DEMO MANUAL DC247
DESIGN-READY SWITCHER
LTC1735 Constant
Frequency Synchronous DC/DC Converter
DESCRIPTIO
Demonstration Circuit DC247 demonstrates a 1.6V/9A
notebook CPU application using the LTC
®
1735 switching
regulator controller. A high performance, constant fre-
quency, current mode architecture generates a precise
low voltage CPU core supply. Protection features include
an externally defeatable overcurrent latchoff and internal
current foldback for overload conditions. A soft-latched
crowbar monitors the output voltage for overvoltage pro-
tection. The circuit was designed for a 5V to 26V input
range but allows a 4.5V to 28V range (limited by the
external MOSFETs). Strong output drivers easily handle
large power MOSFETs efficiently. Other output voltages,
PERFOR A CE SU
PARAMETER
Input Voltage Range
Output
CONDITIONS
(Maximum Input Voltage Limited by External MOSFET and Input Capacitor)
Output Voltage
Max Output Current (Continuous, Thermally Limited)
Max Output Current (Peak)
Typical Output Ripple at 10MHz Bandwidth (Burst Mode Operation) I
O
= 100mA
Typical Output Ripple at 10MHz Bandwidth (Continuous) I
O
= 5A
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
Efficiency
100
V
IN
= 15V
90
EFFICIENCY (%)
V
IN
= 5V
80
70
60
50
EXTV
CC
= 5V
40
0.01
0.1
1
LOAD CURRENT (A)
10
247 TA01
V
IN
= 24V
U
WW
U W
U
to as low as 0.8V, can be configured by modifying the
external resistive divider. External frequency synchroniza-
tion is provided, as are three modes of operation: Burst
Mode
TM
operation to reduce switching losses and maintain
high operating efficiencies, burst inhibit/forced continu-
ous mode and a pulse-skipping mode that provides con-
stant frequency operation down to 1% maximum load
currents with low quiescent current. This results in a
power supply that has very high efficiency, low ripple and
fast transient response.
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
Operating Temperature Range 0°C to 50°C
VALUE
4.5V to 26V
1.6V
9A
10.5A
50mV
P-P
25mV
P-P
Demo Circuit 247A
1
DEMO MANUAL DC247
DESIGN-READY SWITCHER
PERFOR A CE SU
PARAMETER
V
IN
I
OUT
I
Q
I
EXTVCC
V
RUN
Frequency
CONDITIONS
Line Regulation 4.5V to 26V
Load Regulation No Load to Full Rated Output
Supply Current (Typical) with No Load at 15V Input, FCB = INTV
CC
Supply Current in Shutdown (Typical), V
IN
=15V, JP2 Open
EXTV
CC
Pin Current, V
EXTVCC
= 5V, V
IN
= 10V, No Load, FCB = INTV
CC
Run Pin Threshold (Typical)
Operating Frequency (Typical), C
OSC
= 47pF
PACKAGE A D SCHE ATIC DIAGRA S
JP1
BURST MODE
OFF
ON
INTV
CC
C
OSC1
47pF
1
C
SS1
0.1µF
2
C
C2
330pF R
C1
33k
C
C1
100pF
4
C
OSC
FCB/SYNC RUN
E4
E3
JP2
LATCHOFF
R6
680k
CF1
0.1µF
U1
TG
16
R8
0Ω
C
B1
0.22µF
15
M2
FDS6680A
RF1
4.7Ω
C
IN1
22µF
30V
GND
E9
+V
IN
E10
RUN/SS
BOOST
3
I
TH
LTC1735
FCB
SW
14
L1
2µH
R
CS1
0.005Ω
R
S1
10Ω
C4
1µF
M1
FDS6680A
D2
MBRS340T3
C3
47pF
R7
10k
1%
R3
10k
1%
V
IN
13
D1
CMDSH-3
5
C1
47pF
6
SGND
INTV
CC
12
C2
11 4.7µF
+
V
OSENSE
–
BG
7
R4*
47k
V
SEL
E1
R1*
10k
Q1*
VN2222
C
S1
1000pF
SENSE
PGND
10
8
SENSE
+
EXTV
CC
9
EXTV
CC
E2
R5, 10Ω
R2, 10Ω
247 F01
*NOT INSTALLED
Figure 1. Demo Board Schematic
PARTS LIST
REFERENCE
DESIGNATOR
C1, C3
C2
C4
C
C1
C
C2
CB1
QUANTITY
2
1
1
1
1
1
PART NUMBER
08055A470JAT1A
TACC475M010R
0805ZC105MAT1A
08055A101MAT1A
08055A331MAT1A
08055A224KAT1A
DESCRIPTION
47pF 50V 5% NPO Capacitor
4.7µF 10V 20% Tantalum Capacitor
1µF 10V 20% X7R Capacitor
100pF 50V 5% NPO Capacitor
330pF 50V 5% NPO Capacitor
0.22µF 50V 20% X7R Capacitor
VENDOR
AVX
AVX
AVX
AVX
AVX
AVX
TELEPHONE
(843) 946-0362
(207) 282-5111
(843) 946-0362
(843) 946-0362
(843) 946-0362
(843) 946-0362
2
W
WW
W
U W
ARY
Operating Temperature Range 0°C to 50°C
VALUE
0.002%/V
– 0.03%
500µA
15µA
460µA
1.3V
270kHz
U
LTC1735CS16
TOP VIEW
C
OSC
1
16 TG
15 BOOST
14 SW
13 V
IN
12 INTV
CC
11 BG
10 PGND
9
EXTV
CC
+
+
C
IN3
22µF
30V
RUN/SS 2
I
TH
3
FCB 4
SGND 5
V
OSENSE
6
SENSE
–
7
SENSE
+
8
V
OUT
1.6V/9A
E7
V
OSNS
E11
S PACKAGE
16-LEAD PLASTIC SO
+
CO3
820µF
4V
+
CO2
100µF
2V
GND
E8
DEMO MANUAL DC247
DESIGN-READY SWITCHER
PARTS LIST
REFERENCE
DESIGNATOR
CF1, C
SS1
C
IN1
, C
IN2
C
IN3
, C
IN4
CO1, CO2, CO4
CO1, C02, C04
CO3
CO2
C
OSC1
C
S1
C4
D1
D2
E2, E3, E4, E7–E10
JP1
JP2
L1
M1, M2
Q1
R1
R2, R5, R
S1
R3, R7
R4
R6
R8
R
C1
R
CS1
RF1
U1
QUANTITY
2
2
ALTERNATE
ALTERNATE
ALTERNATE
1
1
1
1
1
1
1
7
1
1
1
2
OPTIONAL
OPTIONAL
3
2
OPTIONAL
1
1
1
1
1
1
2
CR16-684JM
CR16-000M
CR16-333FM
LRF2010-01-R005J
CR16-470JM
LTC1735CS16
CCIJ2mm-138-G
PART NUMBER
08055A104KAT1A
30SC22M
THCR70E1H2262T
TS10X4470JAT1A
EEFUE0G181R
4SP820M
EEFCD0D101R
08055A470JAT1A
08055A102MAT1A
0805ZC105MAT1A
CMDSH-3
MBRS340T3
1502-2
2802S-03-G2
2802S-02-G2
ETQP6F2ROHFA
FDS6680A
VN2222
CR16-103JM
CR16-100FM
W0805-03-1002B
DESCRIPTION
0.1µF 50V 20% X7R Capacitor
22µF 30V OS-CON Capacitor
22µF 50V 20% Y5U Capacitor
470µF 6.3V Low ESR Capacitor
180µF 4V SP Capacitor
820µF 4V OS-CON Capacitor
100µF 2V SP Capacitor
47pF 50V 5% NPO Capacitor
1000pF 50V 5% NPO Capacitor
1µF 10V 20% X7R Capacitor
0.1A BVR = 30V Schottky Diode
3A BVR = 40V Schottky Diode
Turret Terminal
2mm Pin Header
2mm Pin Header
2µH Inductor
0.013Ω 30V N-Channel MOSFET
10Ω 20V N-Channel MOSFET
10k 1/10W 5% Chip Resistor
10Ω 1/10W 1% Chip Resistor
10k 1/10W 0.1% Chip Resistor
USER DEF 1/10W 1% Chip Resistor
680k 1/10W 5% Chip Resistor
0Ω 1/10W Chip Resistor
33k 1/10W 5% Chip Resistor
0.005Ω 1W 5% Resistor
4.7Ω 1/10W 5% Chip Resistor
IC, Switching Regulator Controller
Jumper
TAD
TAD
TAD
IRC
TAD
LTC
Comm Con
(800) 508-1521
(800) 508-1521
(800) 508-1521
(512) 992-7900
(800) 508-1521
(408) 432-1900
(626) 301-4200
TAD
TAD
IRC
(800) 508-1521
(800) 508-1521
(361) 992-7900
VENDOR
AVX
SANYO
Marcon
Kemet
Panasonic
SANYO
Panasonic
AVX
AVX
AVX
Central
Motorola
Keystone
Comm Con
Comm Con
Panasonic
Fairchild
TELEPHONE
(843) 946-0362
(619) 661-6835
(847) 696-2000
(408) 986-0424
(201) 348-7522
(619) 661-6835
(201) 348-7522
(843) 946-0362
(843) 946-0362
(843) 946-0362
(516) 435-1110
(800) 441-2447
(718) 956-8900
(626) 301-4200
(626) 301-4200
(201) 348-7522
(408) 822-2126
QUICK START GUIDE
This demonstration board is easily set up to evaluate the
performance of the LTC1735. Please follow the proce-
dure outlined below for proper operation. Soldered wire
connections are required to properly ascertain the per-
formance of this switching regulator.
• Refer to Figure 2 for proper connection of monitoring
and measurement equipment.
• Connect the input power supply to the V
IN
and GND
terminals on the right hand side of the board with
soldered connections. Do not increase V
IN
over 28V or
the MOSFET(s) WILL BE DAMAGED.
• Connect the load between the V
OUT
and GND terminals
on the right side of the board with soldered connec-
tions.
• The RUN pin can be left unconnected. To shut down
the LTC1735, tie this pin to ground.
• If an external 5V supply is used, connect it to EXTV
CC
.
3
DEMO MANUAL DC247
DESIGN-READY SWITCHER
QUICK START GUIDE
• Set the jumper JP1 so that FCB selects the desired
mode:
JP1
On
Off
Open
MODE
Burst Mode Operation Enabled
Forced Continuous, Burst Disabled
Apply External Clock to FCB/Sync Pin
enabled. With JP2 installed, this function is disabled.
Remove JP2 to enable.
JP2
Installed
Removed
OVERCURRENT LATCHOFF
Disabled
Enabled
• Jumper JP2 determines if the overcurrent latchoff is
RUN
JP2
FCB/SYNC
• Active loads can cause confusing results. Refer to the
active load discussion in the operation section.
A
+V
IN
CPU POWER CONVERTER
+
–
I
IN
+
–
GND
EXT CLOCK
(REMOVE JP1 IF USED)
EXTV
CC
OFF
ON
JP1
Burst
Mode
GND
V
OSNS
+V
OUT
V
OUT
V
SEL
–
LOAD
+
I
OUT
GND
LTC1735CS
DEMO CIRCUIT DC247A
(408) 432-1900
A
OPTIONAL REMOTE
V
OUT
SENSE CONNECTION
247 F02
Figure 2. Proper Measurement Setup
I TRODUCTIO
The circuit in Figure 1 highlights the capabilities of the
LTC1735.
The LTC1735 is a synchronous step-down switching
regulator controller that 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 80% over
decades of load current range. A maximum high duty cycle
OPERATIO
The operating frequency is set by an external capacitor,
C
OSC1
, allowing maximum flexibility in optimizing effi-
ciency. In this application, the frequency is set to 270kHz.
A multifunction control pin, FCB, inhibits Burst Mode
operation, reducing noise and RF interference and allows
synchronization to an external oscillator.
4
U
limit of 99% provides low dropout operation, which ex-
tends operating time in battery-operated systems.
Do not use spring-clip leads when testing this circuit.
Soldered wire connections are required to properly ascer-
tain the performance of the PC board.
This demonstration circuit is intended for the evaluation of
the LTC1735 switching regulator IC and was not designed
for any other purpose.
Soft-start is provided by an external capacitor, C
SS1
,
which can be used to properly sequence supplies. The
operating current level is user-programmable via an exter-
nal current sense resistor, R
CS1
, and is set to approxi-
mately 10A. Short-circuit current is limited to approxi-
mately 3A by internal current foldback.
U
U
DEMO MANUAL DC247
DESIGN-READY SWITCHER
OPERATIO
Main Control Loop
The LTC1735 uses a constant frequency, current mode
step-down architecture. Current mode operation was
judged to be mandatory for its well known advantages of
clean start-up, accurate current limit and excellent line and
load regulation in wide input voltage range applications.
During normal operation, the top MOSFET is turned on
during 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 the EA to receive an output
feedback voltage, V
FB
, from an external resistive divider.
When the load current increases, it causes a slight de-
crease in V
FB
relative to the 0.8V reference, which, in turn,
causes the I
TH
voltage to increase until the average induc-
tor 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 begin-
ning of the next cycle.
The top MOSFET driver is biased from floating bootstrap
capacitor C
B1
, which is normally recharged during each off
cycle. However, when V
IN
decreases to a voltage close to
V
OUT
, the regulator 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 C
B1
to recharge.
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 LTC1735’s control loop
looks for the information to keep the output voltage
constant. In this demonstration board, it is located be-
tween Pin 5 of the LTC1735, the signal ground, and the
sense side of R
S1
. This point corresponds to the V
OSNS
terminal of the board. Output voltage test leads should be
attached directly to this terminal. The load should be
placed across +V
OUT
(E7) and GND (E8). Measurements
should not
be taken at the end of test leads at the load; refer
U
to Figure 2 for the proper monitoring equipment configu-
ration.
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.
REMOTE OUTPUT VOLTAGE SENSING
Remote output voltage sensing can be accomplished by
connecting the V
OSNS
terminal with another wire directly
to the load. A 10Ω resistor, R
S1
, connects +V
OUT
to V
OSNS
to avoid open sense conditions. Never under any circum-
stance connect the load to V
OSNS
!
INTV
CC
Regulator
An internal P-channel, low dropout regulator produces the
5.2V supply that powers the drivers and internal circuitry
within the LTC1735. The INTV
CC
pin can supply up to
50mA (this includes the gate-drive currents). External
loading of the INTV
CC
pin can be thermally limited (allow
10mA to 20mA for gate-drive currents). At high input
voltages, the maximum junction temperature rating for
the LTC1735 may be exceeded if too large an external load
is placed on INTV
CC
. See the LTC1735 data sheet for
further details.
EXTV
CC
CONNECTION
The LTC1735 contains an internal P-channel MOSFET
switch connected between the EXTV
CC
and INTV
CC
pins.
The switch closes and supplies the INTV
CC
power when-
ever the EXTV
CC
pin is above 4.7V; it remains closed until
EXTV
CC
drops below 4.5V. This allows the MOSFET driver
and control power to be derived from the EXTV
CC
pin
instead of V
IN
. Do not apply greater than 7V to the EXTV
CC
pin and ensure that EXTV
CC
< V
IN
.
Significant efficiency gains can be realized by powering
INTV
CC
from the output, since the V
IN
current resulting
from the driver and control currents will be scaled by a
factor of (Duty Cycle)/(Efficiency). For 5V output regula-
tors, this simply means connecting the EXTV
CC
pin di-
rectly to V
OUT
. However, for 1.6V and other lower voltage
regulators, additional circuitry is required to derive INTV
CC
power from the output.
5
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