Datasheet

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LTC3548A

3548afa

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Dual Synchronous

400mA/800mA, 2.25MHz

Step-Down DC/DC Regulator

The LTC

3548A is a dual, constant frequency, synchronous

step-down DC/DC converter. Intended for low power ap-

plications, it operates from a 2.5V to 5.5V input voltage

range and has a constant 2.25MHz switching frequency,

enabling the use of tiny, low cost capacitors and inductors

1mm or less in height. Each output voltage is adjustable

from 0.6V to 5V. Internal synchronous 0.35Ω, 0.7A/1.2A

power switches provide high efficiency without the need

for external Schottky diodes.

A user-selectable mode input is provided to allow the user

to trade-off ripple noise for low power efficiency. Burst

Mode

operation provides the highest efficiency at light

loads, while pulse-skipping mode provides the lowest

ripple noise at light loads.

To further maximize battery life, the P-channel MOSFETs

are turned on continuously in dropout (100% duty cycle),

and both channels draw a total quiescent current of only

40μA. In shutdown, the device draws <1μA.

2.5V/1.8V at 400mA/800mA Step-Down Regulators

High Efficiency: Up to 95%

Low Ripple (<35mV

P-P

) Burst Mode Operation;

= 40μA

2.25MHz Constant Frequency Operation

High Switch Current: 0.7A and 1.2A

No Schottky Diodes Required

Low R

DS(ON)

Internal Switches: 0.35Ω

Current Mode Operation for Excellent Line

and Load Transient Response

Short-Circuit Protected

Low Dropout Operation: 100% Duty Cycle

Ultralow Shutdown Current: I

<1μA

Output Voltages from 5V down to 0.6V

Power-On Reset Output

Externally Synchronizable Oscillator

Optional External Soft-Start

Small Thermally Enhanced MSOP and 3mm × 3mm

DFN Packages

PDAs/Palmtop PCs

Digital Cameras

Cellular Phones

Wireless and DSL Modems

Efficiency/Power Loss Curves

L, LT, LTC, LTM, Linear Technology, the Linear logo 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 5481178, 6580258, 6304066, 6127815, 6498466,

6611131.

RUN/SS2 V

2.5V TO 5.5V

OUT2

2.5V

400mA

OUT1

1.8V

800mA

RUN/SS1

POR

SW1

FB1

GND

FB2

SW2

MODE/SYNC

LTC3548A

10μF

100k

RESET

22pF

2.2μH4.7μH

887k 887k

442k

280k

4.7μF 10μF

3548A TA01

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (W)

100

0.0001

0.001

0.1

0.01

10 100 1000

3548A TA02

2.5V

= 3.3V

Burst Mode OPERATION

NO LOAD ON OTHER CHANNEL

1.8V

Summary of content (20 pages)

PAGE 1
LTC3548A Dual Synchronous 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator DESCRIPTION FEATURES n n n n n n n n n n n n n n n High Efficiency: Up to 95% Low Ripple (<35mVP-P) Burst Mode Operation; IQ = 40μA 2.25MHz Constant Frequency Operation High Switch Current: 0.7A and 1.2A No Schottky Diodes Required Low RDS(ON) Internal Switches: 0.
PAGE 2
LTC3548A ABSOLUTE MAXIMUM RATINGS (Note 1) VIN Voltage ...................................................– 0.3V to 6V VFB1, VFB2 Voltages ................................... –0.3V to 1.5V RUN/SS1, RUN/SS2 Voltages .......... –0.3V to VIN + 0.3V MODE/SYNC Voltage ....................... –0.3V to VIN + 0.3V SW1, SW2 Voltages ......................... –0.3V to VIN + 0.3V POR Voltage ................................................. –0.3V to 6V Operating Junction Temperature Range (Notes 2, 5) ...............
PAGE 3
LTC3548A ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TJ = 25°C. VIN = 3.6V, unless otherwise speciﬁed. (Note 2) SYMBOL PARAMETER VIN Operating Voltage Range IFB Feedback Pin Input Current VFB Feedback Voltage (Note 3) CONDITIONS MIN ● TYP 2.5 ● 0°C ≤ TA ≤ 85°C –40°C ≤ TA ≤ 125°C (Note 2) ● 0.588 0.585 MAX UNITS 5.5 V 30 nA 0.6 0.6 0.612 0.612 V V 0.
PAGE 4
LTC3548A TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise speciﬁed. Burst Mode Operation Pulse-Skipping Mode SW 5V/DIV SW 5V/DIV VOUT 50mV/DIV VOUT 10mV/DIV IL 200mA/DIV IL 100mA/DIV 3548A G01 2μs/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 50mA FIGURE 3 CIRCUIT 3548A G02 1μs/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 50mA FIGURE 3 CIRCUIT Load Step Soft-Start VOUT1 200mV/DIV VIN 2V/DIV VOUT2 100mV/DIV IL 500mA/DIV VOUT1 1V/DIV ILOAD 500mA/DIV IL 500mA/DIV 3548A G03 20μs/DIV VIN = 3.
PAGE 5
LTC3548A TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Temperature RDS(ON) vs Input Voltage VIN = 3.6V 0.605 400 0.600 0.595 MAIN SWITCH 350 100 250 150 1 125 2 3 4 VIN (V) 5 6 100 –50 –25 7 Efficiency vs Load Current Efficiency vs Load Current 2.7V 3.3V Load Regulation Burst Mode OPERATION 3 80 60 50 40 2 70 VOUT ERROR(%) EFFICIENCY (%) 70 60 PULSE-SKIPPING MODE 50 40 30 30 20 V OUT = 2.
PAGE 6
LTC3548A PIN FUNCTIONS VFB1 (Pin 1): Output Feedback. Receives the feedback voltage from the external resistive divider across the output. Nominal voltage for this pin is 0.6V. RUN/SS1 (Pin 2): Regulator 1 Enable and Soft-Start Input. Forcing this pin to VIN enables regulator 1, while forcing it to GND causes regulator 1 to shut down. Connect external RC network with desired time-constant to enable soft-start feature. This pin must be driven; do not float. VIN (Pin 3): Main Power Supply.
PAGE 7
LTC3548A BLOCK DIAGRAM REGULATOR 1 MODE/SYNC 6 BURST CLAMP VIN SLOPE COMP 0.6V EA VFB1 SLEEP ITH – UVDET 57 UV R OVDET Q SWITCHING LOGIC AND BLANKING CIRCUIT ANTI SHOOTTHRU 4 SW1 OV + + Q RS LATCH + 0.65V + BURST S 0.55V – ICOMP + 0.65V – 1 EN – + – IRCMP – 11 GND SHUTDOWN PGOOD1 3 VIN 8 POR RUN/SS1 2 0.
PAGE 8
LTC3548A OPERATION The LTC3548A uses a constant frequency, current mode architecture. The operating frequency is set at 2.25MHz and can be synchronized to an external oscillator. Both channels share the same clock and run in phase. To suit a variety of applications, the selectable MODE/SYNC pin allows the user to trade-off noise for efficiency. The output voltage is set by an external divider returned to the VFB pins. An error amplifier compares the divided output voltage with a reference voltage of 0.
PAGE 9
LTC3548A OPERATION VIN = 2.5V TO 5.5V CIN R7 VIN BURST* R6 PULSE SKIP* RUN/SS2 POWER-ON RESET POR MODE/SYNC R5 RUN/SS1 C4 C3 LTC3548A L1 L2 VOUT2 VOUT1 SW1 SW2 C2 COUT2 R4 C1 VFB1 VFB2 R3 GND R1 R2 COUT1 3548A F01 *MODE/SYNC = 0V: PULSE SKIP MODE/SYNC = VIN: Burst Mode OPERATION Figure 1.
PAGE 10
LTC3548A APPLICATIONS INFORMATION Table 1. Representative Surface Mount Inductors MANUFACTURER PART NUMBER MAX DC VALUE CURRENT DCR HEIGHT Taiyo Yuden CB2016T2R2M CB2012T2R2M CB2016T3R3M 2.2μH 2.2μH 3.3μH 510mA 530mA 410mA 0.13Ω 0.33Ω 0.27Ω 1.6mm 1.25mm 1.6mm Panasonic ELT5KT4R7M 4.7μH 950mA 0.2Ω 1.2mm Sumida CDRH2D18/LD 4.7μH 630mA 0.086Ω 2mm Murata LQH32CN4R7M23 4.7μH 450mA 0.2Ω 2mm Taiyo Yuden NR30102R2M NR30104R7M 2.2μH 4.7μH 1100mA 750mA 0.1Ω 0.
PAGE 11
LTC3548A APPLICATIONS INFORMATION AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. Aluminum electrolytic capacitors have a significantly larger ESR, and are often used in extremely cost-sensitive applications provided that consideration is given to ripple current ratings and long term reliability.
PAGE 12
LTC3548A APPLICATIONS INFORMATION Power-On Reset The POR pin is an open-drain output which pulls low when either regulator is out of regulation. When both output voltages are within ±8.5% of regulation, a timer is started which releases POR after 216 clock cycles (about 29ms in pulse-skipping mode). This delay can be significantly longer in Burst Mode operation with low load currents, since the clock cycles only occur during a burst and there could be milliseconds of time between bursts.
PAGE 13
LTC3548A APPLICATIONS INFORMATION For approximately a 1ms ramp time, use RSS = 4.7M and CSS = 680pF at VIN = 3.3V. Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Percent efficiency can be expressed as: %Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc.
PAGE 14
LTC3548A APPLICATIONS INFORMATION The junction temperature, TJ, is given by: TJ = TRISE + TAMBIENT As an example, consider the case when the LTC3548A is in dropout on both channels at an input voltage of 2.7V with a load current of 400mA and 800mA and an ambient temperature of 70°C. From the Typical Performance Characteristics graph of Switch Resistance, the RDS(ON) resistance of the main switch is 0.425Ω.
PAGE 15
LTC3548A APPLICATIONS INFORMATION 4. Keep sensitive components away from the SW pins. The input capacitor, CIN, and the resistors R1 to R4 should be routed away from the SW traces and the inductors. signal components returning to the GND pin at one point. Additionally the two grounds should not share the high current paths of CIN or COUT . 6. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components.
PAGE 16
LTC3548A TYPICAL APPLICATIONS VIN = 2.5V TO 5.5V CIN 10μF R6 4.7M R7 100k VIN POR RUN/SS2 C4 680pF V OUT2 2.5V 400mA COUT2 10μF L2 10μH RUN/SS1 L1 4.7μH LTC3548A SW2 SW1 C2 22pF C1 22pF R3 280k C3 VOUT1 680pF 1.2V 800mA VFB1 VFB2 R4 887k R5 4.7M POWER-ON RESET MODE/SYNC R2 R1 604k 604k GND COUT1 10μF 3548A F04a L1: TDK VLF3012ST-4R7MR91 L2: TDK VLF3012ST-100MR59 CIN, COUT1, COUT2: TAIYO YUDEN JMK316BJ106ML Figure 4.
PAGE 17
LTC3548A PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev B) 0.70 p0.05 3.55 p0.05 1.65 p0.05 2.15 p0.05 (2 SIDES) PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 2.38 p0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 p0.10 (4 SIDES) R = 0.125 TYP 6 0.40 p 0.10 10 1.65 p 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) (DD) DFN REV B 0309 5 0.200 REF 1 0.25 p 0.05 0.50 BSC 0.75 p0.05 0.00 – 0.05 2.38 p0.
PAGE 18
LTC3548A PACKAGE DESCRIPTION MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev C) BOTTOM VIEW OF EXPOSED PAD OPTION 2.794 p 0.102 (.110 p .004) 5.23 (.206) MIN 0.889 p 0.127 (.035 p .005) 1 0.05 REF 10 3.00 p 0.102 (.118 p .004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 10 9 8 7 6 DETAIL “A” 0o – 6o TYP 1 2 3 4 5 GAUGE PLANE 0.53 p 0.152 (.021 p .006) DETAIL “A” 0.18 (.007) 0.497 p 0.
PAGE 19
LTC3548A REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 2/10 Changes to Absolute Maximum Ratings 2 Changes to Electrical Characteristics 3 3548afa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 20
LTC3548A TYPICAL APPLICATION 2mm Height Lithium-Ion Single Inductor Buck-Boost Regulator and a Buck Regulator VIN 2.8V TO 4.2V C1 10μF R5 100k RUN/SS2 VIN RUN/SS1 VOUT2 3.3V 200mA D1 + SW2 C6 22μF POWER-ON RESET L1 2.2μH POR MODE/SYNC L2 15μH LTC3548A SW1 C4 22pF M1 C3 4.7μF R4 887k VFB1 VFB2 R3 196k VOUT1 1.