Datasheet

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LTC3407-3

34073fb

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

Dual Synchronous,

1.8V/0.8A and 3.3V/0.8A

2.25MHz Step-Down DC/DC Regulator

The LTC

3407-3 is a dual, constant frequency, synchro-

nous step down DC/DC converter. Intended for low power

applications, it operates from 3.3V to 5.5V input voltage

range and has a constant 2.25MHz switching frequency,

allowing the use of tiny, low cost capacitors and inductors

with a proﬁ le ≤1.0mm. Internal synchronous 0.35Ω, 1.2A

power switches provide high efﬁ ciency without the need

for external Schottky diodes.

A user selectable mode input is provided to allow the user

to trade-off noise ripple for low power efﬁ ciency. Burst

Mode

operation provides high efﬁ ciency at light loads,

while pulse-skipping mode provides low noise ripple 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.

Figure 1. 1.8V/3.3V at 800mA Step-Down Regulators

Very Low Quiescent Current: Only 40μA

1.8V at 800mA/3.3V at 800mA

High Efﬁ ciency: Up to 95%

2.25MHz Constant Frequency Operation

High Switch Current: 1.2A on Each Channel

No Schottky Diodes Required

Low R

DS(ON)

Internal Switches: 0.35Ω

: 3.3V to 5.5V

Current Mode Operation for Excellent Line

and Load Transient Response

Short-Circuit Protected

Low Dropout Operation: 100% Duty Cycle

Ultralow Shutdown Current: I

< 1μA

Power-On Reset Output

Externally Synchronizable Oscillator

Small Thermally Enhanced 3mm × 3mm DFN Package

PDAs/Palmtop PCs

Digital Cameras

Cellular Phones

Portable Media Players

PC Cards

Wireless and DSL Modems

L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo 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.

LTC3407-3 Efﬁ ciency/Power Loss

RUN2

NOTE: IN DROPOUT, THE OUTPUT TRACKS THE INPUT VOLTAGE

3.3V TO 5.5V

OUT2

3.3V

OUT1

1.8V

RUN1

POR

SW1

OUT1

GND

OUT2

SW2

MODE/SYNC

LTC3407-3

10μF

100k

RESET

2.2μH

10μF 10μF

34073 TA01

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (W)

100

10 100 1000

34073 TA01b

0.001

0.1

0.01

0.000

OUT

= 3.3V

OUT

= 3.3V

OUT

= 1.8V

OUT

= 1.8V

= 5V

Burst Mode OPERATION

NO LOAD ON OTHER CHANNEL

Summary of content (16 pages)

PAGE 1
LTC3407-3 Dual Synchronous, 1.8V/0.8A and 3.3V/0.8A 2.25MHz Step-Down DC/DC Regulator DESCRIPTION FEATURES n n n n n n n n n n n n n n n Very Low Quiescent Current: Only 40μA 1.8V at 800mA/3.3V at 800mA High Efﬁciency: Up to 95% 2.25MHz Constant Frequency Operation High Switch Current: 1.2A on Each Channel No Schottky Diodes Required Low RDS(ON) Internal Switches: 0.35Ω VIN: 3.3V to 5.
PAGE 2
LTC3407-3 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VIN Voltages .................................................– 0.3V to 6V VOUT1, VOUT2 Voltages ..................... –0.3V to VIN + 0.3V RUN1, RUN2 Voltages .................................–0.3V to VIN MODE/SYNC Voltage....................................–0.3V to VIN SW1, SW2 Voltage ........................... –0.3V to VIN + 0.3V POR Voltage ................................................. –0.
PAGE 3
LTC3407-3 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3.6V, unless otherwise speciﬁed. (Note 2) SYMBOL PARAMETER CONDITIONS MIN TYP MAX ILIM Peak Switch Current Limit, Channel 1 + Channel 2 VIN = 3V, VOUT1 = 1.5V, VOUT2 = 2.8V, Duty Cycle <35% 0.95 1.2 1.6 A RDS(ON) Top Switch On-Resistance Bottom Switch On-Resistance (Note 6) (Note 6) 0.35 0.30 0.45 0.
PAGE 4
LTC3407-3 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise speciﬁed. Oscillator Frequency vs Temperature Load Step 2.5 VOUT 200mV/DIV Oscillator Frequency Deviation vs Supply Voltage 10 VIN = 3.6V 8 IL 500mA/DIV ILOAD 500mA/DIV 2.3 2.2 2.1 34073 G04 VIN = 5V 20μs/DIV VOUT = 3.3V ILOAD = 80mA TO 800mA CIRCUIT OF FIGURE 1 FREQUENCY DEVIATION (%) FREQUENCY (MHz) 2.4 6 4 2 0 –2 –4 –6 –8 2.
PAGE 5
LTC3407-3 TYPICAL PERFORMANCE CHARACTERISTICS Load Regulation Load Regulation 4 4 3 3 2 Line Regulation 0.80 VOUT = 1.8V 0.60 IOUT = 200mA 0.40 1 0 PULSE-SKIPPING –1 MODE 1 PULSE-SKIPPING –1 MODE –2 –3 –3 VIN = 3.6V, VOUT = 1.8V NO LOAD ON OTHER CHANNEL 1 10 100 LOAD CURRENT (mA) 1000 Burst Mode OPERATION 0 –2 –4 VOUT ERROR (%) 2 Burst Mode OPERATION VOUT ERROR (%) VOUT ERROR (%) TA = 25°C unless otherwise speciﬁed. 0.20 0 –0.20 –0.40 –0.60 VIN = 5V, VOUT = 3.
PAGE 6
LTC3407-3 BLOCK DIAGRAM REGULATOR 1 MODE/SYNC 6 BURST CLAMP VIN SLOPE COMP VOUT1 1 R1 VFB EN – + 0.6V EA SLEEP ITH + 5Ω ICOMP + 0.65V – – BURST R3 S Q RS LATCH R 0.55V – UVDET UV + Q SWITCHING LOGIC AND BLANKING CIRCUIT ANTI SHOOTTHRU 4 SW1 + OVDET – + 0.65V OV IRCMP SHUTDOWN – 11 GND VIN PGOOD1 RUN1 2 RUN2 9 3 VIN 8 POR 0.
PAGE 7
LTC3407-3 OPERATION The LTC3407-3 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 pin allows the user to choose between low noise and light load efﬁciency. The output voltage is set by an internal divider. An error ampliﬁer compares the divided output voltage with a reference voltage of 0.
PAGE 8
LTC3407-3 APPLICATIONS INFORMATION A general LTC3407-3 application circuit is shown in Figure 2. External component selection is driven by the load requirement, and begins with the selection of the inductor L. Once the inductor is chosen, CIN and COUT can be selected. Inductor Selection Although the inductor does not inﬂuence the operating frequency, the inductor value has a direct effect on ripple current.
PAGE 9
LTC3407-3 APPLICATIONS INFORMATION Output Capacitor (COUT) Selection The selection of COUT is driven by the required ESR to minimize voltage ripple and load step transients. Typically, once the ESR requirement is satisﬁed, the capacitance is adequate for ﬁltering. The output ripple (ΔVOUT) is determined by: 1 VOUT IL ESR+ 8fO COUT where f = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor.
PAGE 10
LTC3407-3 APPLICATIONS INFORMATION Since the ESR of a ceramic capacitor is so low, the input and output capacitor must instead fulﬁll a charge storage requirement. During a load step, the output capacitor must instantaneously supply the current to support the load until the feedback loop raises the switch current enough to support the load. The time required for the feedback loop to respond is dependent on the compensation and the output capacitor size.
PAGE 11
LTC3407-3 APPLICATIONS INFORMATION 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. are the individual losses as a percentage of input power.
PAGE 12
LTC3407-3 APPLICATIONS INFORMATION with a load current of 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. Therefore, power dissipated by each channel is: Choosing a vendor’s closest inductor value of 2.2μH, results in a maximum ripple current of: 1.8V 1.8V • 1 IL = = 208mA 2.25MHz • 2.2μH 4.2V For cost reasons, a ceramic capacitor will be used.
PAGE 13
LTC3407-3 APPLICATIONS INFORMATION 2. Are the COUT and L1 closely connected? The (–) plate of COUT returns current to GND and the (–) plate of CIN. 5. A ground plane is preferred, but if not available, keep the signal and power grounds segregated with small signal components returning to the GND pin at one point and should not share the high current path of CIN or COUT. 3.
PAGE 14
LTC3407-3 TYPICAL APPLICATIONS 1mm Height Core Supply VIN 5V C1* 10μF RUN2 VIN MODE/SYNC L2 2.2μH VOUT2 3.3V 800mA POR L1 2.2μH LTC3407-3 POWER-ON RESET SW1 SW2 VOUT1 VOUT2 C3 10μF R5 100k RUN1 VOUT1 1.8V 800mA C2 10μF GND 34073 TA03a C1, C2, C3: TAIYO YUDEN JMK212BJ106MD-B L1, L2: COILCRAFT LDO3310-222MX *IF C1 IS GREATER THAN 3" FROM POWER SOURCE, ADDITIONAL CAPACITANCE MAY BE REQUIRED. Efﬁciency vs Load Current 100 95 EFFICIENCY (%) 90 VOUT = 3.3V 85 80 VOUT = 1.
PAGE 15
LTC3407-3 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 16
LTC3407-3 TYPICAL APPLICATION 2mm Height Lithium-Ion Single Inductor Buck-Boost Regulator and a Buck Regulator VIN 2.8V TO 4.2V C1 10μF RUN2 VIN MODE/SYNC VOUT2 3.3V 200mA L2 10μH D1 C6 47μF POWER-ON RESET POR L1 2.2μH LTC3407-3 VOUT1 1.