Datasheet

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LT1941

1941fb

TYPICAL APPLICATION

DESCRIPTION

Triple Monolithic

Switching Regulator

The LT

1941 is a triple current mode DC/DC converter

with internal power switches. Two of the regulators are

step-down converters with 3A and 2A power switches.

The third regulator can be conﬁ gured as a boost, inverter

or SEPIC converter and has a 1.5A power switch. All

three converters are synchronized to a 1.1MHz oscillator.

The two step-down converters run with opposite phase,

reducing input ripple current. The output voltages are

set with external resistor dividers and each regulator

has independent shutdown and soft-start circuits. Each

regulator generates a power good signal when its output

is in regulation, easing power supply sequencing and

interfacing with microcontrollers and DSPs.

The high switching frequency offers several advantages

by permitting the use of small inductors and ceramic

capacitors, leading to a very small triple output solution.

The constant switching frequency, combined with low

impedance ceramic capacitors, result in low, predictable

output ripple. With its wide input voltage range of 3.5V to

25V, the LT1941 regulates a broad array of power sources

from 4-cell batteries and 5V logic rails to unregulated wall

transformers, lead acid batteries and distributed-power

supplies.

L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other

trademarks are the property of their respective owners.

FEATURES

APPLICATIONS

Wide Input Range: 3.5V to 25V

Three Switching Regulators with Internal Power

Switches: 3A Step-Down, 2A Step-Down,

1.5A Inverting/Boost

Antiphase Switching Reduces Ripple

Independent Shutdown/Soft-Start Pins

Independent Power Good Indicators Ease Supply

Sequencing

Input Voltage Power Good Indicators Monitor Input

Supply

Uses Small Inductors and Ceramic Capacitors

Constant 1.1MHz Switching Frequency

Thermally Enhanced 28-Lead TSSOP Package

Cable Modems

DSL Modems

Distributed Power Regulation

Wall Transformer Regulation

Disk Drives

DSP Power

Figure 1. Triple Output Power Supply: 3.3V, 1.8V, –12V

Start-Up Waveforms

with Sequencing

OUT2

OUT1

LT1941

GND

0.22μF

22μF

1000pF

1.5nF

3300pF

13.7k

133k

2.49k10k

4.7V TO 14V

5GOOD

12GOOD

OUT1

1.8V

2.4A

OUT3

–12V

350mA*

3.3μH

3μH

22μH

10.7k

13.7k

7.32k 3.3k

1.5k

1941 F01

130k 100k 100k 100k 100k

PGOOD1

PGOOD2

PGOOD3

OUT2

3.3V

1.4A

0.22μF

33μF

1μF

10μF

*240mA AT V

= 5V, 550mA AT V

= 12V

10μF

22nF

PGOOD1

PGOOD2

PGOOD3

BOOST2

SW2

FB2

SW1

FB1

RUNSS2

BIAS1

BIAS2

RUNSS3

RUNSS1

SW3

NFB

FB3

5GOOD

12GOOD

BOOST1

1.5nF

RUN/SS

2V/DIV

2ms/DIV

OUT1

2V/DIV

OUT2

5V/DIV

OUT3

10V/DIV

VIN(AVE)

1A/DIV

PGOOD2

5V/DIV

1941 F01b

Summary of content (24 pages)

PAGE 1
LT1941 Triple Monolithic Switching Regulator FEATURES DESCRIPTION n The LT®1941 is a triple current mode DC/DC converter with internal power switches. Two of the regulators are step-down converters with 3A and 2A power switches. The third regulator can be conﬁgured as a boost, inverter or SEPIC converter and has a 1.5A power switch. All three converters are synchronized to a 1.1MHz oscillator. The two step-down converters run with opposite phase, reducing input ripple current.
PAGE 2
LT1941 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW VIN Pin ........................................................(–0.3V), 25V BOOST Pin Voltage ...................................................35V BOOST Above SW Pin ...............................................25V BIAS1, BIAS2 Pins ....................................................25V PGOOD, 5GOOD, 12GOOD Pins.................................25V RUN/SS, VC, FB, NFB Pins ..........................................
PAGE 3
LT1941 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN, VBIAS1, VBIAS2 = 5V, VBOOST1, VBOOST2 = 8V, unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS Switching Phase SW1 to SW2 SW1 to SW3 150 –30 180 0 210 30 Deg Deg Foldback Frequency VFB = 0V 200 RUN/SS Current RUN/SS Threshold 1 2 0.4 kHz 3 μA 0.6 V 5GOOD Threshold VIN Rising 4.
PAGE 4
LT1941 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN, VBIAS1, VBIAS2 = 5V, VBOOST1, VBOOST2 = 8V, unless otherwise noted. (Note 2) PARAMETER CONDITIONS MIN Switch 2 Leakage Current Minimum Boost Voltage Above Switch (Note 4) Maximum Duty Cycle TYP MAX 0.01 10 UNITS μA 1.8 2.5 V l 78 88 l 1.23 1.22 1.25 1.27 1.27 V V 800 1400 nA % 1.
PAGE 5
LT1941 TYPICAL PERFORMANCE CHARACTERISTICS Efﬁciency, VOUT1 = 1.8V 90 Efﬁciency, VOUT2 = 3.3V 90 VIN = 5V TA = 25°C 70 60 60 0 0.5 1.5 1 LOAD CURRENT (A) 2 50 0 0.25 1.25 0.5 0.75 1 LOAD CURRENT (A) 600 100 400 300 200 0.5 1 1.5 2 SWITCH CURRENT (A) 2.5 0 3 0.5 1 1.5 SWITCH CURRENT (A) 0 1941 G02 SW1 Current Limit vs Duty Cycle 200 2.5 CURRENT LIMIT (A) MINIMUM 2.5 2.0 1.5 1.0 0.25 0.5 0.75 1 1.25 SWITCH CURRENT (A) TYPICAL 2.0 1.
PAGE 6
LT1941 TYPICAL PERFORMANCE CHARACTERISTICS BOOST1 Pin Current VFB3 vs Temperature TA = 25°C BOOST CURRENT (mA) 40 0.645 1.265 0.635 VFB (V) 30 VFB1, VFB2 vs Temperature 1.280 VFB (V) 50 1.250 0.625 20 1.235 0.615 10 0 0.5 1 1.5 2 SW1 PIN CURRENT (A) 2.5 3 1.220 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 1941 G04 1.2 1.1 1.0 100 TA = 25°C 0.8 0.6 0.4 0 20 40 60 80 % OF FEEDBACK VOLTAGE 1941 G13 8.0 1.2 7.5 MINIMUM INPUT VOLTAGE (V) RUN/SS THRESHOLDS (V) 1.5 1.
PAGE 7
LT1941 PIN FUNCTIONS VIN (Pins 1, 2, 22, 25): The VIN pins supply current to the LT1941’s internal circuitry and to the internal power switches. These pins must be tied to the same source and locally bypassed. BIAS1 (Pin 15): The BIAS1 pin supplies the current to the LT1941’s internal regulator. Tie this pin to the lowest available voltage source above 2.35V (Either VIN, VOUT or any other available supply). SW1, SW2, SW3 (Pins 3, 4, 23, 27): The SW pins are the outputs of the internal power switches.
PAGE 8
LT1941 BLOCK DIAGRAM The LT1941 is a constant frequency, current mode, triple output regulator with internal power switches. The three regulators share common circuitry including input source, voltage reference and oscillator, but are otherwise independent. Operation can be best understood by referring to the Block Diagram. If the RUN/SS pins are tied to ground, the LT1941 is shut down and draws 50μA from the input source tied to VIN.
PAGE 9
LT1941 BLOCK DIAGRAM VIN 5GOOD + BIAS1 VIN RUN/SS1 4.5V 2μA INT REG AND REF One of Two Step-Down Switching Regulators RUN/SS2 RUN/SS3 CLK1 CLK2 CLK3 MASTER OSC – 12GOOD + 2μA 10.8V – VIN 2μA VIN CIN + 0.9V – + ¤ BOOST SLOPE – S C1 SLAVE OSC CLK D2 R Q C3 SW L1 OUT + – 0.35V FB R1 – VC ERROR AMP RC – RUN/SS CC + CF C1 D1 ILIMIT CLAMP + R2 0.628V 54mV 1.
PAGE 10
LT1941 APPLICATIONS INFORMATION STEP-DOWN CONSIDERATIONS FB Resistor Network The output voltage is programmed with a resistor divider (refer to the Block Diagram) between the output and the FB pin. Choose the resistors according to R1 = R2(VOUT/628mV – 1) R2 should be 10k or less to avoid bias current errors. Input Voltage Range The minimum operating voltage is determined either by the LT1941’s undervoltage lockout of ~3.3V or by its maximum duty cycle.
PAGE 11
LT1941 APPLICATIONS INFORMATION your load is lower than the maximum load current, then you can relax the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor or one with a lower DCR resulting in higher efﬁciency. Be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on input voltage.
PAGE 12
LT1941 APPLICATIONS INFORMATION You can estimate output ripple with the following equations: VRIPPLE = ΔIL/(8 • f • COUT) for ceramic capacitors and Table 2. Low ESR Surface Mount Capacitors VENDOR TYPE SERIES Taiyo-Yuden Ceramic AVX Ceramic Tantalum TPS VRIPPLE = ΔIL • ESR for electrolytic capacitors (tantalum and aluminum) Kemet Tantalum Tantalum Organic Aluminum Organic T491,T494,T495 T520 A700 where ΔIL is the peak-to-peak ripple current in the inductor.
PAGE 13
LT1941 APPLICATIONS INFORMATION Boost Pin Considerations The capacitor and diode tied to the BOOST pin generate a voltage that is higher than the input voltage. In most cases, a 0.18μF capacitor and fast switching diode (such as the CMDSH-3 or MMSD914LT1) will work well. Figure 3 shows four ways to arrange the boost circuit. The BOOST pin must be more than 2.5V above the SW pin for full efﬁciency. For outputs of 3.3V and higher, the standard circuit (Figure 3a) is best. For outputs between 2.8V and 3.
PAGE 14
LT1941 APPLICATIONS INFORMATION Converter with Backup Output Regulator Regulating Negative Output Voltages There is another situation to consider in systems where the output will be held high when the input to the LT1941 is absent. If the VIN and one of the RUN/SS pins are allowed to ﬂoat, then the LT1941’s internal circuitry will pull its quiescent current through its SW pin. This is acceptable if the system can tolerate a few mA of load in this state.
PAGE 15
LT1941 APPLICATIONS INFORMATION can handle at least 1.5A of current without saturating and ensure that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. If using uncoupled inductors, each inductor need only handle one-half of the total switch current so that 0.75A per inductor is sufﬁcient. A 4.7μH to 15μH coupled inductor or two 15μH to 20μH uncoupled inductors will usually be the best choice for most LT1941 inverter designs. A 4.
PAGE 16
LT1941 APPLICATIONS INFORMATION and is largest when VIN = 2 VOUT (50% duty cycle). As the second, lower power channel draws input current, the input capacitor’s RMS current actually decreases as the out-of-phase current cancels the current drawn by the higher power channel. The ripple current contribution from the third channel will be minimal. Considering that the maximum load current from a single channel is ~2.8A, RMS ripple current will always be less than 1.4A.
PAGE 17
LT1941 APPLICATIONS INFORMATION Table 5. Converter Equivalent Model Parameters STEP-DOWN1 STEP-DOWN2 BOOST INVERTER VFB 0.628V 0.628V 1.25 1.24 RO 500kΩ 500kΩ 500kΩ 500kΩ gma 1700μmho 1700μmho 800μmho 800μmho gmp 5mho 3.6mho VIN • 5mho VOUT VIN • 5mho –|VOUT| LT1941 VSW ERROR AMPLIFIER FB + VFB gma 500k GND VC RC OUTPUT R1 – CURRENT MODE POWER STAGE gmp CPL ESR C1 + C1 CF POLYMER R2 OR TANTALUM CERAMIC CC 1941 F05 Figure 5.
PAGE 18
LT1941 APPLICATIONS INFORMATION In Figure 6a, a larger capacitor on RUN/SS2 delays channel 2 with respect to channel 1. The soft-start capacitor on RUN/SS2 should be at least twice the value of the capacitor on RUN/SS1. A larger ratio may be required, depending on the output capacitance and load on each channel. Make sure to test the circuit in the system before deciding on ﬁnal values for these capacitors.
PAGE 19
LT1941 APPLICATIONS INFORMATION VIN VIN SW GND SW GND (a) (b) VSW VIN IC1 C1 L1 SW D1 GND C2 1941 F07 (c) Figure 7. Subtracting the Current when the Switch is ON (a) From the Current when the Switch is OFF (b) Reveals the Path of the High Frequency Switching Current (c) Keep This Loop Small. The Voltage on the SW and BOOST Nodes will also be Switched; Keep these Nodes as Small as Possible.
PAGE 20
LT1941 APPLICATIONS INFORMATION current capability of the third channel can be calculated from the output currents and voltages of the other channels, the switching regulator efﬁciency (η), the ambient temperature (TA), the maximum junction temperature (TJMAX) and the thermal resistance from junction to ambient (θJA) as follows: PDISS = TJMAX – TA θ JA Note that decreasing θJA increases the power output capability.
PAGE 21
LT1941 APPLICATIONS INFORMATION SLIC Power Supply –21.6V, –65V, 3.3V and 1.8V with Soft-Start VOUT1 VIN 5V R2 130k R1 100k 5GOOD 5GOOD 12GOOD R8 100k R9 100k PGOOD1 PGOOD1 PGOOD2 PGOOD2 12GOOD PGOOD3 D1 BOOST1 C1 0.22μF L1 3μH VOUT1 1.8V 2.4A R3 100k VIN VOUT2 R7 13.7k D3 C3 33μF R6 7.32k L3 2.7μH R15 1Ω C5 1μF 35V C6 10μF C12 3300pF R4 3.3k R13 178k C2 0.22μF LT1941 C14 1.5nF SW1 SW2 FB1 FB2 VC1 VC2 RUNSS1 RUNSS2 SW3 BIAS1 NFB BIAS2 R5 10.
PAGE 22
LT1941 TYPICAL APPLICATIONS Quadruple Output Power Supply: ±12V, 3.3V and 2.5V with Soft-Start VOUT1 VIN 5V R2 130k R1 100k 5GOOD 12GOOD C10 1000pF D3 C3 33μF R6 3.4k R4 10k C11 1.5nF PGOOD1 PGOOD2 PGOOD2 C9 D5 4.7μF 1Ω R13 118k C5 4.7μF SW2 C2 0.22μF FB1 FB2 VC1 VC2 SW3 BIAS1 NFB BIAS2 L2 3.3μH R12 10.7k C12 1000pF C13 1.5nF C4 22μF D4 R11 2.49k R10 10k VOUT2 3.3V 1.4A C14 6800pF VC3 D8 C7 10μF SW1 PGOOD3 D2 RUNSS1 RUNSS2 L3 10μH R5 13.
PAGE 23
LT1941 PACKAGE DESCRIPTION FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 2726 25 24 23 22 21 20 19 18 1716 15 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 SEE NOTE 4 0.45 ±0.05 EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 6.40 2.74 (.252) (.108) BSC 1.05 ±0.10 0.65 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1.
PAGE 24
LT1941 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1613 550mA (ISW), 1.4MHz, High Efﬁciency Step-Up DC/DC Converter VIN: 0.9V to 10V, VOUT(MAX) = 34V, IQ = 3mA, ISD < 1μA, ThinSOT™ Package LT1615/LT1615-1 300mA/80mA (ISW), High Efﬁciency Step-Up DC/DC Converter VIN: 1V to 15V, VOUT(MAX) = 34V, IQ = 20μA, ISD < 1μA, ThinSOT Package LT1617/LT1617-1 300mA/100mA (ISW), 1.2MHz/2.2MHz, High Efﬁciency Inverting DC/DC Converter VIN: 1.