LINEAR TECHNOLOGY LTC3809 Technical data

LTC3809

FEATURES

■

No Current Sense Resistor Required

■

Selectable Spread Spectrum Frequency Modulation
for Low Noise Operation

■

Constant Frequency Current Mode Operation for
Excellent Line and Load Transient Response

■

True PLL for Frequency Locking or Adjustment
(Frequency Range: 250kHz to 750kHz)

■

Wide VIN Range: 2.75V to 9.8V

■

Wide V

■

0.6V ±1.5% Reference

■

Low Dropout Operation: 100% Duty Cycle

■

Selectable Burst Mode®/Pulse Skipping/Forced

Range: 0.6V to V

OUT

IN

Continuous Operation

■

Auxiliary Winding Regulation

■

Internal Soft-Start Circuitry

■

Power Good Output Voltage Monitor

■

Output Overvoltage Protection

■

Micropower Shutdown: IQ = 9µA

■

Tiny Thermally Enhanced Leadless (3mm × 3mm)
DFN and 10-lead MSOP Packages

U

APPLICATIO S

■

One or Two Lithium-Ion Powered Devices

■

Portable Instruments

■

Distributed DC Power Systems

No R

SENSE

, Low EMI,

Synchronous DC/DC Controller

U

DESCRIPTIO

The LTC®3809 is a synchronous step-down switching
regulator controller that drives external complementary
power MOSFETs using few external components. The
constant frequency current mode architecture with MOSFET

sensing eliminates the need for a current sense

V

DS

resistor and improves efficiency.

For noise sensitive applications, the LTC3809 can be ex­ternally synchronized from 250kHz to 750kHz. Burst Mode
is inhibited during synchronization or when the SYNC/
MODE pin is pulled low to reduce noise and RF interference.
To further reduce EMI, the LTC3809 incorporates a novel
spread spectrum frequency modulation technique.

Burst Mode operation provides high efficiency operation at
light loads. 100% duty cycle provides low dropout opera­tion, extending operating time in battery-powered systems.

The switching frequency can be programmed up to 750kHz,
allowing the use of small surface mount inductors and
capacitors.

The LTC3809 is available in the tiny footprint thermally
enhanced DFN and 10-lead MSOP packages.

, LTC and LT are registered trademarks of Linear Technology Corporation.

Burst Mode is a registered trademark of Linear Technology Corporation.
No R

is a trademark of Linear Technology Corporation.

SENSE

All other trademarks are the property of their respective owners.
Protected by U.S. Patents including 5481178, 5929620, 6580258, 6304066, 5847554,
6611131, 6498466. Other Patents pending.

TM

TYPICAL APPLICATIO

High Efficiency, 550kHz Step-Down Converter

LTC3809

PLLLPF

15k

SYNC/MODE

V

FB

I

TH

RUN

GND

IPRG

187k

59k

470pF

V

SW

U

Efficiency and Power Loss vs Load Current

100

V

IN

2.75V TO 9.8V

10µF

IN

TG

BG

2.2µH

47µF

V

2.5V
2A

3809 TA01

OUT

EFFICIENCY (%)

VIN = 3.3V

90

80

70

60

50

1 100 1000 10000

EFFICIENCY

VIN = 5V

10

LOAD CURRENT (mA)

VIN = 4.2V

POWER LOSS
V

IN

FIGURE 10 CIRCUIT
V

= 4.2V

OUT

= 2.5V

10k

1k

POWER LOSS (mW)

100

10

1

0.1

3809 TA01b

3809fa

1

LTC3809

WW

W

ABSOLUTE MAXIMUM RATINGS

U

(Note 1)

Input Supply Voltage (VIN)........................ –0.3V to 10V

PLLLPF, RUN, SYNC/MODE,

IPRG Voltages .............................. –0.3V to (V

V

, ITH Voltages...................................... –0.3V to 2.4V

FB

+ 0.3V)

IN

SW Voltage ......................... – 2V to VIN + 1V (10V Max)

TG, BG Peak Output Current (<10µs)........................ 1A

U

W

U

PACKAGE/ORDER INFORMATION

TOP VIEW

PLLLPF

1

SYNC/MODE

2

11

3

V

FB

4

I

TH

5

RUN

10-LEAD (3mm × 3mm) PLASTIC DFN

DD PACKAGE

T

= 125°C, θJA = 43°C/W

JMAX

EXPOSED PAD (PIN 11) IS GND
(MUST BE SOLDERED TO PCB)

10

SW

V

9

IN

TG

8

7

BG

6

IPRG

ORDER PART

NUMBER

LTC3809EDD

DD PART

MARKING

LBQY

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

Storage Temperature Range ................. –65°C to 125°C

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

Lead Temperature (Soldering, 10 sec)

MSOP Package ................................................. 300°C

ORDER PART

NUMBER

LTC3809EMSE

MSE PART

MARKING

LTBQT

PLLLPF

SYNC/MODE

V

I

RUN

TOP VIEW

1
2
3

FB
TH

10-LEAD PLASTIC MSOP

T

JMAX

EXPOSED PAD (PIN 11) IS GND
(MUST BE SOLDERED TO PCB)

11
4
5

MSE PACKAGE

= 125°C, θJA = 40°C/W

10
9
8
7
6

SW
V

IN

TG
BG
IPRG

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

ELECTRICAL CHARACTERISTICS

The ● indicates specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. VIN = 4.2V unless otherwise noted.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Main Control Loops

Input DC Supply Current (Note 4)
Normal Operation 350 500 µA
Sleep Mode 105 150 µA
Shutdown RUN = 0V 9 20 µA
UVLO V

Undervoltage Lockout Threshold (UVLO) VIN Falling ● 1.95 2.25 2.55 V

Shutdown Threshold of RUN Pin 0.8 1.1 1.4 V

Regulated Feedback Voltage (Note 5) ● 0.591 0.6 0.609 V

Output Voltage Line Regulation 2.75V < VIN < 9.8V (Note 5) 0.01 0.04 %/V

Output Voltage Load Regulation ITH = 0.9V (Note 5) 0.1 0.5 %

VFB Input Current (Note 5) 9 50 nA

Overvoltage Protect Threshold Measured at V

= UVLO Threshold – 200mV 3 10 µA

IN

V

Rising ● 2.15 2.45 2.75 V

IN

I

= 1.7V –0.1 –0.5 %

TH

FB

0.66 0.68 0.7 V

2

3809fa

LTC3809

ELECTRICAL CHARACTERISTICS

The ● indicates specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C. VIN = 4.2V unless otherwise noted.

PARAMETER CONDITIONS MIN TYP MAX UNITS

Overvoltage Protect Hysteresis 20 mV

Auxiliary Feedback Threshold 0.325 0.4 0.475 V

Top Gate (TG) Drive Rise Time CL = 3000pF 40 ns

Top Gate (TG) Drive Fall Time CL = 3000pF 40 ns

Bottom Gate (BG) Drive Rise Time CL = 3000pF 50 ns

Bottom Gate (BG) Drive Fall Time CL = 3000pF 40 ns
Maximum Current Sense Voltage (∆V

+

(SENSE

– SW) IPRG = 0V (Note 6) ● 70 85 100 mV

Soft-Start Time (Internal) Time for VFB to Ramp from 0.05V to 0.55V 0.5 0.74 0.9 ms

Oscillator and Phase-Locked Loop

Oscillator Frequency Unsynchronized (SYNC/MODE Not Clocked)

Phase-Locked Loop Lock Range SYNC/MODE Clocked

Phase Detector Output Current
Sinking f
Sourcing f

Spread Spectrum Frequency Range Minimum Switching Frequency 460 kHz

SYNC/MODE Pull-Down Current SYNC/MODE = 2.2V 2.6 µA

SENSE(MAX)

) IPRG = Floating (Note 6) ● 110 125 140 mV

IPRG = V

PLLLPF = Floating 480 550 600 kHz
PLLLPF = 0V 260 300 340 kHz
PLLLPF = V

Minimum Synchronizable Frequency 200 250 kHz
Maximum Synchronizible Frequency 750 1000 kHz

OSC
OSC

Maximum Switching Frequency 635 kHz

(Note 6) ● 185 204 223 mV

IN

650 750 825 kHz

–3 µA

3 µA

> f

SYNC/MODE

< f

SYNC/MODE

IN

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LTC3809E is guaranteed to meet specified performance from
0°C to 70°C. Specifications over the –40°C to 85°C operating range are
assured by design characterization, and correlation with statistical process
controls.

Note 3: T
dissipation P

is calculated from the ambient temperature TA and power

J

according to the following formula:

D

T

= TA + (PD • θJA °C/W)

J

Note 4: Dynamic supply current is higher due to gate charge being
delivered at the switching frequency.

Note 5: The LTC3809 is tested in a feedback loop that servos I
specified voltage and measures the resultant V

Note 6: Peak current sense voltage is reduced dependent on duty cycle to
a percentage of value as shown in Figure 1.

voltage.

FB

to a

TH

3809fa

3

LTC3809

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Efficiency vs Load Current Efficiency vs Load Current

100

FIGURE 10 CIRCUIT

95

V

= 3.3V

OUT

90

85

80

V

= 1.8V

75

EFFICIENCY (%)

70

65

60

1 100 1k 10k

OUT

10

LOAD CURRENT (mA)

V

= 2.5V

OUT

V

= 1.2V

OUT

SYNC/MODE = V

VIN = 5V

IN

3809 G01

100

FIGURE 10 CIRCUIT

95

= 5V, V

V

IN

90

85

BURST MODE
(SYNC/MODE =

80

V

75

70

EFFICIENCY (%)

65

60

55

50

1 100 1k 10k10

= 2.5V

OUT

)

IN

LOAD CURRENT (mA)

FORCED
CONTINUOUS
(SYNC/MODE = 0V)

PULSE SKIPPING
(SYNC/MODE = 0.6V)

T

= 25°C unless otherwise noted.

A

Maximum Current Sense Voltage
vs ITH Pin Voltage

3809 G02

100

80

60

40

20

CURRENT LIMIT (%)

0

–20

Burst Mode OPERATION
(I

TH

Burst Mode OPERATION
(I

TH

FORCED CONTINUOUS
MODE
PULSE SKIPPING
MODE

0.5

RISING)

FALLING)

1 1.5

ITH VOLTAGE (V)

2

3809 G03

V

OUT

200mV/DIV

AC COUPLED

2A/DIV

V

OUT

200mV/DIV

AC COUPLED

2A/DIV

Load Step
(Burst Mode Operation)

I

L

V

= 3.3V

IN

= 1.8V

V

OUT

= 300mA TO 3A

I

LOAD

SYNC/MODE = V
FIGURE 10 CIRCUIT

100µs/DIV

IN

Load Step (Pulse Skipping Mode)

I

L

3809 G04

V

OUT

200mV/DIV

AC COUPLED

2A/DIV

Load Step
(Forced Continuous Mode)

I

L

V

= 3.3V

IN

= 1.8V

V

OUT

= 300mA TO 3A

I

LOAD

SYNC/MODE = 0V
FIGURE 10 CIRCUIT

100µs/DIV

Start-Up with Internal Soft-Start

3809 G05

V

OUT

1.8V

500mV/DIV

4

V

= 3.3V

IN

= 1.8V

V

OUT

= 300mA TO 3A

I

LOAD

SYNC/MODE = V
FIGURE 10 CIRCUIT

FB

100µs/DIV

3809 G06

= 4.2V

V

IN

= 1Ω

R

LOAD

FIGURE 10 CIRCUIT

200µs/DIV

3809 G07

3809fa

UW

TEMPERATURE (°C)

–60

–10

–8

–6

NORMALIZED FREQUENCY (%)

–4

–2

0

4

–20 20

60

100

3809 G13

8

2

6

10

–40 0

40

80

INPUT VOLTAGE (V)

2

70

SLEEP CURRENT (µA)

80

100

46

8

10

3809 G16

120

90

110

130

35

7

9

TYPICAL PERFOR A CE CHARACTERISTICS

LTC3809

TA = 25°C unless otherwise noted.

Regulated Feedback Voltage
vs Temperature

0.606

0.604

0.602

0.600

0.598

FEEDBACK VOLTAGE (V)

0.596

0.594
–60

–20 20

–40 0

TEMPERATURE (°C)

60

40

Maximum Current Sense
Threshold vs Temperature

135

IPRG = FLOAT

130

125

120

MAXIMUM CURRENT SENSE THRESHOLD (mV)

115

–60

–40 0

–20 20

TEMPERATURE (°C)

40

60

80

80

3809 G11

3809 G08

100

100

Undervoltage Lockout Threshold
vs Temperature

2.55

2.50

2.45

2.40

2.35

2.30

INPUT VOLTAGE (V)

2.25

2.20

2.15
–60

–40 0

VIN RISING

VIN FALLING

–20 20

TEMPERATURE (°C)

SYNC/MODE Pull-Down Current
vs Temperature

2.80

2.75

2.70

2.65

2.60

2.55

2.50

2.45

SYNC/MODE PULL-DOWN CURRENT (µA)

2.40
–60

–40 0

–20 20

TEMPERATURE (°C)

Shutdown (RUN) Threshold
vs Temperature

1.20

1.15

1.10

RUN VOLTAGE (V)

1.05

1.00
–60

–40 0

40

60

80

100

3809 G09

–20 20

TEMPERATURE (°C)

40

60

80

100

3809 G10

Oscillator Frequency
vs Temperature

40

60

80

100

3809 G12

Oscillator Frequency
vs Input Voltage

5

4

3

2

1

0

–1

–2

–3

NORMALIZED FREQUENCY SHIFT (%)

–4

–5

2

35

46

INPUT VOLTAGE (V)

7

Shutdown Quiescent Current
vs Input Voltage

18

16

14

12

10

8

6

4

SHUTDOWN CURRENT (µA)

2

8

9

10

3809 G14

0

2

35

46

INPUT VOLTAGE (V)

7

8

9

10

3809 G15

Sleep Current vs Input Voltage

3809fa

5

LTC3809

U

UU

PI FU CTIO S

PLLLPF (Pin 1): Frequency Set/PLL Lowpass Filter. When
synchronizing to an external clock, this pin serves as the
low pass filter point for the phase-locked loop. Normally,
a series RC is connected between this pin and ground.

When not synchronizing to an external clock, this pin serves
as the frequency select input. Tying this pin to GND selects
300kHz operation; tying this pin to VIN selects 750kHz
operation. Floating this pin selects 550kHz operation.

Connect a 2.2nF capacitor between this pin and GND, and
a 1000pF capacitor between this pin and the SYNC/MODE
when using spread spectrum modulation operation.

SYNC/MODE (Pin 2): This pin performs four functions: 1)
auxiliary winding feedback input, 2) external clock syn­chronization input for phase-locked loop, 3) Burst Mode,
pulse skipping or forced continuous mode select, and 4)
enable spread spectrum modulation operation in pulse
skipping mode. Applying a clock with frequency between
250kHz to 750kHz causes the internal oscillator to phase­lock to the external clock and disables Burst Mode opera­tion but allows pulse skipping at low load currents.

To select Burst Mode operation at light loads, tie this pin
to VIN. Grounding this pin selects forced continuous
operation, which allows the inductor current to reverse.
Tying this pin to VFB selects pulse skipping mode. In these
cases, the frequency of the internal oscillator is set by the
voltage on the PLLLPF pin. Tying to a voltage between

1.35V to VIN – 0.5V enables spread spectrum modulation
operation. In this case, an internal 2.6µA pull-down cur-
rent source helps to set the voltage at this pin by tying a
resistor with appropriate value between this pin and VIN.

Do not leave this pin floating.

RUN (Pin 5): Run Control Input. Forcing this pin below

1.1V shuts down the chip. Driving this pin to VIN or
releasing this pin enables the chip to start-up with the
internal soft-start.

IPRG (Pin 6): Three-State Pin to Select Maximum Peak
Sense Voltage Threshold. This pin selects the maximum
allowed voltage drop between the VIN and SW pins (i.e.,
the maximum allowed drop across the external P-channel
MOSFET). Tie to V
or 125mV respectively.

BG (Pin 7): Bottom (NMOS) Gate Drive Output. This pin
drives the gate of the external N-channel MOSFET. This pin
has an output swing from PGND to V

TG (Pin 8): Top (PMOS) Gate Drive Output. This pin drives
the gate of the external P-channel MOSFET. This pin has an
output swing from PGND to VIN.

VIN (Pin 9): Chip Signal Power Supply. This pin powers the
entire chip, the gate drivers and serves as the positive
input to the differential current comparator.

SW (Pin 10): Switch Node Connection to Inductor. This
pin is also the negative input to the differential current
comparator and an input to the reverse current compara­tor. Normally this pin is connected to the drain of the
external P-channel MOSFET, the drain of the external
N-channel MOSFET and the inductor.

GND (Pin 11): Exposed Pad. The Exposed Pad is ground
and must be soldered to the PCB ground for electrical
contact and optimum thermal performance.

, GND or float to select 204mV, 85mV

IN

.

IN

VFB (Pin 3): Feedback Pin. This pin receives the remotely
sensed feedback voltage for the controller from an exter­nal resistor divider across the output.

ITH (Pin 4): Current Threshold and Error Amplifier Com­pensation Point. Nominal operating range on this pin is
from 0.7V to 2V. The voltage on this pin determines the
threshold of the main current comparator.

6

3809fa

LTC3809

U

U

W

FU CTIO AL DIAGRA

V

IN

5

11

2

0.4V

2.6µA

1

RUN

GND

SYNC/MODE

PLLLPF

C

IN

9

V

VOLTAGE

REFERENCE

UNDERVOLTAGE

LOCKOUT

V

IN

0.7µA

t = 1ms
INTERNAL
SOFT-START

BURST DEFEAT

CLOCK DETECT

IN

V

REF

0.6V

SENSE

V

IN

UVSD

SS

BURSTDIS
FCB

PHASE

DETECTOR

V

CO

CLK

SLOPE

+

V

IN

0.3V

+

–

ICMP

0.15V

IPRG

6

CLK

S

Q

R

+

–

SLEEP

BURSTDIS

SWITCHING
LOGIC AND

BLANKING

CIRCUIT

FCB

OV

I

REV

+

UV

–

GND

ANTI-SHOOT-

THROUGH

PV

IN

+

–

+

–

+
+

EAMP

–

+

I

REV

RICMP

–

V

REF

0.6V
SS

SW

GND

8

10

7

0.68V

0.54V

V

FB

4

3

TG

SW

BG

I

V

MP

L

C

OUT

MN

TH

R

C

C

C

FB

3809 FD

V

OUT

R

B

R

A

3809fa

7

LTC3809

OPERATIO

U

(Refer to Functional Diagram)

Main Control Loop

The LTC3809 uses a constant frequency, current mode
architecture. During normal operation, the top external
P-channel power MOSFET is turned on when the clock
sets the RS latch, and is turned off when the current
comparator (ICMP) resets the latch. The peak inductor
current at which ICMP resets the RS latch is determined by
the voltage on the ITH pin, which is driven by the output of
the error amplifier (EAMP). The V
voltage feedback signal from an external resistor divider.
This feedback signal is compared to the internal 0.6V
reference voltage by the EAMP. When the load current
increases, it causes a slight decrease in V

0.6V reference, which in turn causes the ITH voltage to
increase until the average inductor current matches the
new load current. While the top P-channel MOSFET is off,
the bottom N-channel MOSFET is turned on until either the
inductor current starts to reverse, as indicated by the
current reversal comparator IRCMP, or the beginning of
the next cycle.

Shutdown and Soft-Start (RUN Pin)

The LTC3809 is shut down by pulling the RUN pin low. In
shutdown, all controller functions are disabled and the
chip draws only 9µA. The TG output is held high (off) and
the BG output low (off) in shutdown. Releasing the RUN
pin allows an internal 0.7µA current source to pull up the
RUN pin to VIN. The controller is enabled when the RUN pin
reaches 1.1V.

The start-up of V
internal soft-start. During soft-start, the error amplifier
EAMP compares the feedback signal VFB to the internal
soft-start ramp (instead of the 0.6V reference), which
rises linearly from 0V to 0.6V in about 1ms. This allows
the output voltage to rise smoothly from 0V to its final
value while maintaining control of the inductor current.

is controlled by the LTC3809’s

OUT

pin receives the output

FB

relative to the

FB

Light Load Operation (Burst Mode Operation,
Continuous Conduction or Pulse Skipping Mode)
(SYNC/MODE Pin)

The LTC3809 can be programmed for either high effi­ciency Burst Mode operation, forced continuous conduc­tion mode or pulse skipping mode at low load currents. To
select Burst Mode operation, tie the SYNC/MODE pin to
VIN. To select forced continuous operation, tie the SYNC/
MODE pin to a DC voltage below 0.4V (e.g., GND). Tying
the SYNC/MODE to a DC voltage above 0.4V and below

1.2V (e.g., VFB) enables pulse skipping mode. The 0.4V
threshold between forced continuous operation and pulse
skipping mode can be used in secondary winding regula­tion as described in the Auxiliary Winding Control Using
SYNC/MODE Pin discussion in the Applications Informa­tion section.

When the LTC3809 is in Burst Mode operation, the peak
current in the inductor is set to approximately one-fourth
of the maximum sense voltage even though the voltage on
the ITH pin indicates a lower value. If the average inductor
current is higher than the load current, the EAMP will
decrease the voltage on the ITH pin. When the ITH voltage
drops below 0.85V, the internal SLEEP signal goes high
and the external MOSFET is turned off.

In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3809 draws.
The load current is supplied by the output capacitor. As the
output voltage decreases, the EAMP increases the I
voltage. When the ITH voltage reaches 0.925V, the SLEEP
signal goes low and the controller resumes normal opera­tion by turning on the external P-channel MOSFET on the
next cycle of the internal oscillator.

When the controller is enabled for Burst Mode or pulse
skipping operation, the inductor current is not allowed to
reverse. Hence, the controller operates discontinuously.

TH

8

3809fa