LINEAR TECHNOLOGY LTC3808 Technical data

Low EMI Synchronous DC/DC Step-Down Controllers Offer Programmable Output Tracking – Design Note 382

Charlie Zhao

Introduction

The LTC®3808 syn chronous DC/DC step- down controller p a c k s n u m e r o u s f e a t u r e s r e q u ir e d b y t h e l a t e s t e l e c t r o n i c devices into a low proﬁ le (0.75mm) 3mm × 4mm leadless DFN package or a leaded SSOP-16 package. Two similar parts, the LTC3809 and LTC3809-1, are even smaller, but less feature-rich versions of the LTC3808. The LTC3809 family is available in a 3mm × 3mm leadless DFN package or a 10-pin MSOP Exposed Pad package. All three parts can provide output voltages as low as 0.6V and output currents as high as 7A from a 2.75V to 9.8V input range, making them ideal devices for one or two lithium-ion cell inputs as well as distributed DC power systems.

The LTC3808 and LTC3809 also include important features for noise-sensitive applications, including a phase-locked loop (PLL) for frequency synchronization and spread spectrum frequency modulation to minimize generated electromagnetic interference (EMI). The adjustable oper ating frequency (300kH z to 750kHz) allows the use of small surface mount inductors and ceramic capacitors for compact power supply solutions.

Other features include:

• Low operating quiescent current to improve battery life and light load efﬁ ciency

™

• No R

SENSE

current mode technology which senses the voltage across the main (top) power MOSFET to improve efﬁ ciency and reduce the size and cost of the solution

• Current mode control for excellent AC and DC line and load regulation

• Low dropout (100% duty cycle) for maximum energy extraction from a battery source

• Output overvoltage protection and short circuit current limit protection

• Adjustable or ﬁ xed built-in soft-start timer

• Output voltage ramp control and the ability to track other voltage sources (LTC3808 and LTC3809-1)

• PowerGood voltage monitor (LTC3808)

Table 1 compares the features of these three parts.

Table 1.

START-UP CONTROL

Internal

LTC3808

External Tracking

LTC3809 Internal Yes Yes No

Internal

LTC3809-1

External Tracking

SPREAD

SPECTRUM

ADJUSTABLE

FREQ/PLL

Yes Yes Yes

No No No

POWER

GOOD

Three Choices for Start-Up Control

The start-up of V

for the LTC3808 and LTC3809-1 is

OUT

based on the three different connections to the TRACK/ SS pin. A typical application is shown in Figure 2. When TRACK/SS is connected to V

, the start-up of V

OUT

is controlled by the internal soft-start which ramps from 0V to (V

) in about 1ms. A second start up mode

allow s the 1ms sof t- st ar t t ime to i ncre as e or de crea se b y

, LTC and LT are registered trademarks and No R Linear Technology Corporation. All other trademarks are the property of their respective owners.

301k

220pF

1000pF

2200pF

10onF

88.7k

100pF

15k

SYNC/MODE V

PLLLPF

IPRG

LTC3808EDE

TRACK/SS

GND

59k

RUN

Figure 1. Synchronous Converter with Spread Spectrum Frequency Modulation

is a trademark of

SENSE

2.75V TO 9.8V

10µF

MP Si4431BDY

MN Si4860DY

L: VISHAY IHLP-2525CZ-01

1.5µH

150µF

OUT

DN382 F03

1.5V I

OUT

02/06/382

connectin g an external capa citor CSS betwe en the TRACK / SS pin and ground. An internal 1μA current source and the value of C

control the ramp time of TRACK /SS from

0V to above 0.6V. In this case, the LTC3808 and LTC3809-1 regulate the VFB to the voltage at the TRACK/ SS pin instead of the internal soft-start ramp. The third mode allows V track an external voltage, V

10nF

10k

220pF

15k

1.18k

10nF

118k

100pF

Figure 2. The LTC3808 Offers the Flexibility of Start-Up Control Based on the Three Different Connections on the TRACK/SS Pin

Start-Up with Internal Soft-Start (TRACK/SS = V

= 4.2V

LOAD

Start-Up with Coincidental Tracking Start-Up with Ratiometric Tracking

= 1Ω

)

200µs/DIVV

of the LTC3808 and LTC3809-1 to

OUT

, during start-up if a resistor

SYNC/MODE V

PLLLPF

IPRG

PGOOD

LTC3808EDE

TRACK/SS

GND

59k

DN382 F02a

SENSE

10Ω

1µF

–

RUN

Start-Up with External Soft-Start (CSS = 10nF)

OUT

1.8V

500mV/DIV

= 4.2V

LOAD

2.5V

OUT

1.8V

500mV/DIV

10µF

MP Si7540DP

MN Si7540DP

L: VISHAY IHLP-2525CZ-01

< V

OUT

1ms/DIVV

= 1Ω

1.5µH

150µF

OUT

1.8V I

OUT

DN382 F01

OUT

1.8V

500mV/DIV

DN382 F02b

2.5V

OUT

1.8V

500mV/DIV

divider from V 3 shows the start-up of V

is connected to the TRACK /SS pin. Figure

in these tracking modes for

OUT

the circuit shown in Figure 2.

For simplicity, the LTC3809 only offers a 1ms internal soft-start.

Low EMI DC/DC Conversion

The LTC3808 and LTC3809 minimize the need for EMI shields and ﬁ lters in applications such as navigation systems, wireless LANs, data acquisition boards and industrial and military radio devices by optionally spreading the nominal operating frequency (550kHz) over a range of frequencies between 460kHz and 635kHz. Spread spectrum frequency modulation is enabled by biasing the SYNC/MODE pin to a DC voltage between 1.35V and

– 0.5V). An internal 2.6μA pull-down current source

at the SYNC/MODE pin can be used to set the DC voltage at this pin by tying a resistor with an appropriate value betw een SYNC/MODE and V

. Figure 1 shows the appli ca-

tion circuit and Figur e 4 shows the frequency spectral plo ts of the output (V

) with and without spread spectrum

OUT

modulation. Note the signiﬁ cant reduction in peak output noise (>20dBm) with spread spectrum enabled.

Conclusion

The LTC3808, LTC3809 and LTC3809-1 offer ﬂ exibility, high efﬁ ciency, low EMI and many other popular features in small thermally efﬁ cient packages. They offer excellent solutions for low voltage portable and distributed power systems that require a small footprint, high efﬁ ciency and low noise.

RBW: 100Hz

SPREAD SPECTRUM

FREQUENCY (Hz)

DISABLED

DN382 F04

SPREAD SPECTRUM

ENABLED

NOISE (dBm)

–10dBm/DIV

400k 550k 700k

R R

IN TA TB

= 4.2V = 590Ω = 1.18k

10ms/DIVV

DN382 F02c

= 4.2V

= 590Ω = 1.69k

10ms/DIVV

Figure 3. Start-Up Output Voltage Tracking Plots for Circuit in Figure 2

Data Sheet Download

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Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

●

(408) 432-1900

FAX: (408) 434-0507 ● www.linear.com

DN382 F02d

Figure 4. Comparison of the V

Spectrum with and

OUT

without Spread Spectrum Modulation Enabled

For applications help,

call (408) 432-1900, Ext. 2759

dn382 LT/TP 0206 409K • PRINTED IN THE USA

LINEAR TECHNOLOGY LTC3808 Technical data

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