Datasheet

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LTC3447

3447f

DESCRIPTIO

TYPICAL APPLICATIO

■

C Programmable Output with 21.6mV Resolution

■

Overtemperature Protected

■

High Efﬁ ciency: Up to 93%

■

Very Low Quiescent Current: Only 33µA

■

600mA Output Current at V

= 3V

■

2.5V to 5.5V Input Voltage Range

■

1MHz Constant Frequency Operation

■

No Schottky Diode Required

■

Low Dropout Operation: 100% Duty Cycle

■

Stable with Ceramic Capacitors

■

Shutdown Mode Draws <1µA Supply Current

■

±2% Output Voltage Accuracy

■

Standard (100kHz) or Fast Mode (400kHz) I

■

6-Bit Voltage DAC (0.69V to 2.05V)

■

Disable Burst Mode Operation

■

Enable Power Good Blanking

■

Optional External Start-Up Resistors

■

Soft-Start

■

10 Lead, 3mm × 3mm DFN Package

The LTC

3447 is a high efﬁ ciency monolithic synchronous

current mode buck regulator. Using an I

C interface, the

output voltage can be set between 0.69V and 2.05V using

an internal 6-bit DAC.

The buck regulator has optional external feedback resis-

tors that can be used for setting the initial start up voltage.

The feedback voltage reference for this start-up option

is 0.6V. Once the voltage DAC is updated via the I

C, the

buck regulator switches from external to internal feedback

resistors. When there are no external resistors, the default

start-up voltage is 1.38V.

The switching frequency is internally set at 1MHz, al-

lowing the use of small surface mount inductors and

capacitors.

In Burst Mode

operation, supply current is only 33µA,

dropping to <1µA in shutdown. The 2.5V to 5.5V input

voltage range makes the LTC3447 ideally suited for single

cell Li-Ion battery-powered applications. 100% duty cycle

capability provides low dropout operation, extending

battery life in portable systems. Automatic Burst Mode

operation increases efﬁ ciency at light loads, further ex-

tending battery life.

The internal synchronous switch increases efﬁ ciency and

eliminates the need for an external Schottky diode.

APPLICATIO S

■

Distributed Power Supplies

■

Notebook Computers

■

PDAs and Other Handheld Devices

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

Burst Mode is a registered trademark of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

Protected by U.S. Patents, including 5481178, 6580258, 6304066, 6127815,

6498466, 6611131.

FEATURES

C Controllable Buck

Regulator in 3mm × 3mm DFN

Efﬁ ciency and Power Loss vs

Load Current (V

= 3.6V)

RUN

CCD

SDA

SCL

GND

PGOOD

OUT

2.5V TO 5.5V

EXPOSED PADDLE

TO GROUND

LTC3447

10µF

4.7µF

CERAMIC

4.7µF

3.3µH

100k

49.9k

RPU1

20k

3447 TA01

PWREN

10k 10k

SDA

SCL

CCD

OUT

0.69V

TO 2.05V

AT 600mA*

* 600mA AT V

= 3V

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (mW)

100

1000

100

10 100 1000

3447 TA01b

PULSE SKIP EFFICIENCY

Burst Mode EFFICIENCY

LOSS

Summary of content (16 pages)

PAGE 1
LTC3447 I2C Controllable Buck Regulator in 3mm × 3mm DFN U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO I2C Programmable Output with 21.6mV Resolution Overtemperature Protected High Efﬁciency: Up to 93% Very Low Quiescent Current: Only 33µA 600mA Output Current at VIN = 3V 2.5V to 5.
PAGE 2
LTC3447 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) VIN, VCCD Voltages ....................................... –0.3V to 6V RUN, VOUT, FB Voltages .............................. – 0.3V to VIN SW Voltage ................................... –0.3V to (VIN + 0.3V) SCL, SDA Voltages ................................... – 0.3V to VCCD P-Channel Switch Source Current (DC) ...............800mA N-Channel Switch Sink Current (DC) ...................
PAGE 3
LTC3447 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3.6V unless otherwise specified.
PAGE 4
LTC3447 U W TYPICAL PERFOR A CE CHARACTERISTICS Efﬁciency and Power Loss vs Load Current (VIN = 5.5V) 90 0.3 80 0.2 70 EFFICIENCY (%) INL 0.1 0 DNL – 0.1 20 – 0.4 10 – 0.5 0 30 DAC 20 40 50 60 LOSSMAX 10 100 60 50 LOSSMAX 40 1 20 10 100 LOAD CURRENT (mA) 1 1000 1 Output Voltage vs Load Current Bias Current vs Supply Voltage 2.10 0.80 2.05 DAC = MAX 0.75 350 10 LOSSMIN 30 VOUTMAX (V) LOSSMAX VIN = 3.6V VIN = 2.5V 2.00 DAC = MIN 0.70 VIN = 3.6V VIN = 2.5V 0.
PAGE 5
LTC3447 U W TYPICAL PERFOR A CE CHARACTERISTICS RDS(ON) vs Temperature Output Voltage vs Supply Voltage 0.60 0.55 0.55 2.055 0.50 0.50 2.050 0.710 2.045 0.705 2.040 0.700 PFET 0.40 0.35 NFET 0.30 0.25 0.20 2.5 3.5 5.5 4.5 VIN = 3.6V 0.45 PFET VOUTMAX (V) RDS(ON) (Ω) 0.45 2.060 0.40 0.35 NFET 0.720 0.715 DAC = MAX 2.035 0.695 DAC = MIN 0.30 2.030 0.690 0.25 2.025 0.685 0.20 –40 –20 0 SUPPLY VOLTAGE (V) 2.020 2.
PAGE 6
LTC3447 U U U PI FU CTIO S VOUT (Pin 1): Output Voltage Sensing Pin. An internal resistor divider provides the divided down feedback reference for comparison. SW (Pin 6): Switch Node Connector to Inductor. This pin connects the drains of the internal main and synchronous power MOSFET switches. GND (Pin 2): Ground for all Circuits Excluding the Internal Synchronous Power NFET. RUN (Pin 7): Run Control Input. Forcing pin above 1.5V enables the part. Forcing the pin below 0.3V shuts down the device.
PAGE 7
LTC3447 U OPERATIO BUCK OPERATION VIN VFB PEAK CURRENT LEVEL REF EA VREF ICOMP RS LATCH OSC S Q R QB RS PFET SW LOGIC NFET BURST 50mA. When below this level, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 33µA, and the peak current level reference level is held at 150mA. The LTC3447 remains in this sleep state until the output voltage falls below the output voltage setting.
PAGE 8
LTC3447 U OPERATIO the user should calculate the power dissipation when the LTC3447 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information section). for duty cycles >40%; however, the LTC3447 uses a patent-pending scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles. Low Supply Operation DAC The LTC3447 will operate with input supply voltages as low as 2.
PAGE 9
LTC3447 U OPERATIO External Start-Up Option The LTC3447 allows for the use of optional external resistors to determine the start-up voltage. Using this option, the start-up voltage can be set to levels inside or outside the DAC output’s operating range. The output voltage will be regulated at this value until the internal DAC is updated and a STOP command is received. Once the STOP command is received, the internal DAC will retain control of the output voltage until the part is disabled then enabled again.
PAGE 10
LTC3447 U OPERATIO be left HIGH by the slave. The master can then generate a STOP command to abort the transfer. The START and STOP Commands When the bus is not in use, both SCL and SDA must be high. A bus master signals the beginning of a transmission with a START command by transitioning SDA from high to low while SCL is high. When the master has ﬁnished communicating with the slave, it issues a STOP command by transitioning SDA from low to high while SCL is high.
PAGE 11
LTC3447 U OPERATIO Table 1. I2C Fast-Mode Timing Specifications (for Reference) fI2C tBUF tHD,RSTA tSU,RSTA tSU,STOP tHD,DAT tSU,DAT tLOW tHIGH tSP tf tr I2C Operating Frequency Bus free time between Stop and Start Condition Hold Time after (Repeated) Start Condition Repeated Start Condition Setup Time Stop Condition Setup Time Data Hold Time Data Setup Time Clock Low Period Clock High Period Pulse Width of Spikes Suppressed by Input Filter Clock, Data Fall Time 0 1.3 0.6 0.6 0.6 0 100 1.3 0.6 0 20 + 0.
PAGE 12
LTC3447 U W U U APPLICATIO S I FOR ATIO CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: 1/ 2 VOUT (VIN – VOUT )] [ CIN required IRMS ≅ IOMAX VIN (2) This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2.
PAGE 13
LTC3447 U W U U APPLICATIO S I FOR ATIO loss dominates the efﬁciency loss at medium to high load currents. In a typical efﬁciency plot, the efﬁciency curve at very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Figure 8. 1 RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss.
PAGE 14
LTC3447 U W U U APPLICATIO S I FOR ATIO junction temperature of the part. The temperature rise is given by: TR = θJA • PD where PD is the power dissipated by the regulator and θJA is the thermal resistance from the junction of the die to the temperature. possible? This capacitor provides the AC current to the internal power MOSFETs. 5. Keep the switching node, SW, away from the sensitive VOUT and FB nodes. 6. Keep the (–) plates of CIN and COUT as close as possible.
PAGE 15
LTC3447 U W U U APPLICATIO S I FOR ATIO Design Example ⎛ ⎞ 1 ∆VOUT = 0.280 A⎜ 0.25Ω + ⎟= 8(1MHz)(4.7µF)⎠ ⎝ 70mV + 7.4mV = 77.4mV As a design example, assume the LTC3447 is used in a single lithium-ion battery-powered cellular phone application. The VIN will be operating from a maximum of 4.2V down to about 2.7V. The normal load current requirement is a maximum of 500mA at 1.4V, but most of the time it will be in standby mode, requiring only 200µA at 1V.
PAGE 16
LTC3447 U TYPICAL APPLICATIO Li-Ion Battery to 1.4V/1V Regulator Li Ion BATTERY 20k L1 3.3mH VIN CIN 4.7mF FB PGOOD I2C BUS VCCD 10k 10k VOUT = SW LTC3447 PWREN 4.7mF 1.4V AT 500mA IN NORMAL OPERATION 1V AT 200mA IN STBY MODE COUT 4.