Linear Technology DC257, LTC1707 Demo Manual

DESCRIPTIO

DEMO MANUAL DC257

NO-DESIGN SWITCHER

LTC1707 Monolithic

Synchronous Step-Down Regulator

U

Demonstration circuit DC257 is a constant-frequency
step-down converter using an LTC®1707 monolithic syn­chronous 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. De­signed to work at low voltages, the input voltage (VIN) can
range from 2.85V to 8.5V. At VIN < 2.7V, the LTC1707
shuts down and draws just a few microamperes, making

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

TM

to Burst Mode

operation to reduce switching losses and
maintain high operating efficiencies. In switching-noise
sensitive applications, Burst Mode operation can be inhib­ited 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.

UWWW

PERFOR A CE SU ARY

SYMBOL PARAMETER CONDITIONS JUMPER POSITION VALUE

V

IN

V

OUT

I

Q

Input Voltage Range 2.85V to 8.5V
Output Voltage See Figure 2 JP1 = “1.5V” 1.51V ±0.036V

Pulse Skipping Mode Supply Current V
Burst Mode Enabled Supply Current V
Shutdown Current V

= 5V, SYNC/MODE = 0V, RUN/SS = 2V, I

IN

= 5V, SYNC/MODE = 2V, RUN/SS = 2V, I

IN

= 5V, RUN/SS = 0V, I

IN

= 0mA JP1 = “3.3V” 15µA

OUT

= 0mA JP1 = “3.3V” 300µA

OUT

= 0mA JP1 = “3.3V” 200µA

OUT

JP1 = “1.8V” 1.82V ± 0.043V
JP1 = “2.5V” 2.52V ± 0.06V
JP1 = “2.9V” 2.94V ± 0.07V
JP1 = “3.3V” 3.33V ± 0.079V

JP1 = “OPEN” Note 1

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TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO

LTC1707 Efficiency Curve

100

V

= 3.3V

OUT

95

90

85

EFFICIENCY (%)

80

75

70

1

OUTPUT CURRENT (mA)

VIN = 3.6V

= 6V

V

IN

VIN = 8.4V

10 100 1000

U

Demo Board

DC257 TA01

1

DEMO MANUAL DC257

NO-DESIGN SWITCHER

W

A

U
CE SUPERFOR

WW
ARY

SYMBOL PARAMETER CONDITIONS JUMPER POSITION VALUE

I

OUT

f

OSC

Minimum Output Current VIN = 5V, V

= 5V, V

V

IN

V

= 3V, V

IN

= 2.5V 600mA

OUT

= 3.3V 500mA

OUT

= 2.5V 300mA

OUT

Operating Frequency Unsynchronized All 350kHz

Synchronized All 385kHz to 550kHz

V

RIPPLE

V

OUT

V

SYNC

V

RUN/SS

V

REF

I

REF

Note 1: Programmable via optional R6. V

Typical Output Ripple I
Typical Load Regulation 0mA < I

= 500mA, VIN = 5V, V

OUT

< 0.5A, V

OUT

= 3.3V 30mV

OUT

= 5V, V

IN

= 3.3V 0.5%

OUT

Synchronize Threshold Voltage All 1.2V
Minimum Shutdown Threshold Voltage All 0.4V
Reference Output Voltage I
Maximum V

Output Currrent All 100µA

REF

= 0.8V(1 + 210k/R6)

OUT

= 0 All 1.19V

REF

WUW

PACKAGE A D SCHE ATIC DIAGRA S

TOP VIEW

I

C

C2

47pF

OPTIONAL

R

C

C

SS

0.1µF

C

C1

E3

RUN/SS

1

2

3

4

I

I

TH

TH

RUN/SS

V

FB

GND

LTC1707

V

REF

SYNC/
MODE

V

SW

1000pF

1

TH

RUN/SS

2

V

3

FB

GND

4

S8 PACKAGE

8-LEAD PLASTIC SO

LTC1707CS8

8

7

6

IN

5

C

REF

E1
V

E8
SYNC/MODE

L1 15µH

+

REF

C
22µF
16V

R7

210k

+

C

IN

OUT

100µF

6.3V

R1
237k

1.5V

R2
165k

1.8V

2.5V

R3

97.6k

R8
10Ω

R4

78.7k

JP2

2.9V

R5

66.5k

3.3V OPEN

DC257 F01

8

V
SYNC/MODE

7

V

6

SW

5

R6*
OPTIONAL

REF

IN

E2
V

IN

E7
V

OUT

E6
V

OSENSE

E4/E5
GND

≤ 8.5V

P-P

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

DEMO MANUAL DC257

NO-DESIGN SWITCHER

PARTS LIST

REFERENCE
DESIGNATOR QUANTITY PART NUMBER DESCRIPTION VENDOR TELEPHONE

C

IN

C

OUT

C

SS

C

REF

C

C1

C

C2

E1 to E8 8 2501-2 Turret, Testpoint Mill-Max [phone]
JP1 1 22025-12-G1 0.079" Double Row Header COMM CON [phone]
JP1 1 CCIJ2MM-138G 0.079" Center Shunt COMM CON [phone]
L1 1 CD54-150MC 15µH 20% Inductor Sumida [phone]
R1 1 CR16-2373FM 237k 1% 1/16W Chip Resistor TAD [phone]
R2 1 CR16-1653FM 165k 1% 1/16W Chip Resistor TAD [phone]
R3 1 CR16-9762FM 97.6k 1% 1/16W Chip Resistor TAD [phone]
R4 1 CR16-7872FM 78.7k 1% 1/16W Chip Resistor TAD [phone]
R5 1 CR16-6652FM 66.5k 1% 1/16W Chip Resistor TAD [phone]
R7 1 CR16-2103FM 210k 1% 1/16W Chip Resistor TAD [phone]
R8 1 CR16-100JM 10Ω 5% 1/16W Chip Resistor TAD [phone]
R6 Optional
R

C

U1 1 LTC1707CS8 Monolithic Synchronous Step-Down Regulator LTC [phone]

1 TPSC226M016R0375 22µF 16V TPS Tantalum Capacitor AVX [phone]
1 TPSC107M006R0150 100µF 6V TPS Tantalum Capacitor AVX [phone]
1 0603YC104KAT 0.1µF 16V X7R Chip Capacitor AVX [phone]
1 06035A102JAT 1000pF 50V NPO Chip Capacitor AVX [phone]

Optional

1 06035A470JAT 47pF 50V NPO Chip Capacitor AVX [phone]

Optional

3

DEMO MANUAL DC257

1.8V1.5V 2.5V 2.9V 3.3V OPEN

JP1

DC257 F02

NO-DESIGN SWITCHER

QUICK START GUIDE

This demonstration board is easily set up to evaluate the
performance of the LTC1707 IC. Please follow the proce­dure 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 VIN and GND
terminals on the left-hand side of the board. Do not
increase VIN over 10V or the part will be damaged.

• Connect the load between the V

and GND

OUT

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.

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OPERATIO

Table 1. Output Voltage Selection

JP1 POSITION OUTPUT VOLTAGE

“1.5V” 1.51V
“1.8V” 1.82V
“2.5V” 2.52V

“2.9V 2.94V

“3.3V” 3.33V

“OPEN”

Figure 2. Output Voltage Selection (JP1)
(3.3V Position Shown)

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.

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 synchro­nized with an external clock. Burst Mode operation is
automatically disabled when the SYNC/MODE pin is ex­ternally driven. Grounding SYNC/MODE also disables
Burst Mode operation, potentially reducing noise and RF
interference.

Soft-start is provided by an external capacitor, CSS, 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.

4

OPERATIO

DEMO MANUAL DC257

NO-DESIGN SWITCHER

U

SYNC/MODE

7

V

FB

3

V

REF

8

SHUTDOWN

V

IN

0.6V

V

IN

1.19V
REF

UVLO

TRIP = 2.7V

1.5µA

–

+

BURST

DEFEAT

Y = “0” ONLY WHEN X IS A CONSTANT “1”

Y

X

SLOPE

OSC

FREQ

SHIFT

0.8V

2.25µA

V

IN

RUN/SS

2

0.86V

COMP

+

–

+

–

EA

RUN/SOFT

START

OVDET

V

IN

–

EN

+

0.12V

I

1

TH

–

+

BURST

SLEEP

QRS

SWITCHING

Q

LOGIC

AND

BLANKING

CIRCUIT

V

0.4V

IN

–

+

I

COMP

ANTI-

SHOOT-THRU

+

I

RCMP

–

6Ω

5

4

DC257 F03

6

SW

GND

V

IN

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
the RS latch. The peak inductor current at which I
resets the RS latch is controlled by the voltage on the I

COMP

, resets

COMP

TH

pin, which is the output of error amplifier EA. The VFB pin
allows EA to receive an output feedback voltage from an
external resistive divider. When the load current increases,

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

, or the next cycle

RCMP

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 CSS. When
CSS reaches 0.7V, the main control loop is enabled with
the ITH voltage clamped at approximately 5% of its

5

DEMO MANUAL DC257

NO-DESIGN SWITCHER

U

OPERATIO

maxi

mum value. As CSS continues to charge, ITH is gradu-

ally released, allowing normal operation to resume.
Comparator OVDET guards against transient overshoots

> 7.5% by turning the main switch off and keeping it off
until the fault is removed.

SYNC/MODE Pin Function (Frequency Synchronization
and Burst Mode Disable)

The LTC1707 can be synchronized with an external
TTL/CMOS-compatible clock signal driving the SYNC/
MODE pin (E8). The frequency range of this signal must be
from 385kHz to 550kHz. DO NOT attempt to synchronize
the LTC1707 below 385kHz as this may cause abnormal
operation and an undesired frequency spectrum. The top
MOSFET turn-on follows the rising edge of the external
source.

When the LTC1707 is clocked by an external source, Burst
Mode operation is disabled; the LTC1707 then operates in
PWM pulse-skipping mode. In this mode, when the output
load is very low, current comparator I
for more than one cycle and forces the main switch to stay
off for the same number of cycles. Increasing the output
load slightly allows constant-frequency PWM operation to
resume.

remains tripped

COMP

When the converter uses Burst Mode operation, the peak
current of the inductor is set to approximately 200mA,
even though the voltage at the ITH pin indicates a lower
value. The voltage at the I
average current is greater than the load requirement. As
the ITH voltage drops below 0.12V, the BURST comparator
trips, causing the internal sleep line to go high and turn off
both power MOSFETs.

In sleep mode, both power MOSFETs are held off and the
internal circuitry is partially turned off, reducing the quies­cent current to 200µA. The load current is now supplied
from the output capacitor. When the output voltage drops,
causing ITH to rise above 0.22V, the top MOSFET is again
turned on and this process repeats.

SHORT-CIRCUIT PROTECTION

When the output is shorted to ground, the frequency of the
oscillator is reduced to about 35kHz, 1/10 of the nominal
frequency. This frequency foldback ensures that the
inductor current has more time to decay, thereby prevent­ing runaway. The oscillator's frequency will gradually
increase to 350kHz (or the synchronized frequency) when
VFB rises above 0.3V.

DROPOUT OPERATION

pin drops when the inductor’s

TH

Frequency synchronization is inhibited when the feedback
voltage, VFB, is below 0.6V. This prevents the external
clock from interfering with the frequency foldback for
short-circuit protection.

The LTC1707 is capable of Burst Mode operation, in which
the internal power MOSFETs operate intermittently based
on load demand. To enable Burst Mode operation, simply
allow the SYNC/MODE pin to float or connect it to a logic
high. To disable Burst Mode operation and enable PWM
pulse-skipping mode, connect the SYNC/MODE pin to
GND.

When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maxi­mum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle until it reaches 100% duty cycle. The output voltage
will then be determined by the input voltage minus the
voltage drop across the P-channel MOSFET and the inductor.
In Burst Mode operation or pulse skipping mode opera­tion with the outputs lightly loaded, the LTC1707 passes
through continuous mode as it enters dropout.

6

OPERATIO

DEMO MANUAL DC257

NO-DESIGN SWITCHER

U

UNDERVOLTAGE LOCKOUT

A precision undervoltage lockout shuts down the LTC1707
when VIN drops below 2.7V, making it ideal for single
lithium-ion battery applications. In shutdown, the LTC1707
draws only several microamperes, which is low enough to
prevent deep discharge and possible damage to a lithium­ion battery nearing its end of charge. A 150mV hysteresis
ensures reliable operation with noisy supplies.

LOW SUPPLY OPERATION

The LTC1707 is designed to operate from supply voltages as
low as 2.85V. At this voltage, the converter is most likely to
be running at high duty cycles or in dropout, where the main
switch is on continuously. Hence, the I2R loss is due mainly

to the R

of the P-channel MOSFET. See the LTC1707

DS(ON)

data sheet for additional information.

SLOPE COMPENSATION AND
PEAK INDUCTOR CURRENT

Slope compensation provides stability by preventing sub­harmonic oscillations. It works by internally adding a ramp
to the inductor current signal at duty cycles in excess of
40%. As a result, the maximum inductor peak current is

lower for V

OUT/VIN

> 0.4 than when V

OUT/VIN

< 0.4. See the
maximum inductor peak current vs duty cycle graph in
Figure 4.

The graph labeled “With External Clock” shows the worst­case peak current reduction obtained when the oscillator
is synchronized at its minimum frequency, that is, to a
clock just above the oscillator’s free-running frequency.

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 LTC1707’s control loop
looks for the information to keep the output voltage
constant. In this demonstration board, this information
point occurs between Pin 4, the GND of the LTC1707, and
the output side of R7. These points correspond to the GND
(E5) and V

OSENSE

(E6) terminals of the board. Output
voltage test leads should be attached directly to these
terminals. The load should be placed between V
and GND (E5). Measurements

should not

be taken at the

OUT

(E7)

end of test leads at the load. Refer to Figure 5 for the proper
monitoring equipment configuration.

Figure 4. Maximum Inductor Peak Current Vs Duty Cycle

950

900

850

800

750

700

650

600

550

MAXIMUM INDUCTOR PEAK CURRENT (mA)

500

0

WITH

EXTERNAL

CLOCK

20

DUTY CYCLE (%)

WITHOUT

EXTERNAL

CLOCK

40

60

80

100

DC257 F04

7

DEMO MANUAL DC257

NO-DESIGN SWITCHER

U

OPERATIO

MONOLITHIC SYNCHRONOUS

STEP-DOWN REGULATOR

V

REF

I

IN

+

A

+

V

IN

+

V

E1 E8

V

IN

E2

RUN/SS

E3

GND

E4

1.5V

LTC1707CS8

JP1

1.8V

2.5V

2.9V

3.3V

OPEN

DEMO CIRCUIT

Linear Technology

[phone]

DC257

SYNC/
MODE

E7

E6

V

OSENSE

V

OUT

E5

GND

DC257 F05

10Ω

I

OUT

+

A

+

V

LOAD

V

OUT

Figure 5. Proper Measurement Setup

CUT THIS

TRACE

MONOLITHIC SYNCHRONOUS

STEP-DOWN REGULATOR

V

REF

I

IN

+

A

+

V

IN

+

V

E1 E8

V

IN

E2

RUN/SS

E3

GND

E4

1.5V

1.8V

LTC1707CS8

JP1

2.5V

2.9V

3.3V

OPEN

DEMO CIRCUIT

Linear Technology

[phone]

DC257

SYNC/
MODE

V

E7

E6

V

OSENSE

OUT

E5

GND

DC257 F06

10Ω

I

OUT

+

A

+

V

LOAD

V

OUT

This applies to line regulation (input-to-output voltage
regulation) as well as load regulation tests. In doing the
line regulation tests, always look at the input voltage
across the input terminals.

For the purposes of these tests, the demonstration circuit
should be powered from a regulated DC bench supply, so
that variations on the DC input do not add errors to the
regulation measurements.

8

Figure 6. Remote Output Voltage Sense

REMOTE OUTPUT-VOLTAGE SENSING

Remote output-voltage sensing can be accomplished by
modifying the PC board. A small PC trace connecting V
to V

OSENSE

must be cut, as shown in Figure 6. An external

connection from V
made. To prevent uncertainty, there is a 10Ω resistor
placed across the V
under any circumstance, allow V

OSENSE

and V

OUT

OUT

directly to the load must be

OSENSE

terminals. Never,

OSENSE

to float!

OPERATIO

DEMO MANUAL DC257

NO-DESIGN SWITCHER

U

RUN/SOFT-START FUNCTION

is a delay before starting of approximately 310ms/µF,
followed by an additional 490ms/µF to reach full current.

The RUN/SS pin (E3) is a dual-purpose pin that provides
the soft-start function and a means to shut down the
LTC1707. Soft-start reduces surge currents from VIN by
gradually increasing the internal current limit. Power sup­ply sequencing can also be accomplished using this pin.

C

= 0.1µF on this PC board.

SS

Pulling the RUN/SS pin below 0.7V (0.4V min) puts the
LTC1707 into a low quiescent current shutdown mode
(IQ < 15µA). See the LTC1707 data sheet for further
information.

An internal 2.25µA current source charges an external
capacitor, CSS. When the voltage on RUN/SS reaches

0.7V, the LTC1707 begins operating. As the voltage on
RUN/SS continues to increase from 0.7V to 1.8V, the
internal current limit also increases at a proportional linear
rate. The current limit begins at 25mA (at V
and ends at the Figure 4 value (V

RUN/SS

RUN/SS

≥ 1.8V). The output

≤␣ 0.7V)

current thus increases slowly, charging the output capaci­tor. If RUN/SS has been pulled all the way to ground, there

Table 2. List of Alternative Component Manufacturers

MANUFACTURER DEVICE PHONE FAX

AVX Capacitors [phone] [phone]
Central Semiconductor Diodes [phone] [phone]
Coilcraft Inductors [phone] [phone]
Coiltronics Inductors [phone] [phone]
COMM CON Connectors [phone] [phone]
Dale Inductors [phone] [phone]
International Rectifier Diodes [phone] [phone]
Motorola Diodes [phone] [phone]
Murata-Erie Capacitors [phone] [phone]
Sanyo Capacitors [phone] [phone]

Sprague Capacitors [phone] [phone]
Sumida Inductors [phone] [phone]

TDK Inductors [phone] [phone]
TAD Resistors [phone] [phone]
Zetex Diodes [phone] [phone]

COMPONENT MANUFACTURERS

Table 2 is a partial list of manufacturers of components
that can be used in LTC1707 applications. Using compo­nents other than the ones supplied on the demonstration
board will require careful analysis to verify that all com­ponent specifications are not exceeded. Finally,
recharacterizing the circuit for efficiency is necessary.

(81) 0952-82-3959 (81) 0952-82-4655

(81) 03-3607-5111 (81) 03-3607-5114

9

DEMO MANUAL DC257

NO-DESIGN SWITCHER

W

PCB LAYOUT AUD FIL

Component Side Silkscreen

Component Side

Component Side Solder Mask

10

PCB LAYOUT AUD FIL

DEMO MANUAL DC257

NO-DESIGN SWITCHER

W

Solder Side

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Solder Side Solder Mask

Pastemask

11

DEMO MANUAL DC257

NO-DESIGN SWITCHER

U

PC FAB DRAWI

G

2.000

C

DD

A

A

B

2.000

C

NOTES: UNLESS OTHERWISE SPECIFIED

1. MATERIAL: FR4 OR EQUIVALENT EPOXY,

2 OZ COPPER CLAD, THICKNESS 0.062 ±0.006
TOTAL OF 2 LAYERS

2. FINISH: ALL PLATED HOLES 0.001 MIN/0.0015 MAX
COPPER PLATE, ELECTRODEPOSITED TIN-LEAD COMPOSITION
BEFORE REFLOW, SOLDER MASK OVER BARE COPPER (SMOBC)

3. SOLDER MASK: BOTH SIDES USING SR1020 OR EQUIVALENT

4. SILKSCREEN: USING WHITE NONCONDUCTIVE EPOXY INK

5. ALL DIMENSIONS IN INCHES

6. SCORING

0.017

SYMBOL

A
B
C
D

DIAMETER

0.020

0.040

0.072

0.095

TOTAL HOLES

NUMBER

OF HOLES

6

12

2
8

28

PLATED

PLTD
PLTD

NPLTD

PLTD

Linear Technology Corporation

12

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com

dc257f LT/TP 1099 500 • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1999

Mouser Electronics

Authorized Distributor

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