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LTC3606B

3606bfb

TYPICAL APPLICATION

DESCRIPTION

800mA Synchronous

Step-Down DC/DC with

Average Input Current Limit

The LTC

3606B is an 800mA monolithic synchronous

buck regulator using a constant frequency current mode

architecture.

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, extending

the run time in battery-operated systems. Low output

voltages are supported with the 0.6V feedback reference

voltage. The LTC3606B can supply 800mA output current.

The LTC3606B’s programmable average input current limit

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

supplies because the LTC3606B’s limited input current

will still allow its output to deliver high peak load cur-

rents without collapsing the input supply. 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 LTC3606B is available

in an 8-Lead 3mm × 3mm DFN package.

Monolithic Buck Regulator with Input Current Limit

FEATURES

APPLICATIONS

Programmable Average Input Current Limit:

±5% Accuracy

Step-Down Output: Up to 96% Efﬁ ciency

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

Low Dropout Operation: 100% Duty Cycle

Internal Soft-Start

Current Mode Operation for Excellent Line and Load

Transient Response

±2% Output Voltage Accuracy

Short-Circuit Protected

Shutdown Current ≤ 1A

Available in Small Thermally Enhanced 8-Lead

3mm × 3mm DFN Package

High Peak Load Current Applications

USB Powered Devices

Supercapacitor Charging

Radio Transmitters and Other Handheld Devices

GSM Pulse Load

PGOOD

10µF

3.4V

TO 5.5V

499k

255k

1.5µH

2.2mF

SuperCap

116k

1000pF

LIM

= 475mA

OUT

3.4V AT

800mA

1210k

LTC3606B

RLIM

PGOOD

RUN

GND

3606B TA01

L, LT, LTC, LTM, 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

3606B TA01b

OUT

200mV/DIV

OUT

500mA/DIV

AC-COUPLED

1V/DIV

Summary of content (20 pages)

PAGE 1
LTC3606B 800mA Synchronous Step-Down DC/DC with Average Input Current Limit DESCRIPTION FEATURES n n n n n n n n n n n n n n Programmable Average Input Current Limit: ±5% Accuracy Step-Down Output: 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
LTC3606B ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW Input Supply Voltage (VIN) ........................... –0.3V to 6V VFB ................................................... –0.3V to VIN + 0.3V RUN, RLIM ....................................... –0.3V to VIN + 0.3V SW ................................................... –0.3V to VIN + 0.3V PGOOD............................................. –0.3V to VIN + 0.3V P-Channel SW Source Current (DC) (Note 2) ..............
PAGE 3
LTC3606B ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 25°C, VIN = 5V, unless otherwise noted. SYMBOL PARAMETER VIN VIN Operating Voltage Range CONDITIONS MIN VUV VIN Undervoltage Lockout IFB Feedback Pin Input Current VFBREG Feedback Voltage LTC3606BE, –40°C < TJ < 85°C (Note 7) LTC3606BI, –40°C < TJ < 125°C (Note 7) VLINEREG VFB Line Regulation VLOADREG l TYP MAX UNITS 5.
PAGE 4
LTC3606B TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, VIN = 5V, unless otherwise noted. Supply Current vs Temperature Pulse-Skipping Mode Operation 550 SW 2V/DIV 500 Efﬁciency vs Input Voltage 100 RUN = VIN ILOAD = 0A 90 80 IL 100mA/DIV 450 400 EFFICIENCY (%) SUPPLY CURRENT (μA) VIN = 5.5V VOUT 50mV/DIV ACCOUPLED VIN = 2.7V 350 70 60 50 30 300 IOUT = 10mA IOUT = 1mA IOUT = 0.1mA IOUT = 100mA IOUT = 400mA IOUT = 800mA 40 20 3606B G01 5μs/DIV VIN = 5V VOUT = 3.
PAGE 5
LTC3606B TYPICAL PERFORMANCE CHARACTERISTICS Efﬁciency vs Load Current 100 90 TA = 25°C, VIN = 5V, unless otherwise noted. Load Regulation Line Regulation 0.6 3.0 VOUT = 1.2V 2.5 0.4 80 70 60 50 40 30 10 0 0.0001 0.001 0.01 0.1 OUTPUT CURRENT (A) 1.5 1.0 0.5 0.2 0 –0.2 0 VIN = 2.7V VIN = 3.6V VIN = 4.2V VIN = 5V 20 VOUT ERROR (%) 2.0 VOUT ERROR (%) EFFICIENCY (%) VOUT = 1.8V ILOAD = 100mA –1.0 1 –0.4 VOUT = 1.8V VOUT = 2.5V VOUT = 3.3V –0.5 0 –0.6 2.
PAGE 6
LTC3606B PIN FUNCTIONS GND (Pins 1, 3, Exposed Pad Pin 9): Ground. Connect to the (–) terminal of COUT, and the (–) terminal of CIN. The Exposed Pad must be soldered to PCB. PGOOD (Pin 6): Open-Drain Logic Output. PGOOD is pulled to ground if the voltage on the VFB pin is not within power good threshold. RLIM (Pin 2): Average Input Current Limit Program Pin. Place a resistor and capacitor in parallel from this pin to ground. RUN (Pin 7): Regulator Enable.
PAGE 7
LTC3606B OPERATION The LTC3606B uses a constant-frequency, current mode architecture. The operating frequency is set at 2.25MHz. 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 LTC3606B continuously monitors the input current via the voltage drop across the RDS(ON) of the internal P-channel MOSFET.
PAGE 8
LTC3606B 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 9
LTC3606B OPERATION Figure 1a shows VIN (IIN) current below input current limit with a CLIM capacitor of 0.1μF. 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 LTC3606B begins to limit input current. The IIN current is not limited during this 7.5ms time and is more than 725mA.
PAGE 10
LTC3606B APPLICATIONS INFORMATION A general LTC3606B 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 11
LTC3606B 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 12
LTC3606B APPLICATIONS INFORMATION Setting the Output Voltage The LTC3606B regulates the VFB pin to 0.6V during regulation. Thus, the output voltage is set by a resistive divider, Figure 2, according to the following formula: VOUT = 0.6V 1+ R2 R1 (2) The output voltage settling behavior is related to the stability of the closed-loop system and will demonstrate the actual overall supply performance.
PAGE 13
LTC3606B APPLICATIONS INFORMATION 2. The switching current is the sum of the MOSFET driver and control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from VIN to ground. The resulting dQ/dt is a current out of VIN that is typically much larger than the DC bias current.
PAGE 14
LTC3606B APPLICATIONS INFORMATION should be routed away from noisy components and traces, such as the SW line (Pin 4), and their trace length should be minimized. PC Board Layout Considerations When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3606B. These items are also illustrated graphically in the layout diagrams of Figures 3a and 3b. Check the following in your layout: 4. Keep sensitive components away from the SW pin, if possible.
PAGE 15
LTC3606B APPLICATIONS INFORMATION VIA TO VOUT SENSE GND GND VFB RLIM RUN GND PGOOD SW VIN VIN SW GND VOUT Figure 3b.
PAGE 16
LTC3606B APPLICATIONS INFORMATION of CIN = 10μF should sufﬁce, if the source impedance is very low. Design Example As a design example, consider using the LTC3606B in a USB-GSM application, with VIN = 5V, IINMAX = 500mA, with the output charging a SuperCap of 4.4mF. The load requires 800mA in active mode and 1mA in standby mode. The output voltage VOUT = 3.4V. The feedback resistors program the output voltage. To maintain high efﬁciency at light loads, the current in these resistors should be kept small.
PAGE 17
LTC3606B APPLICATIONS INFORMATION 100 90 10 VOUT = 3.4V 1 70 60 0.1 50 40 30 POWER LOSS (W) EFFICIENCY (%) 80 0.01 20 VIN = 3.6V VIN = 4.2V VIN = 5V 10 0 0.0001 0.001 0.01 0.1 OUTPUT CURRENT (A) 0.001 1 3606B F04b Figure 4b. Efﬁciency vs Output Current VOUT 200mV/DIV VIN 1V/DIV AC-COUPLED IOUT 500mA/DIV IIN 500mA/DIV 1ms/DIV VIN = 5V, 500mA COMPLIANT RLIM = 116kΩ, CLIM = 2200pF ILOAD = 0A TO 2A, COUT = 4.4mF, VOUT = 3.4V ILIM = 475mA 3606B F04c Figure 4c.
PAGE 18
LTC3606B PACKAGE DESCRIPTION DD Package 8-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1698) 0.70 p0.05 3.5 p0.05 1.65 p0.05 2.10 p0.05 (2 SIDES) PACKAGE OUTLINE 0.25 p 0.05 0.50 BSC 2.38 p0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 p0.10 (4 SIDES) R = 0.125 TYP 5 0.40 p 0.10 8 1.65 p 0.10 (2 SIDES) PIN 1 TOP MARK (NOTE 6) (DD8) DFN 0509 REV C 0.200 REF 0.75 p0.05 4 0.25 p 0.05 1 0.50 BSC 2.38 p0.10 0.00 – 0.
PAGE 19
LTC3606B REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 3/10 Changes to Electrical Characteristics 3 B 8/12 Changed 750μs to 950μs in the Soft-Start section 7 3606bfb 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
LTC3606B TYPICAL APPLICATIONS 800mA Buck Converter, ILIM = 500mA L1 1.5μH VIN USB INPUT 5V VIN CIN 10μF RPGD 499k LTC3606B RUN PGOOD VFB RLIM PGOOD GND CLIM 1000pF VOUT 3.4V AT 800mA SW R2 1210k + COUT 2.2mF s2 SuperCap R1 255k RLIM 110k L1: COILCRAFT LPS4012-152ML CIN: AVX 08056D106KAT2A COUT: VISHAY 592D228X96R3X2T20H 3606B TA02 800mA Buck Converter, ILIM = 475mA or Disabled L1 1.