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LTC3545/LTC3545-1

35451fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Triple 800mA Synchronous

Step-Down Regulator–2.25MHz

The LTC

3545/LTC3545-1 are triple, high efficiency,

monolithic synchronous buck regulators using a constant

frequency, current mode architecture. The regulators op-

erate independently with separate run pins. The 2.25V to

5.5V input voltage range makes the LTC3545/LTC3545-1

well suited for single Li-Ion battery-powered applications.

Low ripple pulse skip mode or high efﬁ ciency Burst Mode

operation is externally selectable. PWM pulse skip mode

operation provides very low output ripple voltage while

Burst Mode operation increases efﬁ ciency at low output

loads.

Switching frequency is internally set to 2.25MHz, or the

switching frequency can be synchronized to an external

1MHz to 3MHz clock. Power good indicators easily allow

power on sequencing between the three regulators.

The internal synchronous switches increase efﬁ ciency and

eliminate external Schottky diodes. Low output voltages are

supported with the 0.6V feedback reference voltage.

The LTC3545-1 replaces the SYNC/MODE function with a

third PGOOD pin and forces Burst Mode operation.

■

Three 800mA Outputs

■

High Efﬁ ciency: Up to 95%

■

2.25V to 5.5V Input Voltage Range

■

Low Ripple (<20mV

P-P

) Burst Mode

Operation

: 58μA

■

2.25MHz Constant Frequency Operation or

■

Synchronizable to External 1MHz to 3MHz Clock

■

Power Good Indicators Ease Supply Sequencing

■

0.6V Reference Allows Low Output Voltages

■

Current Mode Operation/Excellent Transient Response

■

Low Proﬁ le 16-Lead 3mm × 3mm QFN Package

■

Smart Phones

■

Wireless and DSL Modems

■

Digital Still Cameras

■

Portable Instruments

■

Point of Load Regulation

High Efﬁ ciency Triple Step-Down Converter with Power Sequencing

Efﬁ ciency and Loss vs Load Current

L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners.

Protected by U.S. Patents including 6580258, 5481178, 6127815, 6498466, 6611131.

3545 TA01

1.5µH

10µF

PGNDGNDA

C4 10µF

20pF

255k

511k

10µF

OUT1

1.8V

2.25V TO 5.5V

RUN1

PGOOD1

RUN2

PGOOD2

RUN3

1.5µH

226k

10µF

OUT2

1.2V

SW2

FB2

SYNC/MODE

SW1

FB1

LTC3545

GNDA PGND

500k

20pF

226k

500k

1.5µH

200k

10µF

OUT3

1.5V

SW3

FB3

20pF

301k

LOAD CURRENT (A)

EFFICIENCY (%)

LOSS (W)

100

0.0001 0.01 0.1 1

3545 TA01b

0.001

0.1

0.01

0.0001

0.001

= 2.5V

= 3.6V

= 4.2V

= 25°C

OUT

= 2V

Burst Mode OPERATION

OSC

= 2.25MHz

SINGLE CHANNEL

Summary of content (20 pages)

PAGE 1
LTC3545/LTC3545-1 Triple 800mA Synchronous Step-Down Regulator–2.25MHz FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION Three 800mA Outputs High Efﬁciency: Up to 95% 2.25V to 5.5V Input Voltage Range Low Ripple (<20mVP-P) Burst Mode® Operation IQ: 58μA 2.25MHz Constant Frequency Operation or Synchronizable to External 1MHz to 3MHz Clock Power Good Indicators Ease Supply Sequencing 0.
PAGE 2
LTC3545/LTC3545-1 ABSOLUTE MAXIMUM RATINGS (Note 1) Input Supply Voltage .................................... –0.3V to 6V RUNx, PGOODx............................. –0.3V to (VIN + 0.3V) VFBx, SYNC/MODE ........................ –0.3V to (VIN + 0.3V) SWx .............................................. –0.3V to (VIN + 0.3V) P-Switch Source Current (DC) (Note 8) ...................1.1A N-Channel Sink Current (DC) (Note 8) .....................1.1A Peak SW Sink and Source Current (Note 8) .............1.
PAGE 3
LTC3545/LTC3545-1 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = PVIN = 3.6V unless otherwise noted.
PAGE 4
LTC3545/LTC3545-1 TYPICAL PERFORMANCE CHARACTERISTICS VREF vs Temperature at 2.25V, 3.6V, 5.5V Switching Frequency vs Supply Voltage and Temperature 1.2 TA = 25°C VIN = 3.6V UNTESTED CHANNELS OFF PULSE SKIP MODE 0.600 0.595 0.590 0.585 –50 2.25V 3.6V 5.5V 0 50 100 TEMPERATURE (°C) 0.8 2.5 VOUT ERROR (%) 0.605 SWITCHING FREQUENCY (MHz) 1.0 0.610 VREF (V) Load Regulation, All Channels 3.0 0.615 2.0 2 3 4 5 SUPPLY VOLTAGE (V) 0.4 0.2 0 fOSC = –40°C fOSC = 0°C fOSC = 25°C fOSC = 80°C 1.
PAGE 5
LTC3545/LTC3545-1 TYPICAL PERFORMANCE CHARACTERISTICS Supply Current vs Temperature, Pulse Skipping Efﬁciency vs Load Current, Burst Mode Operation 350 EFFICIENCY (%) SUPPLY CURRENT (μA) 400 300 250 200 100 100 90 90 80 80 70 70 60 50 VIN = 2.7V VIN = 3.6V VIN = 4.2V 40 30 VFB3 = 0.625V ILOAD = 0mA CHANNEL 3 ONLY 150 –50 0 VIN = 5.5V VIN = 4.5V VIN = 3.5V VIN = 2.
PAGE 6
LTC3545/LTC3545-1 TYPICAL PERFORMANCE CHARACTERISTICS Start-Up From Shutdown No Load Start-Up From Shutdown Loaded Load Step Crosstalk VOUT2 2mV/DIV VOUT1 VOUT1 1V/DIV VOUT2 1V/DIV VOUT3 1V/DIV VOUT2 VOUT3 (ALL 1V/DIV) RUNX 5V/DIV VOUT3 2mV/DIV VOUT1 100mV/DIV ILOAD CH1 50mA/DIV ISUPPLY 50mA/DIV ISUPPLY 1A/DIV TA = 25°C 200μs/DIV VIN = 3.6V ILOAD = 600mA, ALL CHANNELS 3545 G13 3545 G14 200μs/DIV TA = 25°C VIN = 3.
PAGE 7
LTC3545/LTC3545-1 PIN FUNCTIONS SW1 (Pin 1): Switch Node Connection to Inductor for Regulator 1. This pin connects to the internal power MOSFET switches. external clock, the part operates in pulse skipping mode with a switching frequency equal to the external clock. PGOOD1 (Pin 2): This open-drain output voltage is pulled to a logic low when VFB1 is below 0.54V (VOUT1 is below 90% of regulated level).
PAGE 8
LTC3545/LTC3545-1 FUNCTIONAL DIAGRAMS RUN3 RUN2 GNDA VIN SYNC/MODE (LTC3545 ONLY) RUN1 SHDN 0.
PAGE 9
LTC3545/LTC3545-1 FUNCTIONAL DIAGRAMS PGOOD REGULATOR BURST CLAMP + PVIN 0.6V SLOPE COMP – + 50mV – – EA 0.6V SLEEP ITH VSLEEP + – + 10Ω ICOMP + BURST S Q RS LATCH SOFT-START R Q SWITCHING LOGIC AND BLANKING CIRCUIT ANTI SHOOTTHRU SWX + IRCMP PGND – VFBX SHUTDOWN 3545 FD02 0.
PAGE 10
LTC3545/LTC3545-1 OPERATION MAIN CONTROL LOOP PULSE SKIPPING/Burst Mode OPERATION The LTC3545/LTC3545-1 use a constant frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP , resets the RS latch.
PAGE 11
LTC3545/LTC3545-1 OPERATION SOFT-START Soft-start reduces surge currents on VIN and output overshoot during start-up. Soft-start on the LTC3545/ LTC3545-1 is implemented by internally ramping the reference signal fed to the error ampliﬁer over approximately a 1ms period. Figure 1 shows the behavior of the regulator channels during start-up. Short-Circuit Protection Short-circuit protection is achieved by monitoring the inductor current.
PAGE 12
LTC3545/LTC3545-1 APPLICATIONS INFORMATION The basic LTC3545/LTC3545-1 application circuit is shown on the ﬁrst page of this data sheet. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. Inductor Selection For most applications, the value of the inductor will fall in the range of 1μH to 10μH. Its value is chosen based on the desired ripple current.
PAGE 13
LTC3545/LTC3545-1 APPLICATIONS INFORMATION The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple ΔVOUT is determined by: ⎛ ⎞ 1 ΔVOUT ≅ ΔIL ⎜ ESR + 8 • ƒ • COUT ⎟⎠ ⎝ where f = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor.
PAGE 14
LTC3545/LTC3545-1 APPLICATIONS INFORMATION 1. The quiescent current is due to two components: the DC bias current as given in the electrical characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from PVIN to ground.
PAGE 15
LTC3545/LTC3545-1 APPLICATIONS INFORMATION Similar situations can occur when all three channels are operating at maximum loads at high ambient temperature. As an example, consider a channel supplying 800mA at 1.8V output and 85% efﬁciency. The dissipated power can be calculated using ⎛ 1– E ⎞ Loss = PO ⎜ = 1.4W • 0.17 = 0.25W ⎝ E ⎟⎠ where PO is the output power and E is the efﬁciency. In this case the temperature rise is 17°C, similar to the dropout scenario described above.
PAGE 16
LTC3545/LTC3545-1 APPLICATIONS INFORMATION PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3545/LTC3545-1. These items are also illustrated graphically in Figures 3 and 4. Figure 3 shows the power path components and traces. In this ﬁgure the feedback networks are not shown since they reside on the bottom side of the board. Check the following in your layout: 1.
PAGE 17
LTC3545/LTC3545-1 TYPICAL APPLICATIONS L1 1.5μH C6 20pF R2 511k E2 PGOOD2 C5 4.7μF R1 511k 1 2 E1 PGOOD1 3 4 5 6 7 VIN 2.7V TO 5.5V C1 10μF 10V 8 SW1 LTC3545 PGOOD1 GNDA VIN RUN2 RUN1 PGOOD2 VFB1 SW2 VFB2 PGND VFB3 PVIN RUN3 SW3 SYNC/MODE R3 191k E3 VOUT1 C2 10μF 6.3V 2.5V AT 0.8A E4 GND R4 60.4k 16 15 14 L2 1.5μH 13 12 C7 20pF 11 R5 100k 10 E7 VOUT2 C3 10μF 6.3V 1.2V AT 0.8A E6 GND R6 100k 9 GND L3 1.5μH 17 C8 20pF R7 165k E5 VOUT3 C4 10μF 6.3V 1.5V AT 0.
PAGE 18
LTC3545/LTC3545-1 TYPICAL APPLICATIONS L1 1.5μH C6 20pF R9 511k E9 PGOOD3 E2 PGOOD2 R2 511k R1 511k 1 2 E1 PGOOD1 3 4 5 6 VIN 2.5V TO 5.5V 7 C1 4.7μF 8 SW1 LTC3545-1 GNDA PGOOD1 VIN RUN1 RUN2 PGOOD2 VFB1 SW2 VFB2 PGND VFB3 PVIN RUN3 SW3 PGOOD3 C2 10μF E3 VOUT1 1.2V AT 0.8A E4 GND R4 100k C5 10μF 16 R3 100k 15 14 L2 1.5μH 13 12 C7 20pF 11 R5 165k C3 10μF 10 R6 110k 9 E7 VOUT2 1.5V AT 0.8A E6 GND GND L3 1.5μH 17 C8 20pF R7 133k C4 10μF E5 VOUT3 1.8V AT 0.
PAGE 19
LTC3545/LTC3545-1 PACKAGE DESCRIPTION UD Package 16-Lead Plastic QFN (3mm s 3mm) (Reference LTC DWG # 05-08-1700 Rev A) Exposed Pad Variation AA 0.70 p0.05 3.50 p 0.05 1.65 p 0.05 2.10 p 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 p0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 s 45o CHAMFER R = 0.115 TYP 0.75 p 0.05 15 16 PIN 1 TOP MARK (NOTE 6) 0.40 p 0.10 1 1.65 p 0.
PAGE 20
LTC3545/LTC3545-1 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3405/LTC3405A 300mA IOUT, 1.5MHz, Synchronous Step-Down DC/DC Converters 95% Efﬁciency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 20μA, ISD < 1μA, ThinSOT™ Package LTC3406/LTC3406B 600mA IOUT, 1.5MHz, Synchronous Step-Down DC/DC Converters 96% Efﬁciency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20μA, ISD < 1μA, ThinSOT Package LTC3407/LTC3407-2 Dual 600mA/800mA IOUT, 1.5MHz/2.