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LT3988

3988f

Typical applicaTion

FeaTures DescripTion

Dual 60V Monolithic 1A

Step-Down Switching

Regulator

The LT

3988 is a dual, current mode, step-down, DC/DC

converter that accepts two input voltages up to 60V (80V

transient), which may be driven from separate supplies or

can be cascaded. High efficiency switches are included on

the die along with internal boost diodes and loop compensa-

tion. Both converters are capable of generating 1A outputs,

are synchronized to a single oscillator programmable up

to 2.5MHz, and run with opposite phases, reducing input

ripple current.

The switching frequency is set with a single resistor yield-

ing a range of 250kHz to 2.5MHz, or a clock signal can

be applied to the SYNC pin. The LT3988’s high switching

frequency allows the use of small inductors and capacitors,

resulting in a very small dual output supply. The constant

switching frequency, combined with low impedance ce-

ramic capacitors, results in low, predictable output ripple.

A current mode PWM architecture provides fast transient

response with cycle-by-cycle current limiting. Diode current

sense and thermal shutdown provide additional protection.

The LT3988 is available in a 16-lead MSOP package with

an exposed pad for low thermal resistance.

Efficiency

applicaTions

Wide Input Range:

Operation from 4.1V to 60V

Overvoltage Lockout Protects Circuit Through

80V Transients

Two 1A Output Switching Regulators with Internal

Power Switches

Short Circuit Robust

Adjustable 250kHz to 2.5MHz Switching Frequency,

Synchronizable Over the Full Range

Integrated Boost Diodes

Integrated Loop Compensation

Anti-Phase Switching Reduces Ripple

Low Shutdown I

(<2µA)

Uses Small Inductors and Ceramic Capacitors

Thermally Enhanced, 16-Lead MSOP Package

Commercial Vehicle Battery Regulation

Industrial Supplies

Distributed Supply Regulation

L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks

of Linear Technology Corporation. All other trademarks are the property of their respective

owners.

OUT

(A)

0 0.2 0.4 0.6 0.80.1 0.3 0.5 0.7 0.9

EFFICIENCY (%)

3988 TA01b

OUT

= 3.3V

= 12V

= 500kHz

OUT

= 5V

1000pF

57.6k 34k

22µH

7V TO 60V

TRANSIENT

TO 80V

OUT1

5V, 1A

OUT2

3.3V, 1A

0.22µF

47µF

3988 TA01

4.7µF

10.2k

1000pF

15µH

0.22µF

47µF

10k

22pF

FB1

DA1

SW1

BOOST1

SYNC

TRACK/SS1

FB2

DA2

SW2

BOOST2

TRACK/SS2EN/UVLO

GND

IN1

IN2

LT3988

200k

Summary of content (24 pages)

PAGE 1
LT3988 Dual 60V Monolithic 1A Step-Down Switching Regulator Description Features n n n n n n n n n n Wide Input Range: Operation from 4.1V to 60V Overvoltage Lockout Protects Circuit Through 80V Transients Two 1A Output Switching Regulators with Internal Power Switches Short Circuit Robust Adjustable 250kHz to 2.
PAGE 2
LT3988 Absolute Maximum Ratings Pin Configuration (Note 1) VIN (Notes 7, 8)........................................... –0.3V to 80V BOOST.......................................................................80V EN/UVLO (Note 7)......................................................80V BOOST above SW......................................................30V EN/UVLO above VIN1....................................................6V RT, SYNC.....................................................................
PAGE 3
LT3988 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN1/VIN2 = 12V, unless otherwise noted. (Notes 2, 5, 6) PARAMETER CONDITIONS TYP MAX VIN1 Quiescent Current VFB1 = 0.9V, VBD = 0V MIN 2.7 4.5 mA VIN1 Quiescent Current VFB1 = 0.9V, VBD = 5V 1.6 3 mA VIN2 Quiescent Current VFB2 = 0.
PAGE 4
LT3988 Typical Performance Characteristics VIN = 12V 75 VIN = 48V 65 60 55 VIN = 24V 75 VIN = 48V 70 65 0 50 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 LOAD CURRENT (A) 70 65 55 f = 500kHz 0 50 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 LOAD CURRENT (A) f = 500kHz 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 LOAD CURRENT (A) 3988 G03 Switch Current Limit vs Temperature 0.755 2.0 0.745 Switch Current Limit vs Duty Cycle 2.5 DUTY CYCLE = 35% 2.0 CURRENT LIMIT (A) 2.5 CURRENT LIMIT (A) 0.
PAGE 5
LT3988 Typical Performance Characteristics No-Load Supply Current vs Input Voltage TA = 25°C, unless otherwise noted. Switch Voltage Drop vs Load Current 3.5 Boost Current vs Load Current 1.0 20 3.4 VOLTAGE DROP (V) IQ (mA) 3.2 3.1 3.0 2.9 2.8 BOOST CURRENT (mA) 0.8 3.3 0.6 0.4 0.2 2.7 15 10 5 2.6 2.5 0 0 5 10 15 20 25 30 35 40 45 50 55 60 VIN (V) 0 0.2 0.4 0.6 LOAD CURRENT (A) 0.8 30 20 40 60 80 BOOST DIODE CURRENT (mA) 100 0.8 200 20 0 1.
PAGE 6
LT3988 Typical Performance Characteristics VIN1 Overvoltage Lockout vs Temperature TRACK/SS Pin Current vs Temperature 1.42 TRACK/SS PIN CURRENT (µA) 65 VIN1 OVLO (V) 64 VIN1 RISING 63 62 1.38 1.34 1.30 1.26 VIN1 FALLING 61 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3988 G18 1.22 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3988 G19 Pin Functions BD: Internal boost diodes are connected between the BD pin and the BOOST pins. Connect BD to a 3V or higher supply, such as VOUT.
PAGE 7
LT3988 Block Diagram RT THERMAL SHUTDOWN SYNC MASTER OSC EN/UVLO INT REG AND REF VIN CIN AV BD CL BOOST SW CTL SLOPE C3 L1 OUT SW CLK SLAVE OSC D1 DA COUT R1 0.675V 1.2µA C2 VC gm TRACK/SS FB R2 TRACK/SS 0.75V ILIMIT CLAMP GND 3988 BD ONE OF TWO SWITCHING REGULATORS SHOWN Figure 1.
PAGE 8
LT3988 Operation The LT3988 is a dual, constant frequency, current mode regulator with internal power switches. Operation can be best understood by referring to the Block Diagram in Figure 1. If the EN/UVLO pin is pulled low, the LT3988 is shut down and draws minimal current from the input source(s) tied to the VIN pins. If the EN/UVLO pin exceeds 0.5V (typ), the internal bias circuits turn on, including the internal regulator, reference and master oscillator.
PAGE 9
LT3988 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:  V  R1= R2  OUT – 1  750mV  The parallel combination of R1 and R2 should be 20k or less to minimize bias current errors.
PAGE 10
LT3988 Applications Information at frequencies above fMAX1. It will continue to regulate but with increased inductor current and increased output ripple. fMAX2 is the frequency at which the maximum duty cycle is exceeded. If there is sufficient charge on the BOOST capacitor, the regulator will skip OFF periods to increase the overall duty cycle at frequencies above fMAX2.
PAGE 11
LT3988 Applications Information should be at least 30% higher. For highest efficiency, the series resistance (DCR) should be less than 0.1Ω. Table 1 lists several vendors and types that are suitable. The current in the inductor is a triangle wave with an average value equal to the load current. The peak switch current is equal to the output current plus half the peak-topeak inductor ripple current. The LT3988 limits its switch current in order to protect itself and the system from overload faults.
PAGE 12
LT3988 Applications Information The low ESR and small size of ceramic capacitors make them the preferred type for LT3988 applications. Not all ceramic capacitors are the same, however. Many of the higher value capacitors use poor dielectrics with high temperature and voltage coefficients. In particular, Y5V and Z5U types lose a large fraction of their capacitance with applied voltage and at temperature extremes.
PAGE 13
LT3988 Applications Information VIN3 > 3V BD VIN BOOST VIN BD C3 VOUT SW VIN BOOST VIN GND BD C3 VOUT SW VIN VIN GND VBOOST – VSW ≅ VOUT MAX VBOOST ≅ VIN + VOUT VOUT SW GND VBOOST – VSW ≅ VIN MAX VBOOST ≅ 2VIN (2a) BOOST VBOOST – VSW ≅ VIN3 MAX VBOOST ≅ VIN3 + VIN MIN VALUE FOR VIN3 = 3V (2b) 3988 F02 (2c) Figure 2. Generating the Boost Voltage The boost circuit also limits the minimum input voltage for proper start-up.
PAGE 14
LT3988 Applications Information Input Capacitor Selection Bypass the input of the LT3988 circuit with a 4.7μF or higher ceramic capacitor of X7R or X5R type. A lower value or a less expensive Y5V type will work if there is additional bypassing provided by bulk electrolytic capacitors, or if the input source impedance is low. The following paragraphs describe the input capacitor considerations in more detail.
PAGE 15
LT3988 Applications Information finite output impedance. The power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the VC node. Note that the output capacitor integrates this current, and that the capacitor on the VC node (CC) integrates the error amplifier output current, resulting in two poles in the loop. RC provides a zero.
PAGE 16
LT3988 Applications Information OUTPUT VOLTAGE VOUT1 I I 1.36µA VOUT2 TRACK/SS 0.75V FB D1 + D2 gm – D3 3988 F09 Figure 9. Equivalent Input Circuit of Error Amplifier TIME Coincident Tracking OUTPUT VOLTAGE VOUT1 VOUT2 3988 F07 TIME Ratiometric Tracking Figure 7.
PAGE 17
LT3988 Applications Information the input current at VIN2 when VOUT2 is at maximum load. Figure 10 shows a 12V to 5V, and 1.8V 2-stage converter using this approach. Independent Input Voltages VIN1 and VIN2 are independent and can be powered with different voltages provided VIN1 is present when VIN2 is present. Each supply must be bypassed as close to the VIN pins as possible.
PAGE 18
LT3988 Applications Information components should be as small as possible. Place these components, along with the inductor and output capacitor, on the same side of the circuit board and connect them on that layer. Place a local, unbroken ground plane below these components and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor. Additionally, keep the SW and BOOST nodes as small as possible. Figure 12 shows an example of proper PCB layout.
PAGE 19
LT3988 Typical Applications 400kHz, 5V and 3.3V Outputs VIN 7V TO 40V 80V TRANSIENT C1 4.7µF C2 2200pF VIN1 VIN2 EN/UVLO TRACK/SS1 TRACK/SS2 RT SYNC LT3988 BD VOUT1 5V, 1A L1 22µH R2 57.6k C6 47µF C4 0.22µF BOOST1 BOOST2 SW1 SW2 DA1 FB1 DA2 FB2 D1 R3 10.2k C3 2200pF R1 118k fSW = 400kHz C5 0.22µF L2 15µH D2 GND VOUT2 3.3V, 1A R4 34k R5 10k C7 47µF 3988 TA02 C1 TO C7: X5R OR X7R D1, D2: DIODES, INC.
PAGE 20
LT3988 Typical Applications 1MHz, Wide Input Range 5V and 1.8V Outputs VIN 7V TO 24V 80V TRANSIENT VOUT1 C1 4.7µF C2 2200pF 4.7µF VIN1 VIN2 EN/UVLO TRACK/SS1 TRACK/SS2 RT SYNC LT3988 BD VOUT1 5V, 0.5A L1 6.8µH R2 57.6k C6 22µF C4 0.22µF BOOST1 BOOST2 SW1 SW2 DA1 FB1 DA2 FB2 D1 R3 10.2k C3 2200pF R1 40.2k fSW = 1MHz C5 0.22µF L2 3.3µH D2 GND VOUT2 1.8V, 0.5A R4 14k R5 10k C7 22µF 3988 TA03 C1 TO C7: X5R OR X7R D1: DIODES, INC. B160 D2: DIODES, INC.
PAGE 21
LT3988 Typical Applications 700kHz, 24V and 12V Outputs with Coincident Tracking VIN1 26V TO 60V 80V TRANSIENT C1 4.7µF C2 4.7µF VIN2 VIN1 EN/UVLO TRACK/SS1 C3 2200pF RT SYNC R2 10k LT3988 TRACK/SS2 R4 4.7k R3 309k VOUT1 24V, 1A L1 47µH C6 10µF VIN2 14V TO 60V C4 0.22µF BOOST1 BD BOOST2 SW1 SW2 DA1 FB1 DA2 FB2 D1 R5 309k R6 10k R1 61.9k fSW = 700kHz C5 0.22µF D2 GND L2 22µH VOUT2 12V, 1A R7 150k R8 10k C7 10µF 3988 TA04 C1 TO C7: X5R OR X7R D1, D2: DIODES, INC. B160 R4: USE 0.
PAGE 22
LT3988 Typical Applications 400kHz, 3.3V and 2.5V Outputs VIN 5.5V TO 32V 80V TRANSIENT C1 4.7µF C2 2200pF VIN1 VIN2 EN/UVLO TRACK/SS1 TRACK/SS2 RT SYNC LT3988 BD VOUT1 3.3V, 1A L1 10µH R2 34k C6 47µF C4 0.22µF BOOST1 BOOST2 SW1 SW2 DA1 FB1 DA2 FB2 D1 R3 10k C3 2200pF R1 118k fSW = 400kHz C5 0.22µF D2 GND VOUT1 L2 10µH VOUT2 2.5V, 1A R4 23.2k R5 10k C7 47µF 3988 TA05 C1 TO C7: X5R OR X7R D1, D2: DIODES, INC.
PAGE 23
LT3988 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev E) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 5.23 (.206) MIN 2.845 ±0.102 (.112 ±.004) 0.889 ±0.127 (.035 ±.005) 8 1 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 0.305 ±0.038 (.0120 ±.0015) TYP 16 0.50 (.0197) BSC 4.039 ±0.102 (.159 ±.
PAGE 24
LT3988 Typical Application 500kHz External Sync, 5V and 3.3V Outputs with 6V UVLO VIN 7V TO 30V 80V TRANSIENT R6 40.2k R7 10k C1 4.7µF C2 1000pF VIN1 VIN2 EN/UVLO TRACK/SS1 TRACK/SS2 RT LT3988 500kHz CLOCK SYNC C3 1000pF R1 100k BD L1 15µH VOUT1 5V, 1A R2 57.6k C6 22µF C4 0.22µF BOOST1 BOOST2 SW1 SW2 DA1 FB1 DA2 FB2 D1 C5 0.22µF L2 10µH D2 R3 10.2k GND 22pF VOUT2 3.3V, 1A R4 34k R5 10k C7 47µF 3988 TA06 C1 TO C7: X5R OR X7R D1, D2: DIODES, INC. B180 EN/UVLO THRESHOLD = 6.