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LTC3619B

3619bfb

Typical applicaTion

DescripTion

400mA/800mA Synchronous

Step-Down DC/DC with

Average Input Current Limit

The LT C

3619B is a dual monolithic synchronous buck

regulator using a constant frequency current mode ar-

chitecture.

The input supply voltage range is 2.5V to 5.5V, making it

ideal for Li-Ion and USB powered applications. 100% duty

cycle capability provides low dropout operation, extend-

ing the run time in battery-operated systems. Low output

voltages are supported with the 0.6V feedback reference

voltage. Channel 1 and channel 2 can supply 400mA and

800mA output current, respectively.

The LTC3619B’s programmable average input current limit

is ideal for USB applications and for point-of-load power

supplies because the LTC3619B’s limited input current

will still allow its output to deliver high peak load currents

without collapsing the input supply. When the sum of

both channels’ currents exceeds the input current limit,

channel 2 is current limited while channel 1 remains regu-

lated. The operating frequency is internally set at 2.25MHz

allowing the use of small surface mount inductors. Internal

soft-start reduces in-rush current during start-up. The

LTC3619B is available in small MSOP and 3mm × 3mm

DFN packages. The LTC

3619B is also available in a low

quiescent current, high efficiency Burst Mode

version,

LTC3619.

Dual Monolithic Buck Regulator in 10-Lead 3mm × 3mm DFN

FeaTures

applicaTions

Programmable Average Input Current Limit:

±5% Accuracy

Dual Step-Down Outputs: Up to 96% Efficiency

Low Noise Pulse-Skipping Operation at Light Loads

Input Voltage Range: 2.5V to 5.5V

Output Voltage Range: 0.6V to 5V

2.25MHz Constant-Frequency Operation

Power Good Output Voltage Monitor for Each Channel

Low Dropout Operation: 100% Duty Cycle

Independent Internal Soft-Start for Each Channel

Current Mode Operation for Excellent Line and Load

Transient Response

±2% Output Voltage Accuracy

Short-Circuit Protected

Shutdown Current ≤ 1μA

Available in Small Thermally Enhanced 10-Lead MS

and 3mm × 3mm DFN Packages

High Peak Load Current Applications

USB Powered Devices

Supercapacitor Charging

Radio Transmitters and Other Handheld Devices

GSM Pulse Load

RUN2 RUN1

LTC3619B

FB2

SW2

SW1

PGOOD1PGOOD2

FB1

22pF

RLIM GND

3.4V TO 5.5V

OUT2

3.4V AT

800mA

OUT1

1.8V AT

400mA

3619B TA01

1190k

511k

255k

1.5µH 3.3µH

2.2mF

×2

SuperCap

10µF

1000pF

116k

LIM

= 475mA

L, LT , LT C , LT M , Linear Technology, the Linear logo and Burst Mode are registered trademarks

and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners. Protected by U.S. Patents, including 5481178, 6127815,

6304066, 6498466, 6580258, 6611131.

1ms/DIV

= 5V, 500mA COMPLIANT

LOAD

= 0A to 2.2A, CHANNEL 1 UNLOADED

3619B TA01b

OUT

200mV/DIV

OUT

500mA/DIV

AC-COUPLED

1V/DIV

Summary of content (20 pages)

PAGE 1
Features n n n n n n n n n n n n n n Programmable Average Input Current Limit: ±5% Accuracy Dual Step-Down Outputs: Up to 96% Efficiency Low Noise Pulse-Skipping Operation at Light Loads Input Voltage Range: 2.5V to 5.5V Output Voltage Range: 0.6V to 5V 2.
PAGE 2
LTC3619B Absolute Maximum Ratings (Note 1) Input Supply Voltage (VIN).............................. –0.3 to 6V VFB1, VFB2......................................... –0.3V to VIN + 0.3V RUN1, RUN2, RLIM........................... –0.3V to VIN + 0.3V SW1, SW2......................................... –0.3V to VIN + 0.3V PGOOD1, PGOOD2............................ –0.3V to VIN + 0.3V P-channel SW Source Current (DC) (Note 2) Channel 1......................................................... 600mA Channel 2......
PAGE 3
LTC3619B Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 3) SYMBOL PARAMETER IFB Feedback Pin Input Current MAX UNITS ±30 nA VFBREG Feedback Voltage (Channels 1, 2) LTC3619BE, –40°C < TJ < 85°C (Note 7) LTC3619BI, –40°C < TJ < 125°C (Note 7) 0.600 0.600 0.612 0.618 V V ΔVLINEREG VFB Line Regulation VIN = 2.5V to 5.5V (Note 7) 0.01 0.
PAGE 4
LTC3619B Typical Performance Characteristics Supply Current vs Temperature Pulse-Skipping Mode Operation 900 SUPPLY CURRENT (µA) 800 VOUT 50mV/DIV ACCOUPLED IL 100mA/DIV VIN = 5V VOUT = 3.3V ILOAD = 5mA Efficiency vs Input Voltage 100 RUN1 = RUN2 = VIN ILOAD = 0A 90 80 VIN = 5.5V 700 600 EFFICIENCY (%) SW 2V/DIV 5µs/DIV TA = 25°C, VIN = 5V, unless otherwise noted. VIN = 2.7V 500 400 60 50 30 10 200 –50 0 25 50 75 TEMPERATURE (°C) –25 100 VOUT = 3.3V CHANNEL 2 0 3.
PAGE 5
LTC3619B Typical Performance Characteristics Efficiency vs Load Current Efficiency vs Load Current 100 100 80 80 80 70 70 70 VOUT = 3.3V 90 CHANNEL 2 60 50 40 30 20 0 0.0001 60 50 40 30 20 VIN = 3.6V VIN = 4.2V VIN = 5V 10 0.001 0.01 0.1 OUTPUT CURRENT (A) VOUT = 1.2V 90 CHANNEL 1 EFFICIENCY (%) EFFICIENCY (%) EFFICIENCY (%) Efficiency vs Load Current 100 VOUT = 3.3V 90 CHANNEL 1 10 0 0.0001 1 0.001 0.01 0.
PAGE 6
LTC3619B Typical Performance Characteristics Average Input Current Limit vs Temperature TA = 25°C, VIN = 5V, unless otherwise noted. Load Step (Channel 1) Load Step (Channel 1) 8 VIN = 5V 6 ILIM = 475mA VOUT 200mV/DIV AC-COUPLED IINLIM ERROR (%) 4 VOUT 200mV/DIV AC-COUPLED 2 0 –2 –4 IL 500mA/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV 20µs/DIV –6 –8 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 125 VIN = 5V, VOUT = 3.3V ILOAD = 0A TO 400mA CL = 4.
PAGE 7
LTC3619B Functional Diagram REGULATOR 1 + – 8 + – PGOOD1 0.654V VFB1 0.546V – VFB1 10 6 VIN SLOPE COMP ITH EA 0.6V MIN CLAMP – VSLEEP RS LATCH R + ICOMP + S SOFT-START – SLEEP + Q + Q SWITCHING LOGIC AND BLANKING CIRCUIT ICOMP – ANTI SHOOTTHRU 7 SW1 + IRCMP SHUTDOWN 11 GND – RUN1 RUN2 2 9 SLEEP2 0.6V REF SLEEP1 OSC TO REGULATOR 2 ONLY + – 3 RLIM 1V OSC MIN CLAMP 6 VIN SLOPE COMP 1 0.
PAGE 8
LTC3619B Operation The LTC3619B uses a constant-frequency, current mode architecture. The operating frequency is set at 2.25MHz. Both channels share the same clock and run in-phase. The output voltage is set by an external resistor divider returned to the VFB pins. An error amplifier compares the divided output voltage with a reference voltage of 0.6V and regulates the peak inductor current accordingly. The LTC3619B continuously monitors the input current of both channels.
PAGE 9
LTC3619B Operation Short-Circuit Protection Programming Input Current Limit When either regulator output is shorted to ground, the corresponding internal N-channel switch is forced on for a longer time period for each cycle in order to allow the inductor to discharge, thus preventing inductor current runaway. This technique has the effect of decreasing switching frequency. Once the short is removed, normal operation resumes and the regulator output will return to its nominal voltage.
PAGE 10
LTC3619B Operation Figure 1a shows VIN (IIN) current below input current limit with a CLIM capacitor of 0.1µF. Channel 1 is unloaded to simplify calculations. When the load pulse is applied, under the specified condition, ILIM current is 1.1A/55k • 0.66 = 13.2µA, where 0.66 is the duty cycle. It will take a little more than 7.5ms to charge the CLIM capacitor from 0V to 1V, after which the LTC3619B begins to limit input current. The IIN current is not limited during this 7.5ms time and is more than 725mA.
PAGE 11
LTC3619B Applications Information A general LTC3619B 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 12
LTC3619B Applications Information Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave with a duty cycle of approximately VOUT / VIN . To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maximum RMS current must be used.
PAGE 13
LTC3619B Applications Information Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to DILOAD • ESR, where ESR is the effective series resistance of COUT. DILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value.
PAGE 14
LTC3619B Applications Information 4. Other “hidden” losses, such as copper trace and internal battery resistances, can account for additional efficiency degradations in portable systems. It is very important to include these “system” level losses in the design of a system. The internal battery and fuse resistance losses can be minimized by making sure that CIN has adequate charge storage and very low ESR at the switching frequency.
PAGE 15
LTC3619B Applications Information VIN 2.5V TO 5.5V RUN2 VIN RUN1 C1 PGOOD2 PGOOD1 L2 SW2 CF2 VOUT2 R4 COUT2 L1 SW1 VFB1 VFB2 RLIM R3 RLIM VOUT1 CF1 LTC3619B GND R1 R2 COUT1 CLIM 3619B F03a BOLD LINES INDICATE HIGH CURRENT PATHS Figure 3a.
PAGE 16
LTC3619B Applications Information Design Example As a design example, consider using the LTC3619B in a USB-GSM application, with VIN = 5V, IINMAX = 500mA, with the output of channel 2 charging a SuperCap of 4.4mF. The load on each channel requires a maximum of 400mA and 800mA in active mode and 2mA in standby mode. The output voltages are VOUT1 = 1.8V and VOUT2 = 3.4V. Start with channel 1. First, calculate the inductor value for about 40% ripple current (160mA in this example) at maximum VIN.
PAGE 17
LTC3619B Applications Information 80 1 60 0.1 50 40 30 0.01 20 10 0 0.0001 1 70 60 0.1 50 40 30 0.01 20 VIN = 3.6V VIN = 4.2V VIN = 5V 10 VOUT = 1.8V 0.001 0.01 0.1 OUTPUT CURRENT (A) 10 VOUT = 3.4V 1 0.001 POWER LOSS (W) 70 90 POWER LOSS (W) EFFICIENCY (%) 80 100 10 VIN = 2.7V VIN = 3.6V VIN = 4.2V VIN = 5V 90 EFFICIENCY (%) 100 0 0.0001 0.001 0.01 0.1 OUTPUT CURRENT (A) 1 0.001 3619B F04b Figure 4b.
PAGE 18
LTC3619B Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) R = 0.125 TYP 6 0.40 ±0.10 10 0.70 ±0.05 PIN 1 TOP MARK (SEE NOTE 6) 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.25 ±0.05 3.00 ±0.10 (4 SIDES) 1.65 ±0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.
PAGE 19
LTC3619B Revision History REV DATE DESCRIPTION A 4/10 Changes to Temperature Range in Order Information section 2 Updates in the Electrical Characteristics table 3 B 10/12 PAGE NUMBER Edit on y-axis on graph G18 5 Updated DD package drawing 18 Clarified Load Step on Typical Performance Characteristics curves 6 Modified soft-start timing in Soft-Start section 8 Clarified device orientation on Suggested Layout 15 3619bfb Information furnished by Linear Technology Corporation is believed
PAGE 20
LTC3619B Typical Applications Dual 400mA/800mA Buck Converter, ILIM = 500mA VIN 3.3V TO 5.5V C1 10µF RUN2 VIN RUN1 L2 1.5µH VOUT2 3.3V AT 800mA R4 1150k + COUT2 2.2mF ×2 SuperCap PGOOD2 PGOOD1 L1 3.3µH LTC3619B SW2 SW1 VFB2 R3 RLIM 255k RLIM 110k CF1, 22pF VFB1 R2 R1 255k 511k GND VOUT1 1.