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LTC3605

3605fc

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

15V, 5A Synchronous

Step-Down Regulator

The LTC

3605 is a high efﬁciency, monolithic synchronous

buck regulator using a phase lockable controlled on-time

constant frequency, current mode architecture. PolyPhase

operation allows multiple LTC3605 regulators to run out of

phase while using minimal input and output capacitance.

The operating supply voltage range is from 15V down to

4V, making it suitable for dual lithium-ion battery inputs

as well as point of load power supply applications from

a 12V or 5V rail.

The operating frequency is programmable from 800kHz to

4MHz with an external resistor. The high frequency capabil-

ity allows the use of small surface mount inductors. For

switching noise sensitive applications, it can be externally

synchronized from 800kHz to 4MHz. The PHMODE pin

allows user control of the phase of the outgoing clock

signal. The unique constant frequency/controlled on-time

architecture is ideal for high step-down ratio applications

that are operating at high frequency while demanding

fast transient response. Two internal phase-lock loops

synchronize the internal oscillator to the external clock

and also servos the regulator on-time to lock on to either

the internal clock or the external clock if it’s present.

High Efﬁciency 1MHz, 5A Step-Down Regulator

High Efﬁciency: Up to 96%

5A Output Current

4V to 15V V

Range

Integrated Power N-Channel MOSFETs

(70mΩ Top and 35mΩ Bottom)

Adjustable Frequency 800kHz to 4MHz

PolyPhase

Operation (Up to 12 Phases)

Output Tracking

0.6V ±1% Reference Accuracy

Current Mode Operation for Excellent Line and Load

Transient Response

Shutdown Mode Draws Less Than 15µA Supply

Current

Available in 24-Pin (4mm × 4mm) QFN Package

Point of Load Power Supply

Portable Instruments

Distributed Power Systems

Battery-Powered Equipment

LTC3605

CLKOUT

CLKIN

PGOOD

RUN

INTV

BOOST

2.2µF

47µF

×2

3605 TA01a

OUT

3.3V

0.1µF

220pF

11.5k

2.55k

16k

162k

1µH

22µF

×2

4V TO 15V

ITH

PGND

Efﬁciency and Power Loss

OUTPUT CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (W)

100 1000 10000

3605 TA01b

100

0.1

= 12V

f = 1MHz

OUT

= 3.3V

OUT

= 3.3V

OUT

= 1.2V

OUT

= 1.2V

L, LT, LTC, LTM, PolyPhase, 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, 5847554, 6580258, 6304066, 6476589,

6774611.

Summary of content (22 pages)

PAGE 1
LTC3605 15V, 5A Synchronous Step-Down Regulator FEATURES n n n n n n n n n n n DESCRIPTION High Efficiency: Up to 96% 5A Output Current 4V to 15V VIN Range Integrated Power N-Channel MOSFETs (70mΩ Top and 35mΩ Bottom) Adjustable Frequency 800kHz to 4MHz PolyPhase® Operation (Up to 12 Phases) Output Tracking 0.
PAGE 2
LTC3605 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) SVIN BOOST INTVCC SGND CLKIN CLKOUT TOP VIEW PVIN, SVIN, SW Voltage............................... –0.3V to 15V SW Transient Voltage.................................. –2V to 17.5V BOOST Voltage........................... –0.3V to PVIN + INTVCC RUN Voltage.............................................. –0.3V to SVIN VON Voltage................................................ –0.3V to SVIN INTVCC Voltage.......................................
PAGE 3
LTC3605 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. SYMBOL PARAMETER CONDITIONS MIN PVIN VIN Supply Range IQ Input DC Supply Current Active Shutdown (Note 3) Mode = 0, RT = 162k VIN =12V, RUN = 0 VFB Feedback Reference Voltage ITH =1.2V (Note 4) –40°C to 85°C ITH =1.2V (Note 4) –40°C to 125°C l l DVFB(LINE) Feedback Voltage Line Regulation VIN = 4V to 15V, ITH = 1.
PAGE 4
LTC3605 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise specified. Quiescent Current vs VIN Shutdown Current vs VIN MODE = 0V NO LOAD 2.16 SHUTDOWN CURRENT (µA) QUIESCENT CURRENT (mA) 2.12 2.10 2.08 2.06 2.04 MODE = 3.3V NO LOAD 20 12 11 10 9 8 15 10 5 7 2.02 0 6 3 9 VIN (V) 12 6 15 3 0 6 9 VIN (V) 0 15 12 3605 G01 0 1.5 120 100 INTERNAL ITH COMPENSATION (ITH = 3.3V) 1.0 TOP FET NORMALIZED (%) 60 BOTTOM FET 40 20 5 0.5 4.
PAGE 5
LTC3605 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C unless otherwise specified. 300 96 200 150 50 86 0 4 0 8 12 ILOAD = 1A 84 20 16 ILOAD = 5A 0 3 6 9 VIN (V) Current Limit Foldback 101 120 100 0 15 12 2 4 8 6 VON (V) 10 14 RUN Pin Threshold vs Temperature TA = 25°C 1.25 98 RUN THRESHOLD (V) NORMALIZED INTVCC (%) 12 1.30 97 96 95 94 93 92 20 0 3605 G12 99 40 0.6 0.4 INTVCC Load Regulation 100 60 0.8 3605 G11 3605 G10 80 1.0 0.
PAGE 6
LTC3605 PIN FUNCTIONS RT (Pin 1): Oscillator Frequency Programming Pin. Connect an external resistor (between 200k to 40k) from RT to SGND to program the frequency from 800kHz to 4MHz. Since the synchronization range is ±30% of set frequency, be sure that the set frequency is within this percentage range of the external clock to ensure frequency lock. VON (Pin 9): On-Time Voltage Input. Voltage trip point for the on-time comparator.
PAGE 7
LTC3605 BLOCK DIAGRAM VOUT VON 100K 0.6V 3pF 9 3 MODE 6V 35pF SVIN 3.3V REG ION PLL-SYNC (±50%) CIN2 CIN 17-18 ION INTVCC OST 21 V tON = VON (0.64pF) IION VIN INTVCC R S BOOST Q 20 CB TG 12 x OSC RT 1 + PHMODE 2 RT CLKIN 24 + IREV ICMP – OSC PLL-SYNC (±30%) – RUN CLKOUT 11–16 SENSE– BG CVCC 10, 25 FOLDBACK DISABLED AT START-UP 3.3µA + 0µA TO 10µA M2 PGND 3pF 35pF COUT DB SENSE+ –3.3µA TO 6.
PAGE 8
LTC3605 OPERATION Main Control Loop The LTC3605 is a current mode monolithic step-down regulator. In normal operation, the internal top power MOSFET is turned on for a fixed interval determined by a one-shot timer, OST. When the top power MOSFET turns off, the bottom power MOSFET turns on until the current comparator, ICMP , trips, restarting the one-shot timer and initiating the next cycle. Inductor current is determined by sensing the voltage drop across the bottom power MOSFET ’s VDS.
PAGE 9
LTC3605 OPERATION Output Voltage Programming Output Power Good The output voltage is set by an external resistive divider according to the following equation: When the LTC3605’s output voltage is within the ±10% window of the regulation point, which is reflected back as a VFB voltage in the range of 0.54V to 0.66V, the output voltage is good and the PGOOD pin is pulled high with an external resistor. Otherwise, an internal open-drain pulldown device (12Ω) will pull the PGOOD pin low.
PAGE 10
LTC3605 OPERATION Minimum Off-Time and Minimum On-Time Considerations The minimum off-time, tOFF(MIN), is the smallest amount of time that the LTC3605 is capable of turning on the bottom power MOSFET, tripping the current comparator and turning the power MOSFET back off. This time is generally about 70ns. The minimum off-time limit imposes a maximum duty cycle of tON/(tON + tOFF(MIN)).
PAGE 11
LTC3605 OPERATION Using Ceramic Input and Output Capacitors Inductor Selection Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at the input and output.
PAGE 12
LTC3605 OPERATION inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Table 1. Inductor Selection Table INDUCTANCE DCR MAX CURRENT DIMENSIONS HEIGHT 6.7mm × 7mm 3mm Vishay IHLP-2525CZ-01 Series 0.33µH 4.1mW 18A 0.47µH 6.5mW 13.5A 0.68µH 9.4mW 11A 0.82µH 11.8mW 10A 1.0µH 14.
PAGE 13
LTC3605 OPERATION steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The initial output voltage step may not be within the bandwidth of the feedback loop, so the standard second order overshoot/DC ratio cannot be used to determine phase margin. The gain of the loop increases with the R and the bandwidth of the loop increases with decreasing C.
PAGE 14
LTC3605 OPERATION Thermal Considerations In a majority of applications, the LTC3605 does not dissipate much heat due to its high efficiency and low thermal resistance of its exposed-back QFN package. However, in applications where the LTC3605 is running at high ambient temperature, high VIN, high switching frequency and maximum output current load, the heat dissipated may exceed the maximum junction temperature of the part.
PAGE 15
LTC3605 OPERATION measure the junction temperature directly is to use the internal junction diode on one of the pins (PGOOD) to measure its diode voltage change based on ambient temperature change. First remove any external passive component on the PGOOD pin, then pull out 100µA from the PGOOD pin to turn on its internal junction diode and bias the PGOOD pin to a negative voltage.
PAGE 16
LTC3605 OPERATION Design Example As a design example, consider using the LTC3605 in an application with the following specifications: VIN = 10.8V to 13.2V, VOUT = 1.8V, IOUT(MAX) = 5A, IOUT(MIN) = 500mA, f = 2MHz Because efficiency is important at both high and low load current, discontinuous mode operation will be utilized. First select from the characteristic curves the correct RT resistor value for 2MHz switching frequency. Based on that RT should be 80.6k.
PAGE 17
LTC3605 TYPICAL APPLICATIONS 12V to 1.2V 1MHz Buck Regulator C1 2.2µF 0.1µF D1 10Ω 23 24 1 16.2k 2 3 4 5 12k 330pF 6 10pF 0.1µF 22 21 20 CLKIN CLKOUT SGND INTVCC BOOST SVIN RT PVIN PHMODE PVIN MODE SW LTC3605 FB SW TRACK/SS SW ITH SW RUN PGOOD 7 SVIN VON PGND 9 10 8 SW 11 CIN 22µF ×2 19 18 VIN 4V TO 15V 0.1µF 17 16 15 L1 0.68µH 14 4.99k COUT 47µF ×2 13 SW VOUT 1.2V 5A 4.
PAGE 18
LTC3605 TYPICAL APPLICATIONS 12V, 10A 2-Phase Single Output Regulator 0.1µF C1 2.2µF D1 10Ω 23 24 1 162k 2 3 4 5 6 16.2k 220pF 0.1µF 22 21 20 19 CLKIN CLKOUT SGND INTVCC BOOST SVIN RT PVIN PHMODE PVIN MODE 17 16 LTC3605 SW TRACK/SS SW ITH SW 10pF PGOOD 7 VON PGND 9 10 8 SVIN SW 11 VIN 4V TO 15V 0.1µF 18 SW FB RUN CIN1 22µF 15 L1 1µH 14 22.6k 13 COUT1 47µF VOUT 3.3V 10A 4.99k SW 12 100k PGND SGND 0.1µF C2 2.2µF D2 10Ω 24 1 162k 2 3 4 5 6 16.
PAGE 19
LTC3605 TYPICAL APPLICATIONS Dual Output Tracking Application C1 2.2µF 0.1µF D1 10Ω 23 24 1 162k 2 3 4 5 16.2k 100pF 6 10pF 0.1µF 22 21 20 CLKIN CLKOUT SGND INTVCC BOOST SVIN RT PVIN PHMODE PVIN MODE 18 TRACK/SS SW ITH SW PGOOD 7 VON PGND 9 10 8 SVIN1 SW 11 15 L1 0.33µH 14 7.5k COUT1 47µF 13 4.99k PGND SGND 0.1µF D2 1 162k 2 3 4 5 16.
PAGE 20
LTC3605 PACKAGE DESCRIPTION UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697 Rev B) 0.70 ±0.05 4.50 ± 0.05 2.45 ± 0.05 3.10 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 ± 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD R = 0.115 TYP 0.75 ± 0.05 PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER 23 24 PIN 1 TOP MARK (NOTE 6) 0.40 ± 0.10 1 2 2.45 ± 0.10 (4-SIDES) (UF24) QFN 0105 0.200 REF 0.00 – 0.05 0.25 ± 0.05 0.
PAGE 21
LTC3605 REVISION HISTORY (Revision history begins at Rev B) REV DATE DESCRIPTION B 10/09 Edit to Typical Application 1 Changes to Absolute Maximum Ratings 2 Add I-grade part to Order Information 2 Changes to Electrical Characteristics and Notes 3 Text changes to Pin Functions 6 Text changes 9 Edits to equations 14 Updates to Typical Application 17 C 10/12 PAGE NUMBER 3605fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
PAGE 22
LTC3605 TYPICAL APPLICATION –3.6V Negative Converter C1 2.2µF 0.1µF D1 CIN 22µF ×2 10Ω 23 24 1 162k 2 3 4 5 16.2k 6 470pF 47pF 0.1µF 22 21 20 CLKIN CLKOUT SGND INTVCC BOOST SVIN RT PVIN PHMODE PVIN MODE SW LTC3605 FB SW TRACK/SS SW ITH SW RUN PGOOD 7 SVIN VON PGND 9 10 8 SW 11 VIN 3V TO 10V 19 0.1µF 18 17 16 15 L1 1µH 14 13 SW 24.9k COUT 47µF ×2 4.99k 12 100k 3605 TA05 VOUT –3.6V 2A RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3602 10V, 2.