6
R
C
U1
1
LTC1707CS8
1
8
1
1
1
1
1
1
1
1
1
1
06035A470JAT
2501-2
22025-12-G1
CCIJ2MM-138G
CD54-150MC
CR16-2373FM
CR16-1653FM
CR16-9762FM
CR16-7872FM
CR16-6652FM
CR16-2103FM
CR16-100JM
QUANTITY
1
1
1
1
PART NUMBER
TPSC226M016R0375
TPSC107M006R0150
0603YC104KAT
06035A102JAT
DESCRIPTION
22µF 16V TPS Tantalum Capacitor
100µF 6V TPS Tantalum Capacitor
0.1µF 16V X7R Chip Capacitor
1000pF 50V NPO Chip Capacitor
Optional
47pF 50V NPO Chip Capacitor
Turret, Testpoint
0.079" Double Row Header
0.079" Center Shunt
15µH 20% Inductor
237k 1% 1/16W Chip Resistor
165k 1% 1/16W Chip Resistor
97.6k 1% 1/16W Chip Resistor
78.7k 1% 1/16W Chip Resistor
66.5k 1% 1/16W Chip Resistor
210k 1% 1/16W Chip Resistor
10Ω 5% 1/16W Chip Resistor
Optional
Optional
Monolithic Synchronous Step-Down Regulator
LTC
(408) 432-1900
AVX
Mill-Max
COMM CON
COMM CON
Sumida
TAD
TAD
TAD
TAD
TAD
TAD
TAD
(843) 946-0362
(516) 922-6000
(626) 301-4200
(626) 301-4200
(847) 956-0667
(800) 508-1521
(800) 508-1521
(800) 508-1521
(800) 508-1521
(800) 508-1521
(800) 508-1521
(800) 508-1521
VENDOR
AVX
AVX
AVX
AVX
TELEPHONE
(207) 282-5111
(207) 282-5111
(843) 946-0362
(843) 946-0362
3
DEMO MANUAL DC257
NO-DESIGN SWITCHER
QUICK START GUIDE
This demonstration board is easily set up to evaluate the
performance of the LTC1707 IC. Please follow the proce-
dure outlined below for proper operation.
• Refer to Figure 5 for proper connection of monitoring
equipment to ensure correct measurement.
• Connect the input power supply to the V
IN
and GND
terminals on the left-hand side of the board. Do not
increase V
IN
over 10V or the part will be damaged.
• Connect the load between the V
OUT
and GND
terminals on the right side of the board.
• The RUN/SS pin can be left unconnected. To shut
down the LTC1707, tie this pin to GND.
• Set the desired output voltage with jumper JP1, as
shown in Figure 2 and Table 1.
1.5V
1.8V
Table 1. Output Voltage Selection
JP1 POSITION
“1.5V”
“1.8V”
“2.5V”
“2.9V
“3.3V”
“OPEN”
OUTPUT VOLTAGE
1.51V
1.82V
2.52V
2.94V
3.33V
JP1
2.5V
2.9V
3.3V OPEN
DC257 F02
Figure 2. Output Voltage Selection (JP1)
(3.3V Position Shown)
OPERATIO
The circuit in Figure 1 highlights the capabilities of the
LTC1707. The application circuit is set up for a variety of
output voltages. Output voltages from 1.5V to 3.3V or user
programmable voltages can be obtained by selecting the
appropriate jumper position.
The LTC1707 is a monolithic synchronous step-down
switching regulator using a fixed-frequency architecture.
Burst Mode operation provides high efficiency at low load
currents. Operating efficiencies typically exceed 90% over
two decades of load current range. 100% duty cycle
provides low dropout operation, which extends operating
time in battery-operated systems.
Do not use small spring-clip leads when testing this
circuit. Soldered wire connections are required to properly
verify the performance of the PC board.
This demonstration board is intended for the evaluation of
the LTC1707 switching regulator IC and was not designed
for any other purpose.
4
U
The operating frequency of this demo circuit is 350kHz,
the frequency of the LTC1707’s internal oscillator. For
higher frequencies, SYNC/MODE (E8) can be synchro-
nized with an external clock. Burst Mode operation is
automatically disabled when the SYNC/MODE pin is ex-
ternally driven. Grounding SYNC/MODE also disables
Burst Mode operation, potentially reducing noise and RF
interference.
Soft-start is provided by an external capacitor, C
SS
, which
can be used to properly sequence supplies. The maximum
operating current level is 0.6A.
This demo board is optimized for 3.3V outputs and 5V
input. Output voltages from 1.5V to 3.3V are available by
selecting the appropriate position of JP1. For other output
voltages, select the OPEN position and add an appropriate
resistor value in the space provided. The output voltage
must never exceed 3.3V because the output capacitor may
be damaged. The input supply can range from 2.85V to
8.5V.
DEMO MANUAL DC257
NO-DESIGN SWITCHER
OPERATIO
V
IN
1.5µA
SYNC/MODE
7
BURST
DEFEAT
X
0.6V
V
FB
3
V
REF
8
V
IN
0.8V
+
EA
0.12V
I
TH
1
S
RUN/SOFT
START
R
Q
Q
–
1.19V
REF
2.25µA
V
IN
RUN/SS 2
UVLO
TRIP = 2.7V
+
OVDET
0.86V
SHUTDOWN
–
+
I
RCMP
5 SW
4 GND
DC257 F03
Figure 3. Functional Block Diagram
Main Control Loop (Refer to Functional Diagram)
The LTC1707 uses a constant-frequency, current mode
step-down architecture. Both the main and synchronous
switches, consisting of top P-channel and bottom
N-channel power MOSFETs, are internal. During normal
operation, the internal top power MOSFET is turned on
during each cycle when the oscillator sets the RS latch,
and turned off when the current comparator, I
COMP
, resets
the RS latch. The peak inductor current at which I
COMP
resets the RS latch is controlled by the voltage on the I
TH
pin, which is the output of error amplifier EA. The V
FB
pin
allows EA to receive an output feedback voltage from an
external resistive divider. When the load current increases,
+
–
+
FREQ
SHIFT
–
U
Y = “0” ONLY WHEN X IS A CONSTANT “1”
Y
V
IN
V
IN
SLOPE
COMP
OSC
0.4V
6 V
IN
EN
SLEEP
+
–
–
+
6Ω
I
COMP
BURST
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
ANTI-
SHOOT-THRU
–
it causes a slight decrease in the feedback voltage relative
to the 0.8V reference, which, in turn, causes the I
TH
voltage to increase until the average inductor current
matches the new load current. While the top MOSFET is
off, the bottom MOSFET is turned on until either the
inductor current starts to reverse, as indicated by the
current reversal comparator I
RCMP
, or the next cycle
begins.
The main control loop is shut down by pulling the RUN/
SS pin low. Releasing RUN/SS allows an internal 2.25µA
current source to charge soft-start capacitor C
SS
. When
C
SS
reaches 0.7V, the main control loop is enabled with
the I
TH
voltage clamped at approximately 5% of its
5
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