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LTC1702A

1702afa

Dual 550kHz Synchronous

2-Phase Switching Regulator Controller

The LTC

1702A is a dual switching regulator controller

optimized for high efficiency with low input voltages. It

includes two complete, on-chip, independent switching regu-

lator controllers each designed to drive a pair of external N-

channel MOSFETs in a voltage mode feedback, synchronous

buck configuration. The LTC1702A uses a constant-fre-

quency, true PWM design switching at 550kHz, minimizing

external component size and cost and maximizing load

transient performance. The synchronous buck architecture

automatically shifts to discontinuous and then to Burst

Mode

operation as the output load decreases, ensuring

maximum efficiency over a wide range of load currents.

The LTC1702A features an onboard reference trimmed to

0.5% and can provide better than 1% regulation at the

converter outputs. Open-drain logic outputs indicate whether

either output has risen to within 5% of the final output voltage

and an optional latching FAULT mode protects the load if the

output rises 15% above the intended voltage. Each channel

can be enabled independently; with both channels disabled,

the LTC1702A shuts down and supply current drops below

100µA.

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Pin Compatible with the LTC1702 with Improved

Current Limit Control

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Two Sides Run Out-of-Phase to Minimize C

■

No External Current Sense Resistors

■

Excellent Output Regulation: 1% Total Output

Accuracy

■

Two Independent Controllers in One Package

■

All N-Channel External MOSFET Architecture

■

550kHz Switching Frequency Minimizes External

Component Size

■

1A to 25A Output Current per Channel

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High Efficiency over Wide Load Current Range

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Less Than 100µA Shutdown Current

■

Small 24-Pin Narrow SSOP Package

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

Burst Mode is a registered trademark of Linear Technology Corporation. All other trademarks

are the property of their respective owners. Protected by U.S. Patents including 6144194.

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Microprocessor, DSP Core and I/O Supplies

■

Multiple Logic Supply Generator

■

Distributed Power Applications

■

High Efficiency Power Conversion

Dual Output High Power 3.3V/2.5V Logic Supply

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATIO

LTC1702A

BOOST1

BG1

TG1

SW1

MAX1

PGOOD1

FCB

RUN/SS

COMP1

SGND

FB1

MAX2

BOOST2

BG2

TG2

SW2

PGND

PGOOD2

FAULT

RUN/SS2

COMP2

FB2

1µF

10Ω

1µF

11.8k

1.6k

68k

1702 TA01

1.2k

10k

OUT1

180µF

×4

820pF

27pF

680pF

47k

1µF

OUT1

2.5V

AT 15A

1µF

4.75k

Q2Q1

Q4Q3

OUT1

, C

OUT2

: PANASONIC EEFUE0G181R

: KEMET TS10X337M010AS

D1, D2: MOTOROLA MBR0520LT1

D3, D4: MOTOROLA MBRS320T3

L1, L2: SUMIDA CEP125-1R0

Q1 TO Q8: FAIRCHILD FDS6670A

PWRGD1

1µH

10µF

1µF

= 5V ±10%

1µF

330µF

×3

680pF

27pF

3300pF

15.8k

4.99k

10k

1µH

OUT2

180µF

×4

1µF

OUT2

3.3V

AT 15A

PWRGD2

FAULT

10k

Summary of content (36 pages)

PAGE 1
LTC1702A Dual 550kHz Synchronous 2-Phase Switching Regulator Controller DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Pin Compatible with the LTC1702 with Improved Current Limit Control Two Sides Run Out-of-Phase to Minimize CIN No External Current Sense Resistors Excellent Output Regulation: 1% Total Output Accuracy Two Independent Controllers in One Package All N-Channel External MOSFET Architecture 550kHz Switching Frequency Minimizes External Component Size 1A to 25A Output Current per Channel High Eff
PAGE 2
LTC1702A U W U U W W W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) TOP VIEW Supply Voltage VCC ........................................................................................... 7V BOOSTn ............................................................... 15V BOOSTn – SWn .................................................... 7V Input Voltage SWn .......................................................... – 1V to 8V All Other Inputs ......................... – 0.3V to VCC + 0.
PAGE 3
LTC1702A ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = 5V unless otherwise specified. (Note 3) SYMBOL PARAMETER VFCB FCB Threshold ∆VFCB FCB Feedback Hysteresis IFCB FCB Pin Current ● VRUN RUN/SS Pin RUN Threshold ● ISS Soft-Start Source Current RUN/SSn = 0V Maximum Soft-Start Sink Current VIMAX = 0V, VSW = – 0.
PAGE 4
LTC1702A U W TYPICAL PERFOR A CE CHARACTERISTICS Efficiency vs Load Current VIN = 5V VOUT = 2.5V 90 35 VIN = 5V VOUT = 1.8V ILOAD = 0A-10A-0A ±2.2% MAX DEVIATION VOUT = 3.3V VOUT = 1.
PAGE 5
LTC1702A U U U PIN FUNCTIONS PVCC (Pin 1): Driver Power Supply Input. PVCC provides power to the two BGn output drivers. PVCC must be connected to a voltage high enough to fully turn on the external MOSFETs QB1 and QB2. PVCC should generally be connected directly to VIN. PVCC requires at least a 1µF bypass capacitor directly to PGND. BOOST1 (Pin 2): Controller 1 Top Gate Driver Supply. The BOOST1 pin supplies power to the floating TG1 driver. BOOST1 should be bypassed to SW1 with a 1µF capacitor.
PAGE 6
LTC1702A U U U PIN FUNCTIONS FB1 (Pin 12): Controller 1 Feedback Input. FB1 should be connected through a resistor network to VOUT1 to set the output voltage. The loop compensation network for controller 1 also connects to FB1. turning on the bottom MOSFET switches and pulling down the high output voltage. The LTC1702A will remain latched in this state until the power is cycled. When FAULT mode is active, the FAULT pin will be pulled up with an internal 10µA current source.
PAGE 7
LTC1702A TEST CIRCUIT Test Circuit 1 5V 0.1µF IBOOST1 ICC VCC BOOST1 fOSC MEASURED 5V CL CL 10k IBOOST2 TG2 BG1 BG2 SW1 SW2 IMAX1 IMAX2 CL LTC1702A PGOOD2 RUN/SS1 RUN/SS2 FB2 GND 5V VPGOOD2 VFAULT 2k COMP2 FB1 VFB1 CL 10k PGOOD1 COMP1 100µF BOOST2 FAULT 2k + PVCC TG1 FCB VPGOOD1 IPVCC VFB2 PGND 1702A TC U W U U APPLICATIONS INFORMATION OVERVIEW The LTC1702A is a dual, step-down (buck), voltage mode feedback switching regulator controller.
PAGE 8
LTC1702A U W U U APPLICATIONS INFORMATION 2-Step Conversion “2-step” architectures use a primary regulator to convert the input power source (batteries or AC line voltage) to an intermediate supply voltage, often 5V. This intermediate voltage is then converted to the low voltage, high current supplies required by the system using a secondary regulator— the LTC1702A.
PAGE 9
LTC1702A U W U U APPLICATIONS INFORMATION remember that the power losses are proportional to IRMS2, meaning that the actual power wasted is reduced by a factor of 3.75. The reduced input ripple voltage also means less power is lost in the input power path, which could include batteries, switches, trace/connector resistances and protection circuitry. Improvements in both conducted and radiated EMI also directly accrue as a result of the reduced RMS input current and voltage.
PAGE 10
LTC1702A U W U U APPLICATIONS INFORMATION amp of continuous current with peak currents up to 5A to slew large MOSFET gates quickly. The external MOSFETs are connected with the drain of QT attached to the input supply and the source of QT at the switching node SW. QB is the synchronous rectifier with its drain at SW and its source at PGND. SW is connected to one end of the inductor, with the other end connected to VOUT. The output capacitor is connected from VOUT to PGND.
PAGE 11
LTC1702A U W U U APPLICATIONS INFORMATION Feedback Amplifier PGOOD Flags Each side of the LTC1702A senses the output voltage at VOUT with an internal feedback op amp (see Block Diagram). This is a real op amp with a low impedance output, 85dB open-loop gain and 25MHz gain-bandwidth product. The positive input is connected internally to an 800mV reference, while the negative input is connected to the FB pin.
PAGE 12
LTC1702A U W U U APPLICATIONS INFORMATION SHUTDOWN/SOFT-START Each half of the LTC1702A has a RUN/SS pin. The RUN/SS pins perform two functions: when pulled to ground, each shuts down its half of the LTC1702A, and each acts as a conventional soft-start pin, enforcing a maximum duty cycle limit proportional to the voltage at RUN/SS. An internal 3.5µA current source pull-up is connected to each RUN/SS pin, allowing a soft-start ramp to be generated with a single external capacitor to ground. The 3.
PAGE 13
LTC1702A U W U U APPLICATIONS INFORMATION Any time QB is on and the current flowing to the output is reasonably large, the SW node at the drain of QB will be somewhat negative with respect to PGND. The LTC1702A senses this voltage and inverts it to allow it to compare the sensed voltage with a positive voltage at the IMAX pin. The IMAX pin includes a trimmed 10µA pull-up, enabling the user to set the voltage at IMAX with a single resistor, RIMAX, to ground.
PAGE 14
LTC1702A U W U U APPLICATIONS INFORMATION LTC1702A remains in continuous mode. See the Inductor Selection section for a detailed description of ripple current. As the output load current decreases in continuous mode, the average current in the inductor will reach a point where it drops below half the ripple current. At this point, the inductor current will reverse during a portion of the switching cycle, or begin to flow from the output back to the input.
PAGE 15
LTC1702A U W U U APPLICATIONS INFORMATION Burst Mode Operation Discontinuous mode removes the resistive loss drop term in QB, but the LTC1702A is still switching QT and QB on and off once a cycle. Each time an external MOSFET is turned on, the internal driver must charge its gate to VCC. Each time it is turned off, that charge is lost to ground. At the high switching frequencies that the LTC1702A operates at, the charge lost to the gates can add up to tens of milliamps from VCC.
PAGE 16
LTC1702A U W U U APPLICATIONS INFORMATION remains in this state until both RUN/SS pins are pulled low simultaneously, the power supply is recycled, or the FAULT pin is pulled low externally. This behavior is intended to protect a potentially expensive load from overvoltage damage at all costs. Under some conditions, this behavior can cause the output voltage to undershoot below ground.
PAGE 17
LTC1702A U W U U APPLICATIONS INFORMATION where the charge is going, the fact remains that it all has to come out of VCC to turn the MOSFET gate on, and when the MOSFET is turned back off, that charge all ends up at ground. In the meanwhile, it travels through the LTC1702A’s gate drivers, heating them up. More power lost! In this case, the power is lost in little bite-sized chunks, one chunk per switch per cycle, with the size of the chunk set by the gate charge of the MOSFET.
PAGE 18
LTC1702A U W U U APPLICATIONS INFORMATION The input bypass capacitor in an LTC1702A circuit is common to both channels. Consider our 10A example case with the other side of the LTC1702A disabled. The input bypass capacitor gets exercised in three ways: its ESR must be low enough to keep the initial drop as QT turns on within reason (100mV or so); its RMS current capability must be adequate to withstand the 4.
PAGE 19
LTC1702A U U W U APPLICATIONS INFORMATION from CIN (time point A). 50% of the way through, TG2 turns on and the total current is 13A (time point B). Shortly thereafter, TG1 turns off and the current drops to 10A (time point C). Finally, TG2 turns off and the current spends a short time at 0 before TG1 turns on again (time point D). ( ) ( ) (10A • 0.16) + (0A • 0.18) = 5.18A IAVG = 3A • 0.5 + 13A • 0.16 + Now we can calculate the RMS current.
PAGE 20
LTC1702A U W U U APPLICATIONS INFORMATION INDUCTOR 32% 10A 68% Q1 CURRENT, SIDE 1 ONLY (FOR 1-PHASE, 2 SIDES: MULTIPLY CURRENT BY 2) 0 32% 6.8A 0 –3.2A 68% CURRENT IN CIN, SIDE 1 ONLY ICIN = 4.66ARMS, (1-PHASE, 2 SIDES: ICIN = 9.3ARMS) Q11 CURRENT Q21 CURRENT BOTH SIDES EQUAL LOAD 2-PHASE OPERATION 0 32% 18% 32% 18% 3.6A 0 tON(Q2) (VOUT ) L In our hypothetical 1.
PAGE 21
LTC1702A U U W U APPLICATIONS INFORMATION output, with the attendant 180° phase shift. This roll-off is what filters the PWM waveform, resulting in the desired DC output voltage, but the phase shift complicates the loop compensation if the gain is still higher than unity at the pole frequency.
PAGE 22
LTC1702A U U W U APPLICATIONS INFORMATION Type 2 loops work well in systems where the ESR zero in the LC roll-off happens close to the LC pole, limiting the total phase shift due to the LC. The additional phase compensation in the feedback amplifier allows the 0dB point to be at or above the LC pole frequency, improving loop bandwidth substantially over a simple type 1 loop.
PAGE 23
LTC1702A U U W U APPLICATIONS INFORMATION 5V 10Ω MBR0530T + CIN + 10µF VCC PVCC BOOST2 VCOMP TO ANALYZER 0.1µF NC AC SOURCE FROM ANALYZER RB 1µF TG 1/2 LTC1702A COMP SW QT FB QB RUN/SS 10k BG LEXT + VOUT TO ANALYZER COUT FCB FAULT SGND PGND BOOST = – (PHASE + 30°) 1702A F12 Figure 12. Modulator Gain/Phase Measurement Set-Up If breadboard measurement is not practical, a SPICE simulation can be used to generate approximate gain/ phase curves.
PAGE 24
LTC1702A U W U U APPLICATIONS INFORMATION Type 3 Loop: ⎞ ⎛ BOOST K = Tan2 ⎜ + 45°⎟ ⎠ ⎝ 4 1 C2 = 2πƒGR1 C1 = C2 K – 1 ( ) R2 = K 2πƒC1 R1 R3 = (K – 1) C3 = 1 RB = chosen properly. The CF term is an approximate factor that corrects for errors caused by ringing on the switch node (illustrated in Figure 6). This correction factor will change depending on the layout and the components used, but –10mV is usually a good starting point.
PAGE 25
LTC1702A U W U U APPLICATIONS INFORMATION POWER GOOD/FAULT FLAGS VIN VOUT(AUX) + + CIN TG COUT(AUX) QT LTC1702A FCB BG + QB VOUT COUT RFCB1 RFCB2 1702A F08 Figure 13. Regulating an Auxiliary Output with the FCB Pin be added, either unregulated or with additional external circuitry at the FCB pin. The extra auxiliary output is taken from a second winding on the core of the inductor on one channel, converting it into a transformer (Figure 13).
PAGE 26
LTC1702A U U W U APPLICATIONS INFORMATION OPTIMIZING PERFORMANCE either system can generate it from the 3.3V output using an LDO linear regulator, without the 75% linear efficiency making much of an impact on total system efficiency. 2-Step Conversion The LTC1702A is ideally suited for use in 2-step conversion systems. 2-step systems use a primary regulator to convert the input power source (batteries or AC line voltage) to an intermediate supply voltage, often 5V.
PAGE 27
LTC1702A U W U U APPLICATIONS INFORMATION 2-Step Efficiency Calculation Maximizing High Load Current Efficiency Calculating the efficiency of a 2-step converter system involves some subtleties. Simply multiplying the efficiency of the primary 5V or 3.3V supply by the efficiency of the 1.8V or 1.5V supply underestimates the actual efficiency, since a significant fraction of the total power is drawn from the 3.3V and 5V rails in a typical system.
PAGE 28
LTC1702A U W U U APPLICATIONS INFORMATION Maximizing Low Load Current Efficiency Low load current efficiency depends strongly on proper operation in discontinuous and Burst Mode operations. In an ideally optimized system, discontinuous mode reduces conduction losses but not switching losses, since each power MOSFET still switches on and off once per cycle.
PAGE 29
LTC1702A U U W U APPLICATIONS INFORMATION cycles. Over just a few cycles, however, the external components conspire to limit the speed that the output can move. Consider our typical 5V to 1.6V circuit, subjected to a 1A to 5A load transient. Initially, the loop is in regulation and the DC current in the output capacitor is zero. Suddenly, an extra 4A start flowing out of the output capacitor while the inductor is still supplying only 1A.
PAGE 30
LTC1702A U W U U APPLICATIONS INFORMATION Making L smaller than this optimum value yields little or no improvement in transient response. As the output voltage recovers, the inductor current will briefly rise above the level of the output current to replenish the charge lost from the output capacitor. With a properly compensated loop, the entire recovery time will be inside of 10µs.
PAGE 31
LTC1702A U W U U APPLICATIONS INFORMATION it goes out of spec. Similarly, at full load, the output current can only decrease, causing a positive shift in the output voltage; the initial low value allows it to rise further before the spec is exceeded. The primary benefit of voltage positioning is it increases the allowable ESR of the output capacitors, saving cost. An additional bonus is that at maximum load, the output voltage is near the minimum allowable, decreasing the power dissipated in the load.
PAGE 32
LTC1702A U W U U APPLICATIONS INFORMATION FAULT BEHAVIOR Changing the Output Voltage on the Fly Some applications use a switching scheme attached to the feedback resistors to allow the system to adjust the LTC1702A output voltage. The voltage can be changed on the fly if desired, but care must be taken to avoid tripping the overvoltage fault circuit.
PAGE 33
C11 330pF COUT1 470µF ×2 VOUT1 1.8V 10A + R21, 13k R11 10k 0.1% Q21 Q11 CSS1 0.1µF RIMAX1, 15k D2 MBR330T CCP1 1µF 1µF CIN, COUT1, COUT2: KEMET T510X477M006AS C21 680pF RB1 7.96k 0.1% C31 560pF R31, 4.7k LEXT1 1µH 12A DCP1 MBR0530T RUN/SS1 GND CCP2 1µF CSS2 0.1µF + CIN 470µF ×2 LEXT1: MURATA LQT12535C1ROM12 LEXT2: COILTRONICS UP2B-2R2 RUN/SS2 PGND FB2 COMP2 COMP1 FB1 IMAX2 IMAX1 BG2 SW2 TG2 RIMAX2, 28.
PAGE 34
100k 10k 10k 1% VIN 4.75k 0.1% 0.01µF 0.01µF 0.1µF PGOOD 10k 4.7k 0.1µF IMAX1 RUN/SS1 FAULT SGND FCB BG2 PGND BG1 IMAX2 SW2 PGOOD2 RUN/SS2 BOOST2 SW1 BOOST1 TG2 TG1 + 15k 15k 1µF D1 0.1µF 0.1µF 10Ω CIN2 100µF PGOOD1 COMP2 FB2 COMP1 FB1 PVCC LTC1702A VCC CVCC 10µF 10Ω LT1006 + 34 – 1µF 10k 10k D3 D2 CIN1 10µF ×2 QB1A QT1A 0.004Ω 0.5W QB1B QT1B 0.004Ω 0.
PAGE 35
LTC1702A U PACKAGE DESCRIPTION GN Package 24-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .337 – .344* (8.560 – 8.738) 24 23 22 21 20 19 18 17 16 15 1413 .033 (0.838) REF .045 ±.005 .229 – .244 (5.817 – 6.198) .254 MIN .150 – .157** (3.810 – 3.988) .150 – .165 1 .0165 ± .0015 2 3 4 5 6 7 8 9 10 11 12 .0250 BSC RECOMMENDED SOLDER PAD LAYOUT .015 ± .004 × 45° (0.38 ± 0.10) .0075 – .0098 (0.19 – 0.25) .0532 – .0688 (1.35 – 1.75) .004 – .0098 (0.102 – 0.
PAGE 36
LTC1702A U TYPICAL APPLICATION Low Cost Dual Supply with 2.5V Keepalive VIN 5V ±10% IN + 10Ω 4.75k 0.1% 10k 0.1% + 10µF VCC OUT LT1761 16.2k 0.1% GND ADJ MBR0530T 330pF 68k 1µF CIN PVCC FB2 16.9k 0.1% BOOST2 56pF 1µF QT2 TG2 COMP2 VOUT2 2.5V/7A 2.45V/100mA STANDBY L2 SW2 RUN/SS2 RUN/SS1 1k QSS1 QSS2 STBY/ON 0.1µF BG2 36.5k + QB2 COUT2 IMAX2 FAULT LTC1702A BOOST1 1k 20k 1% 56k 220pF MBR0530T 1µF QT1B FB1 TG1 39pF COMP1 QT1A BG1 FCB 16k 1% L1 VOUT1 1.