LINEAR TECHNOLOGY LTC3704 Technical data

LTC3704

Positive-to-Negative DC/DC Controller

FEATURES

■

High Efficiency Operation (No Sense
Resistor Required)

■

Wide Input Voltage Range: 2.5V to 36V

■

Current Mode Control Provides Excellent Transient
Response

■

High Maximum Duty Cycle (Typ 92%)

■

±1% Internal Voltage Reference

■

±2% RUN Pin Threshold with 100mV Hysteresis

■

Micropower Shutdown: IQ = 10µA

■

Programmable Switching Frequency
(50kHz to 1MHz) with One External Resistor

■

Synchronizable to an External Clock Up to 1.3 × f

■

User-Controlled Pulse Skip or Burst Mode® Operation

■

Internal 5.2V Low Dropout Voltage Regulator

■

Capable of Operating with a Sense Resistor for High
Output Voltage Applications (V

■

Small 10-Lead MSOP Package

>36V)

DS

U

APPLICATIO S

■

SLIC Power Supplies

■

Telecom Power Supplies

■

Portable Electronic Equipment

■

Cable and DSL Modems

■

Router Supplies

OSC

Wide Input Range, No R

SENSE

U

DESCRIPTIO

The LTC®3704 is a wide input range, current mode,
positive-to-negative DC/DC controller that drives an
N-channel power MOSFET and requires very few external
components. Intended for low to high power applications,
it eliminates the need for a current sense resistor by
utilizing the power MOSFET’s on-resistance, thereby maxi­mizing efficiency.

The IC’s operating frequency can be set with an external
resistor over a 50kHz to 1MHz range, and can be synchro­nized to an external clock using the MODE/SYNC pin.
Burst Mode operation at light loads, a low minimum
operating supply voltage of 2.5V and a low shutdown
quiescent current of 10µA make the LTC3704 ideally
suited for battery-operated systems.

For applications requiring constant frequency operation,
the Burst Mode operation feature can be defeated using
the MODE/SYNC pin. Higher than 36V switch voltage
applications are possible with the LTC3704 by connecting
the SENSE pin to a resistor in the source of the power
MOSFET.

The LTC3704 is available in the 10-lead MSOP package.

TM

, LTC, LT and LTM are registered trademarks of Linear Technology Corporation. Burst
Mode is a registered trademark of Linear Technology Corporation. No R
registered trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents including 5847554, 5731694.

SENSE

is a

TYPICAL APPLICATIO

R1

1M

R

C

3k

C

C1

4.7nF

C

, C

: TDK C5750X5R1C476M

IN

DC

: TDK C5750X5R0J107M

C

OUT

: TAIYO YUDEN LMK316BJ475ML

C

VCC

Figure 1. High Efficiency Positive to Negative Supply

R

T

80.6k
1%

1.21k

R

FB1

1%

RUN

I

TH

NFB

FREQ

MODE/SYNC

LTC3704

R

FB2

3.65k
1%

SENSE

INTV

GATE

GND

U

L1*

V

IN

CC

C

C

VCC

47µF

4.7µF

D1: MBRD835L (ON SEMICONDUCTOR)
L1, L2: BH ELECTRONICS BH510-1009
M1: Si4884 (SILICONIX/VISHAY)

V

IN

3704 TA01

5V to 15V

V

OUT

–5.0V
3A to 5A

C

OUT

100µF
(X2)

GND

Conversion Efficiency

100

90

VIN = 5V

80

70

60

50

EFFICIENCY (%)

40

30

20

0.001

VIN = 15V

VIN = 10V

0.01 1.0
OUTPUT CURRENT (A)

0.1

10

3704 TA01b

3704fa

•

•

L2*

C

DC

47µF

M1

D1

IN

1

LTC3704

1
2
3
4
5

RUN

I

TH

NFB

FREQ

MODE/

SYNC

10
9
8
7
6

SENSE
V

IN

INTV

CC

GATE
GND

TOP VIEW

MS PACKAGE

10-LEAD PLASTIC MSOP

WWWU

ABSOLUTE AXI U RATI GS

PACKAGE/ORDER I FOR ATIO

UU

W

(Note 1)

VIN Voltage ............................................... –0.3V to 36V

INTV
INTV

GATE Voltage ........................... – 0.3V to V

I

NFB Voltage .............................................. –2.7V to 2.7V

RUN, MODE/SYNC Voltages ....................... –0.3V to 7V

FREQ Voltage ............................................– 0.3V to 1.5V

SENSE Pin Voltage ................................... – 0.3V to 36V

Voltage ........................................... –0.3V to 7V

CC

Output Current ........................................ 50mA

CC

+ 0.3V

INTVCC

Voltage ............................................... – 0.3V to 2.7V

TH

ORDER PART

NUMBER

T

= 125°C, θJA = 120°C/ W

JMAX

MS PART MARKING

Operating Temperature Range (Note 2) .. – 40°C to 85°C

LTC3704E ............................................ –40°C to 85°C

LTC3704I........................................... –40°C to 125°C

Junction Temperature (Note 3)............................ 125°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

LTC3704EMS
LTC3704IMS

Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF

Lead Free Part Marking: http://www.linear.com/leadfree/

Consult LTC Marketing for parts specified with wider operating temperature ranges.

LTYT
LTCFW

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.
V

= V

IN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Main Control Loop

V

IN(MIN)

I

Q

+

V

RUN

–

V

RUN

V

RUN(HYST)

I

RUN

V

NFB

I

NFB

∆V

NFB

∆V

IN

∆V

NFB

∆V

ITH

g

m

V

ITH(BURST)

V

SENSE(MAX)

I

SENSE(ON)

I

SENSE(OFF)

2

= 5V, V

INTVCC

Minimum Input Voltage 2.5 V

Input Voltage Supply Current (Note 4)
Continuous Mode V
Burst Mode Operation, No Load V
Shutdown Mode V

Rising RUN Input Threshold Voltage 1.348 V

Falling RUN Input Threshold Voltage 1.223 1.248 1.273 V

RUN Pin Input Threshold Hysteresis 50 100 150 mV

RUN Input Current 1 100 nA

Negative Feedback Voltage V

NFB Pin Input Current V
Line Regulation 2.5V ≤ VIN ≤ 30V 0.002 0.02 %/V

Load Regulation V

Error Amplifier Transconductance ITH Pin Load = ±5µA (Note 5) 650 µmho

Burst Mode Operation ITH Pin Voltage Falling ITH Voltage (Note 5) 0.3 V

Maximum Current Sense Input Threshold Duty Cycle < 20% 120 150 180 mV

SENSE Pin Current (GATE High) V

SENSE Pin Current (GATE Low) V

= 1.5V, RT = 80k, V

RUN

MODE/SYNC

= 0V, unless otherwise specified.

MODE/SYNC
MODE/SYNC
RUN

ITH

V

ITH

LTC3704I (Note 2)

ITH

MODE/SYNC

SENSE

SENSE

= 5V, V
= 0V, V

= 0V 10 20 µA

= 0.2V (Note 5) –1.218 –1.230 –1.242 V
= 0.2V (Note 5)

= 0.2V (Note 5) 7.5 15 µA

= 0V, VTH = 0.5V to 0.90V (Note 5)

= 0V 40 75 µA

= 30V 0.1 5 µA

= 0.75V 550 1000 µA

ITH

= 0.2V (Note 5) 250 500 µA

ITH

●

1.198 1.298 V

●

–1.212 –1.248 V

●

–1.205 –1.255 V

●

–1 –0.1 %

3704fa

LTC3704

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C.

= V

V

IN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

Oscillator

f

OSC

D

MAX

f

SYNC/fOSC

t

SYNC(MIN)

t

SYNC(MAX)

V

IL(MODE)

V

IH(MODE)

R

MODE/SYNC

V

FREQ

Low Dropout Regulator

V

INTVCC

∆V

INTVCC

∆V

IN1

∆V

INTVCC

∆V

IN2

V

LDO(LOAD)

V

DROPOUT

I

INTVCC

GATE Driver

t

r

t

f

Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliabilty and lifetime.

Note 2: The LTC3704E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LTC3704I is guaranteed over the full
–40°C to 125°C operating temperature range.

Note 3: T
dissipation P

T

J

= 5V, V

INTVCC

Oscillator Frequency R

= 1.5V, R

RUN

FREQ

= 80k, V

MODE/SYNC

= 0V, unless otherwise specified.

= 80k 250 300 350 kHz

FREQ

Oscillator Frequency Range 50 1000 kHz
Maximum Duty Cycle 87 92 97 %
Recommended Maximum Synchronized f

= 300kHz (Note 6) 1.25 1.30

OSC

Frequency Ratio
MODE/SYNC Minimum Input Pulse Width V

MODE/SYNC Maximum Input Pulse Width V

= 0V to 5V 25 ns

SYNC

= 0V to 5V 0.8/f

SYNC

OSC

Low Level MODE/SYNC Input Voltage 0.3 V
High Level MODE/SYNC Input Voltage 1.2 V
MODE/SYNC Input Pull-Down Resistance 50 kΩ
Nominal FREQ Pin Voltage 0.62 V

INTVCC Regulator Output Voltage VIN = 7.5V 5.0 5.2 5.4 V

INTVCC Regulator Line Regulation 7.5V ≤ VIN ≤ 15V 8 25 mV

INTVCC Regulator Line Regulation 15V ≤ VIN ≤ 30V 70 200 mV

INTVCC Load Regulation 0 ≤ I

≤ 20mA –2 – 0.2 %

INTVCC

INTVCC Regulator Dropout Voltage VIN = 5V, INTVCC Load = 20mA 280 mV

Bootstrap Mode INTVCC Supply RUN = 0V, SENSE = 5V 10 20 µA
Current in Shutdown

GATE Driver Output Rise Time CL = 3300pF (Note 7) 17 100 ns
GATE Driver Output Fall Time CL = 3300pF (Note 7) 8 100 ns

Note 4: The dynamic input supply current is higher due to power MOSFET

• f

gate charging (Q

). See Applications Information.

G

OSC

Note 5: The LTC3704 is tested in a feedback loop that servos V
reference voltage with the I

pin forced to a voltage between 0V and 1.4V

TH

(the no load to full load operating voltage range for the I

1.23V).
Note 6: In a synchronized application, the internal slope compensation

gain is increased by 25%. Synchronizing to a significantly higher ratio will
reduce the effective amount of slope compensation, which could result in

is calculated from the ambient temperature TA and power

J

according to the following formula:

D

subharmonic oscillation for duty cycles greater than 50%.
Note 7: Rise and fall times are measured at 10% and 90% levels.

= TA + (PD • 120°C/W)

NFB

pin is 0.3V to

TH

to the

ns

3704fa

3

LTC3704

UW

TYPICAL PERFOR A CE CHARACTERISTICS

NFB Voltage vs Temp NFB Voltage Line Regulation NFB Pin Current vs Temperature

–1.25

–1.24

–1.23

NFB VOLTAGE (V)

–1.22

–1.21

–50

–25

0

50

25

TEMPERATURE (°C)

–1.231

–1.230

NFB VOLTAGE (V)

–1.229

0

75

100

125

150

3704 G01

5101520

VIN (V)

25 30 35

3704 G02

8.0

7.9

7.8

7.7

7.6

7.5

7.4

NFB CURRENT (µA)

7.3

7.2

7.1

7.0
–50

–25

250 50 10075

TEMPERATURE (°C)

125 150

3704 G03

Shutdown Mode IQ vs V

30

(µA)

Q

20

10

SHUTDOWN MODE I

0

0

10 20

VIN (V)

Burst Mode IQ vs Temperature

500

400

(µA)

300

Q

200

Burst Mode I

100

0

–50

–25 25

0

50

TEMPERATURE (°C)

IN

30

40

3704 G04

Shutdown Mode IQ vs Temperature

20

VIN = 5V

15

(µA)

Q

10

5

SHUTDOWN MODE I

0

–50

–25 0 25 50

TEMPERATURE (°C)

75 100 125 150

3704 G05

Burst Mode IQ vs V

600

500

400

(µA)

Q

300

200

Burst Mode I

100

0

0

10 20

IN

30 40

VIN (V)

3704 G06

Gate Drive Rise and Fall Time

Dynamic IQ vs Frequency

18

CL = 3300pF

= 550µA + Qg • f

I

16

Q(TOT)

14

12

10

(mA)

Q

8

I

6

4

2

125

100

75

150

3704 G07

0

0

400 1200

200 1000

FREQUENCY (kHz)

600

800

3704 G08

vs C

60

50

40

30

TIME (ns)

20

10

0

0

L

4000 6000 8000

2000

RISE TIME

FALL TIME

10000 12000

CL (pF)

3704 G09

4

3704fa

INTVCC LOAD (mA)

0

0

DROPOUT VOLTAGE (mV)

50

150

200

250

500

350

5

10

3704 G18

100

400

450

300

15

20

150°C

75°C

125°C

25°C

–50°C

0°C

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LTC3704

RUN Thresholds vs V

1.5

1.4

1.3

RUN THRESHOLDS (V)

1.2
0

10 20

VIN (V)

Frequency vs Temperature

325

320

315

310

305

300

295

290

GATE FREQUENCY (kHz)

285

280

275

–50

–25 25

0

TEMPERATURE (°C)

INTV

Load Regulation

CC

TA = 25°C

IN

30

40

3704 G10

RUN Thresholds vs Temperature

1.40

1.35

1.30

RUN THRESHOLDS (V)

1.25

1.20
–50

–25

0

50

25

TEMPERATURE (°C)

75

100

125

150

3704 G11

RT vs Frequency

1000

100

(kΩ)

T

R

10

100

0

200 1000

400

500

300

FREQUENCY (kHz)

800700600

900

3704 G12

Maximum Sense Threshold
vs Temperature

160

155

150

145

MAX SENSE THRESHOLD (mV)

140

–50

125

50

100

75

150

3704 G13

–25 0 25 50

INTV

5.4
TA = 25°C

TEMPERATURE (°C)

Line Regulation

CC

75 100 125 150

3704 G14

SENSE Pin Current vs Temperature

45

GATE HIGH

= 0V

V

SENSE

40

SENSE PIN CURRENT (µA)

35

–50

–25 25

INTV

0

Dropout Voltage

CC

50

TEMPERATURE (°C)

100

75

vs Current, Temperature

125

150

3704 G15

5.2

VOLTAGE (V)

CC

5.1

INTV

5.0
0

10 20

40

30 50 80

INTVCC LOAD (mA)

60 70

3704 G16

5.3

VOLTAGE (V)

CC

5.2

INTV

5.1
0

515

10 20

VIN (V)

25

30

35

40

3704 G17

3704fa

5

LTC3704

U

UU

PI FU CTIO S

RUN (Pin 1): The RUN pin provides the user with an
accurate means for sensing the input voltage and pro­gramming the start-up threshold for the converter. The
falling RUN pin threshold is nominally 1.248V and the
comparator has 100mV of hysteresis for noise immunity.
When the RUN pin is below this input threshold, the IC is
shut down and the VIN supply current is kept to a low
value (typ 10µA). The Absolute Maximum Rating for the
voltage on this pin is 7V.

ITH (Pin 2): Error Amplifier Compensation Pin. The cur­rent comparator input threshold increases with this
control voltage. Nominal voltage range for this pin is 0V
to 1.40V.

NFB (Pin 3): Receives the feedback voltage from the
external resistor divider across the output. Nominal
voltage for this pin in regulation is –1.230V.

FREQ (Pin 4): A resistor from the FREQ pin to ground
programs the operating frequency of the chip. The nomi­nal voltage at the FREQ pin is 0.62V.

MODE/SYNC (Pin 5): This input controls the operating
mode of the converter and allows for synchronizing the

operating frequency to an external clock. If the MODE/
SYNC pin is connected to ground, Burst Mode operation
is enabled. If the MODE/SYNC pin is connected to INTV
or if an external logic-level synchronization signal is
applied to this input, Burst Mode operation is disabled
and the IC operates in a continuous mode.

GND (Pin 6): Ground Pin.

GATE (Pin 7): Gate Driver Output.

I

NTVCC (Pin 8): The Internal 5.20V Regulator Output. The

gate driver and control circuits are powered from this
voltage. Decouple this pin locally to the IC ground with a
minimum of 4.7µF low ESR tantalum or ceramic
capacitor.

V

(Pin 9): Main Supply Pin. Must be closely decoupled

IN

to ground.

SENSE (Pin 10): The Current Sense Input for the Control
Loop. Connect this pin to the drain of the power MOSFET
for V
SENSE pin may be connected to a resistor in the source
of the power MOSFET. Internal leading edge blanking is
provided for both sensing methods.

sensing and highest efficiency. Alternatively, the

DS

CC

,

6

3704fa

BLOCK DIAGRA

FREQ

4

MODE/SYNC

5

NFB

200k

3

1.230V

I

TH

2

INTV

CC

8

2.00V

W

0.62V

5.2V

–

BUFFER

+

–

+

–

+

200k

g

m

EA

LDO

UV

I

OSC

0.30V

1.230V

TO
START-UP
CONTROL

SLOPE

COMPENSATION

OSCV-TO-I

50k

+

–

BURST

COMPARATOR

V-TO-I

SLOPE

BIAS V

I

LOOP

1.230V

REF

BIAS AND

START-UP

CONTROL

S

Q

R

PWM LATCH

100mV
HYSTERESIS
(1.348V RISING)

LOGIC

C1

CURRENT

COMPARATOR

LTC3704

RUN

1

+

C2

1.248V

–

V

IN

9

INTV

CC

GATE

7

GND

SENSE

+

–

R

10

LOOP

GND

6

3704 BD

V

IN

3704fa

7

LTC3704

OPERATIO

U

Main Control Loop

The LTC3704 is a constant frequency, current mode
controller for DC/DC positive-to-negative converter appli­cations. The LTC3704 is distinguished from conventional
current mode controllers because the current control loop
can be closed by sensing the voltage drop across the
power MOSFET switch instead of across a discrete sense
resistor, as shown in Figure 2. This sensing technique
improves efficiency, increases power density, and re­duces the cost of the overall solution.

V

IN

V

IN

SENSE

GATE

GND

GND

2a. SENSE Pin Connection for
Maximum Efficiency (V

V

IN

V

IN

GATE

SENSE

GND

GND

2b. SENSE Pin Connection for Precise
Control of Peak I

Figure 2. Using the SENSE Pin On the LTC3704

IN/IOUT

V

SW

< 36V)

SW

V

SW

R

SENSE

3704 F02

or for VSW > 36V

For circuit operation, please refer to the Block Diagram of
the IC and Figure 1. In normal operation, the power
MOSFET is turned on when the oscillator sets the PWM
latch and is turned off when the current comparator C1
resets the latch. The divided-down output voltage is com­pared to an internal 1.230V reference by the error amplifier
EA, which outputs an error signal at the ITH pin. The voltage
on the ITH pin sets the current comparator C1 input
threshold. When the load current increases, a fall in the
NFB voltage relative to the reference voltage causes the I

TH

pin to rise, which causes the current comparator C1 to trip
at a higher peak inductor current value. The average
inductor current will therefore rise until it equals the load
current, thereby maintaining output regulation.

The nominal operating frequency of the LTC3704 is pro­grammed using a resistor from the FREQ pin to ground
and can be controlled over a 50kHz to 1000kHz range. In
addition, the internal oscillator can be synchronized to an
external clock applied to the MODE/SYNC pin and can be
locked to a frequency between 100% and 130% of its
nominal value. When the MODE/SYNC pin is left open, it is
pulled low by an internal 50k resistor and Burst Mode
operation is enabled. If this pin is taken above 2V or an
external clock is applied, Burst Mode operation is disabled
and the IC operates in continuous mode. With no load (or
an extremely light load), the controller will skip pulses in
order to maintain regulation and prevent excessive output
ripple.

The RUN pin controls whether the IC is enabled or is in a
low current shutdown state. A micropower 1.248V refer­ence and comparator C2 allow the user to program the
supply voltage at which the IC turns on and off (compara­tor C2 has 100mV of hysteresis for noise immunity). With
the RUN pin below 1.248V, the chip is off and the input
supply current is typically only 10µA.

The LTC3704 can be used either by sensing the voltage
drop across the power MOSFET or by connecting the
SENSE pin to a conventional shunt resistor in the source
of the power MOSFET, as shown in Figure 2. Sensing the
voltage across the power MOSFET maximizes converter
efficiency and minimizes the component count, but limits
the output voltage to the maximum rating for this pin
(36V). By connecting the SENSE pin to a resistor in the
source of the power MOSFET, the user is able to program
output voltages significantly greater than the 36V maxi­mum input voltage rating for the IC.

Programming the Operating Mode

For applications where maximizing the efficiency at very
light loads (e.g., <100µA) is a high priority, Burst Mode
operation should be applied (i.e., the MODE/SYNC pin
should be connected to ground). In applications where
fixed frequency operation is more critical than low cur­rent efficiency, or where the lowest output ripple is
desired, pulse-skip mode operation should be used and
the MODE/SYNC pin should be connected to the INTV

CC

pin. This allows discontinuous conduction mode (DCM)
operation down to near the limit defined by the chip’s

3704fa

8

OPERATIO

LTC3704

U

minimum on-time (about 175ns). Below this output
current level, the converter will begin to skip cycles in
order to maintain output regulation. Figures 3 and 4 show
the light load switching waveforms for Burst Mode and
Pulse-Skip Mode operation for the converter in Figure 1.

Burst Mode Operation

Burst Mode operation is selected by leaving the MODE/
SYNC pin unconnected or by connecting it to ground. In
normal operation, the range on the ITH pin corresponding
to no load to full load is 0.30V to 1.2V. In Burst Mode
operation, if the error amplifier EA drives the I

voltage

TH

below 0.525V, the buffered ITH input to the current com­parator C1 will be clamped at 0.525V (which corresponds
to 25% of maximum load current). The inductor current
peak is then held at approximately 30mV divided by the
power MOSFET R

. If the ITH pin drops below 0.30V,

DS(ON)

the Burst Mode comparator B1 will turn off the power
MOSFET and scale back the quiescent current of the IC to
250µA (sleep mode). In this condition, the load current will
be supplied by the output capacitor until the I

voltage

TH

rises above the 50mV hysteresis of the burst comparator.
At light loads, short bursts of switching (where the aver­age inductor current is 25% of its maximum value) fol­lowed by long periods of sleep will be observed, thereby
greatly improving converter efficiency. Oscilloscope wave­forms illustrating Burst Mode operation are shown in
Figure 3.

MODE/SYNC = 0V
(Burst Mode OPERATION)

V

OUT

50mV/DIV

I

L

5A/DIV

buffered I

burst clamp is removed, allowing the ITH pin

TH

to directly control the current comparator from no load to
full load. With no load, the I

pin is driven below 0.30V,

TH

the power MOSFET is turned off and sleep mode is
invoked. Oscilloscope waveforms illustrating this mode of
operation are shown in Figure 4.

MODE/SYNC = INTV
(PULSE-SKIP MODE)

V

OUT

50mV/DIV

I

L

5A/DIV

2µs/DIV

Figure 4. LTC3704 Low Output Current Operation with Burst
Mode Operation Disabled (MODE/SYNC = INTV

CC

3704 F04

CC

)

When an external clock signal drives the MODE/SYNC pin
at a rate faster than the chip’s internal oscillator, the
oscillator will synchronize to it. In this synchronized mode,
Burst Mode operation is disabled. The constant frequency
associated with synchronized operation provides a more
controlled noise spectrum from the converter, at the
expense of overall system efficiency of light loads.

When the oscillator’s internal logic circuitry detects a
synchronizing signal on the MODE/SYNC pin, the internal
oscillator ramp is terminated early and the slope compen­sation is increased by approximately 30%. As a result, in
applications requiring synchronization, it is recommended
that the nominal operating frequency of the IC be pro­grammed to be about 75% of the external clock frequency.
Attempting to synchronize to too high an external fre­quency (above 1.3fO) can result in inadequate slope com­pensation and possible subharmonic oscillation (or jitter).

10µs/DIV 3704 F03

Figure 3. LTC3704 Burst Mode Operation
(MODE/SYNC = 0V) at Low Output Current

Pulse-Skip Mode Operation

With the MODE/SYNC pin tied to a DC voltage above 2V,
Burst Mode operation is disabled. The internal, 0.525V

The external clock signal must exceed 2V for at least 25ns,
and should have a maximum duty cycle of 80%, as shown
in Figure 5. The MOSFET turn on will synchronize to the
rising edge of the external clock signal.

3704fa

9