DEMO MANUAL DC094
DESIGN READY SWITCHER
LTC1435 Constant Frequency
Synchronous DC/DC Converter
This results in a power supply that has very high
efficiency, low ripple and fast transient response. At
low output currents the LTC1435 automatically switches
to Burst Mode
TM
operation to reduce switching losses
and maintain high operating efficiencies. Additionally,
the supply current can be shut down to less than
20µA with an input voltage of 10V. This feature is an
absolute necessity to maximize battery life in portable
applications.
Gerber files for this circuit board are avail-
able. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
DESCRIPTIO
Demonstration Circuit 094 is a constant frequency step-
down (buck) regulator implemented entirely in surface
mount using the LTC
®
1435 switching regulator controller.
The output voltage is programmable from 1.8V to 5V via
a jumper. The input voltage can range from 4.5V to 28V
(limited by the external MOSFETs). The circuit highlights
the capabilities of the LTC1435 which uses a current
mode, constant frequency architecture to switch a pair of
N-channel power MOSFETs while providing 99% maxi-
mum duty cycle. Operating efficiencies exceeding 90%
are obtained.
PERFOR A CE SU
Input Voltage Range
Output
Maximum Input Voltage (Limited by External MOSFET and Input Capacitor)
Output Voltage (Jumper Selectable)
Maximum Output Current (Continuous)
Maximum Output Current (Peak)
Typical Output Ripple at 10MHz Bandwidth (Burst Mode Operation) I
O
= 100mA
Typical Output Ripple at 10MHz Bandwidth (Continuous) I
O
= 1A
V
IN
I
OUT
I
Q
I
EXTVCC
V
RUN
Frequency
Line Regulation, 6V to 20V
Load Regulation, No Load to Full Rated Output
Supply Current with No Load at 10V Input (Typical), SFB = INT V
CC
, EXT V
CC
= 5V
Supply Current in Shutdown (Typical), V
IN
= 10V
EXT V
CC
Pin Current, V
EXTVCC
= 5V, V
IN
= 10V, No Load
Run Pin Threshold (Typical)
Operating Frequency (Typical), C
OSC
= 68pF
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
Efficiency
100
V
IN
= 10V
SFB = INT V
CC
90
V
OUT
= 5V
EFFICIENCY (%)
80
V
OUT
= 3.3V
70
V
OUT
= 2.5V
60
50
1
1000
10
100
LOAD CURRENT (mA)
4000
DM094 BP
DM094 TPC01
U
WW
U W
U
ARY
U W
Operating Temperature Range 0
°
C to 50
°
C
4.5V to 28V
1.8V, 2.5V, 2.9V, 3.3V, 5V
3A
3.5A
50mV
P-P
35mV
P-P
0.002%/V
– 1%
50µA
15µA
650µA
1.3V
170kHz
1
DEMO MANUAL DC094
DESIGN READY SWITCHER
PACKAGE A D SCHE ATIC DIAGRA SM
TOP VIEW
C
OSC
1
16 TG
15 BOOST
14 SW
13 V
IN
12 INT V
CC
11 BG
10 PGND
9
EXT V
CC
E4
E3
RUN SFB
INT V
CC
GND
JP1
V
IN
4.5V TO 28V
E2
E1
C
OSC
68pF
1
C
SS
, 0.1µF
R
C
, 10k
C
C2
, 51pF
C
C
330pF
2
3
4
5
6
7
C5
1000pF
8
C
OSC
RUN/SS
I
TH
SFB
SGND
V
OSENSE
SENSE
–
SENSE
+
U1
LTC1435
C2
0.1µF
TG
BOOST
SW
V
IN
INT V
CC
BG
PGND
EXT V
CC
16
15
14
13
12
11
10
9
C4
D1
0.1µF
CMDSH-3
L1
10µH
D2
MBRS140T3
Q2
Si4412DY
Q1
Si4412DY
C1, 100pF
+
C3
4.7µF
16V
R2
69.8k
1%
JP2A
1.8V
R3
32.4k
1%
JP2B
2.5V
R4
24.9k
1%
JP2C
2.9V
R5
20k
1%
JP2D
3.3V
R6
11k
1%
JP2E
5V
E5
EXT V
CC
Figure 1. LTC1435 Constant Frequency, High Efficiency Converter
PARTS LIST
REFERENCE
DESIGNATOR
C
C
C
C2
C
IN1
, C
IN2
C
OSC
C
OUT1
, C
OUT2
C1, C6
C2, C4, C
SS
C3
C5
D1
D2
E1 to E8
QUANTITY
1
1
2
1
2
2
3
1
1
1
1
8
PART NUMBER
08055A331KAT1A
08055A510KAT1A
TPSE226M035
08055A680JAT1A
TPSD107M010R0080
08055A101KAT1A
08055G104KAT1A
TAJB475M016
08055C102KAT1A
CMDSH-3
MBRS140T3
1502-2
DESCRIPTION
330pF 50V 10% NPO Chip Capacitor
51pF 50V 10% NPO Chip Capacitor
22µF 35V 20% Tantalum Capacitor
68pF 50V 5% NPO Chip Capacitor
100µF 10V 20% Tantalum Capacitor
100pF 50V 10% NPO Chip Capacitor
0.1µF 50V 10% Y5V Chip Capacitor
4.7µF 16V 20% Tantalum Capacitor
1000pF 50V 10% X7R Chip Capacitor
BVR = 30V Schottky Diode
BVR = 40V Schottky Diode
Turret Terminal
VENDOR
AVX
AVX
AVX
AVX
AVX
AVX
AVX
AVX
AVX
Central
Motorola
Keystone
TELEPHONE
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(516) 435-1110
(602) 244-3576
(718) 956-8900
2
W
W
U
RUN/SS 2
I
TH
3
SFB 4
SGND 5
+
C
IN
22µF
35V
×
2
V
OSENSE
6
SENSE
–
7
SENSE
+
8
S PACKAGE
16-LEAD PLASTIC SO
LTC1435CS
R
SENSE
0.033Ω
E6
V
OUT
E7
V
OSENSE
+
SGND
C6
100pF
C
OUT
100µF
10V
×
2
R1
35.7k
1%
E8
GND
DM 094 F01
DEMO MANUAL DC094
DESIGN READY SWITCHER
PARTS LIST
REFERENCE
DESIGNATOR
JP1
JP2
L1
Q1, Q2
R1
R2
R3
R4
R5
R6
R
C
R
SENSE
U1
QUANTITY
1
1
1
2
1
1
1
1
1
1
1
1
1
2
PART NUMBER
2802S-03-G2
2802S-10-G2
CDRH125-10
CDRH127-10
Si4412DY
CR21-3572F-T
CR21-6982F-T
CR21-3242F-T
CR21-2492F-T
CR21-2002F-T
CR21-1102F-T
CR21-103J-T
LR2010-01-R033-F
LTC1435CS
CCIJ2mm-138-G
DESCRIPTION
2mm Pin Header
2mm Pin Header
10µH Inductor
10µH Inductor (Alternate)
N-Channel MOSFET
35.7k 1/10W 1% Chip Resistor
69.8k 1/10W 1% Chip Resistor
32.4k 1/10W 1% Chip Resistor
24.9k 1/10W 1% Chip Resistor
20k 1/10W 1% Chip Resistor
11k 1/10W 1% Chip Resistor
10k 1/10W 5% Chip Resistor
0.033Ω 1/2W 1% Resistor
16-Lead Narrow Small Outline IC
Jumper
VENDOR
Comm Con
Comm Con
Sumida
Sumida
Siliconix
AVX
AVX
AVX
AVX
AVX
AVX
AVX
IRC
LTC
Comm Con
TELEPHONE
(818) 301-4200
(818) 301-4200
(708) 956-0666
(800) 554-5565
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(803) 448-9411
(512) 992-7900
(408) 432-1900
(818) 301-4200
QUICK START GUIDE
This demonstration board is easy to set up to evaluate the
performance of the LTC1435. Please follow the procedure
outlined below for proper operation.
1. Refer to Figure 3 for proper arrangement of monitoring
equipment for correct measurement equipment setup.
2. 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 28V or the MOSFET(s)
will be dam-
aged.
At lower output voltages (V
OUT
< 2.5V) the maxi-
mum input voltage must be decreased. See Table 1.
3. Connect the load between the V
OUT
and GND terminals
on the right side of the board.
4. The RUN pin can be left unconnected. To shut down the
LTC1435 tie this pin to ground.
5. When the 5V output voltage option is selected externally
connect EXT V
CC
to V
OUT
.
6. Set the jumper JP1 so that SFB is connected to
INT V
CC
.
A
B
7. Set the desired output voltage with jumper JP2 shown
in Figure 2 and Table 1.
Table 1. Maximum Allowable Input Voltage vs Selected
Output Voltage
POSITION
A
B
C
D
E
OUTPUT VOLTAGE
1.8V
2.5V
2.9V
3.3V
5V
MAXIMUM INPUT
VOLTAGE
18V
24V
28V
28V
28V
JP2
C
D
E
DM094 F02
1.8V 2.5V 2.9V 3.3V 5V
Figure 2. Output Voltage Selection (JP2)
(3.3V Position Shown)
3
DEMO MANUAL DC094
DESIGN READY SWITCHER
I TRODUCTIO
The circuit in Figure 1 highlights the capabilities of the
LTC1435. The application circuit is set up for a variety of
output voltages. Output voltages from 1.8V to 5V are
available by selecting the appropriate jumper position.
The LTC1435 is a synchronous step-down switching
regulator controller which drives external N-channel power
MOSFETs using a fixed frequency architecture. Burst
Mode operation provides high efficiency at low load cur-
rents. Operating efficiencies typically exceed 90% over
three decades of load current range. A maximum high duty
OPERATIO
The operating frequency is set by an external capacitor
C
OSC
, allowing maximum flexibility in optimizing effi-
ciency. In this application the frequency is set to 170kHz.
A secondary winding feedback control pin SFB inhibits
Burst Mode which reduces noise and RF interference.
Soft start is provided by an external capacitor C
SS
which
can be used to properly sequence supplies. The operating
current level is user-programmable via an external current
sense resistor and is set to 3A. Short-circuit current limit
is set approximately to 4A.
This demo board is optimized for 3.3V outputs. A wide
input supply range allows operation from 4.5V to 28V for
V
OUT
voltages of 3.3V and 5V. Because this board allows
for a wide output voltage range (1.8V to 5V) and the
operating frequency remains constant at 170kHz, there is
a duty cycle induced limit on the maximum input voltage
when low output voltages are selected (V
OUT
< 2.9V). This
is necessary for an adequate turn-on time for the top
MOSFET with the required duty cycle at a given frequency.
If a higher input supply voltage is required together with
low output voltage, the operating frequency can be de-
creased by increasing C
OSC
.
Main Control Loop
The LTC1435 uses a constant frequency, current mode
step-down architecture. Current mode operation was
judged to be mandatory for its well-known advantages of
4
U
cycle limit of 99% provides low dropout operation which
extends operating time in battery-operated systems.
The use of small spring-clip leads are very convenient for
small-signal bench testing and voltage measurements,
but should not be used with the high currents associated
with this circuit. Soldered wire connections are required to
properly ascertain the performance of the PC board.
This demonstration unit is intended for the evaluation of
the LTC1435 switching regulator IC and was not designed
for any other purpose.
clean start-up, accurate current limit and excellent line and
load regulation.
During normal operation, the top MOSFET is turned on
each cycle when the oscillator sets a latch and turned off
when the main current comparator resets the latch. The
peak inductor current is controlled by the voltage on the
I
TH
pin, which is the output of error amplifier EA.
The V
OSENSE
pin allows EA to receive an output feedback
voltage V
FB
from an external resistive divider. When the
load current increases, it causes a slight decrease in V
FB
relative to the 1.19V 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 or the beginning of the
next cycle.
The top MOSFET driver is biased from floating bootstrap
capacitor C4, which normally is recharged during each off
cycle. However, when V
IN
decreases to a voltage close to
V
OUT
, the loop may enter dropout and attempt to turn on
the top MOSFET continuously. The dropout detector counts
the number of oscillator cycles that the top MOSFET
remains on and periodically forces a brief off period to
allow C4 to recharge.
A built-in comparator guards against transient overshoots
> 7.5% by turning off the top MOSFET and keeping it off
until the fault is removed.
U
U
DEMO MANUAL DC094
DESIGN READY SWITCHER
OPERATIO
Low Current Operation
The LTC1435 is capable of Burst Mode operation in which
the external MOSFETs operate intermittently based on
load demand. If the voltage across R
SENSE
does not exceed
approximately 20mV for one full cycle, then on following
cycles the top and bottom drives are disabled. This contin-
ues until the I
TH
voltage exceeds 0.6V, which causes drive
to be returned to the top MOSFET on the next cycle.
Two conditions can force continuous synchronous opera-
tion, even when the load current would otherwise dictate
low current operation. One is when the common mode
voltage of the Sense
+
and Sense
–
pins is below 1.4V, and
the other is when the SFB pin is below 1.19V. See the SFB
pin function description.
INT V
CC
/EXT V
CC
Power
Power for the top and bottom MOSFET drivers and most
of the other LTC1435 circuitry is derived from INT V
CC
pin.
When the EXT V
CC
pin is left open, an internal 5V low
dropout regulator supplies INT V
CC
power. If EXT V
CC
is
taken above 4.7V, the 5V regulator is turned off and an
internal switch is turned on to connect EXT V
CC
to INT V
CC
.
This allows the INT V
CC
power to be derived from a high
efficiency external source such as the output of the regu-
lator itself or a secondary winding, as described in the
LTC1435 data sheet.
When the 5V output voltage option is selected (JP2E
installed) the EXT V
CC
pin should be externally connected
to V
OUT
.
HOW TO MEASURE VOLTAGE REGULATION
When trying to measure voltage regulation, remember
that all measurements must be taken at the point of
regulation. This point is where the LTC1435’s control loop
looks for the information to keep the output voltage
constant. In this demonstration board this information
point occurs between Pin 5 of the LTC1435, the signal
ground, and the output side of R1. These points corre-
spond to the V
OSENSE
(E7) terminal of the board. Output
voltage test leads should be attached directly to this
U
terminal. The load should be placed across V
OUT
(E6) to
GND (E8). Measurements
should not
be taken at the end
of test leads at the load. Refer to Figure 3 for the proper
monitoring equipment configuration.
This applies to line regulation (input to output voltage
regulation) as well as load regulation tests. In doing line
regulation tests always look at the input voltage across the
input terminals.
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.
BURST MODE ENABLED
BURST MODE DISABLED
V
IN
JP1
GND
V
OUT
EXT V
CC
EXTERNAL
SOURCE
V
≤
10V
+
+
V
IN
I
IN
A
+
V
+
I
OUT
A
LOAD
OFF
RUN
RUN
JP2
V
OSENSE
A B C D E
V
OUT
V
+
SFB
OPEN
GND
LINEAR TECHNOLOGY DEMO CIRCUIT 094A
(408) 432-1900
CONSTANT FREQUENCY
HIGH EFFICIENCY CONVERTER
LTC1435
DM094 F03
Figure 3. Proper Measurement Setup
REMOTE OUTPUT VOLTAGE SENSING
Remote output voltage sensing can be accomplished by
modifying the PC board. A small PC trace connecting V
OUT
to V
OSENSE
must be cut as shown in Figure 4. An external
connection from V
OSENSE
directly across the load must be
made. To prevent uncertainty, solder a 10Ω resistor
across the V
OUT
and V
OSENSE
terminals. Never, under any
circumstance, allow V
OSENSE
to float!
5
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