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LT1977

1977fa

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High Voltage Power Conversion

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14V and 42V Automotive Systems

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Industrial Power Systems

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Distributed Power Systems

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Battery-Powered Systems

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

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Wide Input Range: 3.3V to 60V

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1.5A Peak Switch Current

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Burst Mode

Operation: 100µA Quiescent Current**

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Low Shutdown Current: I

< 1µA

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Power Good Flag with Programmable Threshold

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Load Dump Protection to 60V

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500kHz Switching Frequency

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Saturating Switch Design: 0.2Ω On-Resistance

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Peak Switch Current Maintained Over

Full Duty Cycle Range*

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1.25V Feedback Reference Voltage

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Easily Synchronizable

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Soft-Start Capability

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Small 16-Pin Thermally Enhanced TSSOP Package

High Voltage 1.5A, 500kHz

Step-Down Switching Regulator

with 100µA Quiescent Current

Burst Mode is a registered trademark of Linear Technology Corporation.

*Protected by U.S. Patents including 6498466, 6531909.

**See Burst Mode Operation section for conditions

The LT

1977 is a 500kHz monolithic buck switching

regulator that accepts input voltages up to 60V. A high

efficiency 1.5A, 0.2Ω switch is included on the die along

with all the necessary oscillator, control and logic cir-

cuitry. Current mode topology is used for fast transient

response and good loop stability.

Innovative design techniques along with a new high volt-

age process achieve high efficiency over a wide input

range. Efficiency is maintained over a wide output current

range by employing Burst Mode operation at low currents,

utilizing the output to bias the internal circuitry, and by

using a supply boost capacitor to fully saturate the power

switch. Patented circuitry maintains peak switch current

over the full duty cycle range.* Shutdown reduces input

supply current to less than 1µA. External synchronization

can be implemented by driving the SYNC pin with logic-level

inputs. A single capacitor from the C

pin to the output

provides a controlled output voltage ramp (soft-start). The

device also has a power good flag with a programmable

threshold and time-out and thermal shutdown protection.

The LT1977 is available in a 16-pin TSSOP package with

exposed pad leadframe for low thermal resistance. The

LT1976, a 200kHz reduced switch frequency version of the

LT1977, is also available. See the Applications Information

section for selection criteria between the LT1976 and

LT1977.

SHDN

BOOST

LT1977

2.2µF

100V

CER

330pF

0.1µF

10µH

1N4148

1µF

SYNC

GND

100µF

6.3V

TANT

OUT

3.3V

10MQ100N

1500pF

26k

165k

100k

1977 TA01

10pF

BIAS

PGFB

14V to 3.3V Step-Down Converter with

100µA No Load Quiescent Current

Efficiency and Power Loss

vs Load Current

Supply Current vs Input Voltage

INPUT VOLTAGE (V)

SUPPLY CURRENT (µA)

100

30 40

1977 F05

150

125

OUT

= 3.3V

= 25°C

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATIO

LOAD CURRENT (A)

0.0001

EFFICIENCY (%)

1977 TA02

0.001

0.01

0.1

100

POWER LOSS (W)

0.001

= 12V

OUT

= 5V

OUT

= 3.3V

TYPICAL

POWER LOSS

Summary of content (24 pages)

PAGE 1
LT1977 High Voltage 1.5A, 500kHz Step-Down Switching Regulator with 100µA Quiescent Current U FEATURES DESCRIPTIO ■ The LT®1977 is a 500kHz monolithic buck switching regulator that accepts input voltages up to 60V. A high efficiency 1.5A, 0.2Ω switch is included on the die along with all the necessary oscillator, control and logic circuitry. Current mode topology is used for fast transient response and good loop stability. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Wide Input Range: 3.3V to 60V 1.
PAGE 2
LT1977 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) TOP VIEW VIN, SHDN, BIAS, SW, PG....................................... 60V BOOST Pin Above SW ............................................ 35V BOOST Pin Voltage ................................................. 68V SYNC, CSS, PGFB, FB ................................................ 6V Operating JunctionTemperature Range LT1977EFE (Note 2) ........................ – 40°C to 125°C LT1977IFE (Note 2) .......................
PAGE 3
LT1977 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TJ = 25°C. VIN = 12V, SHDN = 12V, BIAS = 5V, FB/PGFB = 1.25V, CSS/SYNC = 0V unless otherwise noted.
PAGE 4
LT1977 U W TYPICAL PERFOR A CE CHARACTERISTICS Oscillator Frequency 1.29 540 1.28 530 1.27 520 1.26 1.25 1.24 1.23 490 460 100 50 25 0 75 TEMPERATURE (°C) 100 1.00 –50 –25 125 Shutdown Supply Current 200 180 20 4.5 3.0 2.5 140 CURRENT (µA) CURRENT (µA) CURRENT (µA) 4.0 3.5 VBIAS = 0V 160 VIN = 60V 15 10 2.0 120 100 80 VBIAS = 5V 60 1.5 5 40 VIN = 42V 1.0 0 10 30 40 20 SHDN VOLTAGE (V) 50 0 –50 –25 60 VIN = 12V 50 25 0 75 TEMPERATURE (°C) 100 0 –50 –25 125 1.
PAGE 5
LT1977 U W TYPICAL PERFOR A CE CHARACTERISTICS Soft-Start Current Threshold vs FB Voltage 3.5 TJ = 25°C 45 90 40 3.0 35 CURRENT (µA) PEAK SWITCH CURRENT (A) Frequency Foldback Percentage 100 50 FOLDBACK PERCENTAGE (%) Switch Peak Current Limit 2.5 SOFT-START DEFEATED 30 25 20 15 2.0 10 75 –0 25 50 TEMPERATURE (°C) 100 0 0.2 0.6 0.8 0.4 FB VOLTAGE (V) 1.0 1977 G10 Switch On Voltage (VCESAT) 300 TJ = 125°C 250 TJ = 25°C 200 150 0 0.25 0.75 0.5 FB PIN VOLTAGE (V) 1 1.
PAGE 6
LT1977 U W TYPICAL PERFOR A CE CHARACTERISTICS Dropout Operation 4.0 VOUT = 3.3V BOOST DIODE = DIODES INC B1100 3.5 Burst Mode Operation VOUT = 5V BOOST DIODE = DIODES INC B1100 5 3.0 LOAD CURRENT = 250mA OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) Dropout Operation 6 2.5 LOAD CURRENT = 1.25A 2.0 1.5 1.0 3 ISW 500mA/DIV LOAD CURRENT = 1.25mA 2 VIN = 12V VOUT = 3.3V IQ = 100µA 1 0.5 0 VOUT 20mV/DIV LOAD CURRENT = 250mA 4 2.5 2 3 3.5 INPUT VOLTAGE (V) 4 4.5 0 2 2.5 3 3.5 4 4.5 5 5.
PAGE 7
LT1977 U U U PI FU CTIO S voltage on the PGFB pin drops below VPGFB, CCT will be discharged rapidly to 0V and PG will be active low with a 200µA sink capability. If the CT pin is clamped (Power Good condition) during normal operation and SHDN is taken low, the CT pin will be discharged and a delay period will occur when SHDN is returned high. See the Power Good section in Applications Information for details.
PAGE 8
LT1977 W BLOCK DIAGRA 4 VIN INTERNAL REF UNDERVOLTAGE LOCKOUT 10 14 15 BIAS THERMAL SHUTDOWN 2.4V SLOPE COMP Σ 500kHz OSCILLATOR + CURRENT COMP – SYNC SHDN BOOST ANTISLOPE COMP + R SHDN COMP S – SWITCH Q LATCH DRIVER CIRCUITRY SW 1.3V 9 12 CSS SOFT-START BURST MODE DETECT FOLDBACK DETECT VC CLAMP 6 2 – FB ERROR 1.25V 11 13 + AMP VC PG PGFB 16 + PG COMP 1.12V 1.2V CT CLAMP – GND 17 7 PGND 8 CT 1977 BD Figure 1.
PAGE 9
LT1977 W BLOCK DIAGRA power from the VIN pin, but if the BIAS pin is connected to an external voltage higher than 3V bias power will be drawn from the external source (typically the regulated output voltage). This improves efficiency. High switch efficiency is achieved by using the BOOST pin to provide a voltage to the switch driver which is higher than the input voltage, allowing switch to be saturated. This boosted voltage is generated with an external capacitor and diode.
PAGE 10
LT1977 U W U U APPLICATIO S I FOR ATIO VOUT LT1977 and Soft-Start Current graphs in Typical Performance Characteristics). SW 2 CSS SOFT-START 500kHz OSCILLATOR C1 9 FOLDBACK DETECT R1 FB 12 – ERROR AMP R2 + 1.25V VC 11 1977 F02 Figure 2. Feedback Network Table 2 OUTPUT VOLTAGE (V) R2 (kΩ, 1%) R1 NEAREST (1%) (kΩ) OUTPUT ERROR (%) 2.5 100 100 0 3 100 140 0 3.3 100 165 0.38 5 100 300 0 6 100 383 0.63 8 100 536 – 0.63 10 100 698 – 0.25 12 100 866 0.
PAGE 11
LT1977 U W U U APPLICATIO S I FOR ATIO INPUT CAPACITOR Step-down regulators draw current from the input supply in pulses. The rise and fall times of these pulses are very fast. The input capacitor is required to reduce the voltage ripple this causes at the input of LT1977 and force the switching current into a tight local loop, thereby minimizing EMI. The RMS ripple current can be calculated from: IRIPPLE(RMS) = IOUT VOUT ( VIN – VOUT ) VIN Ceramic capacitors are ideal for input bypassing.
PAGE 12
LT1977 U W U U APPLICATIO S I FOR ATIO triangular with a typical value of 200mARMS. The formula to calculate this is: Output capacitor ripple current (RMS) IRIPPLE(RMS) = 0.29( VOUT )( VIN – VOUT ) IP-P = 12 (L)( f)( VIN ) CERAMIC CAPACITORS Higher value, lower cost ceramic capacitors are now becoming available. They are generally chosen for their good high frequency operation, small size and very low ESR (effective series resistance).
PAGE 13
LT1977 U W U U APPLICATIO S I FOR ATIO The LT1977 is able to maintain peak switch current limit over the full duty cycle range by using patented circuitry to cancel the effects of slope compensation on peak switch current without affecting the frequency compensation it provides.
PAGE 14
LT1977 U W U U APPLICATIO S I FOR ATIO Table 4. Inductor Selection Criteria VENDOR/ PART NO. Short-Circuit Considerations VALUE (µH) IDC(MAX) (Amps) DCR (Ohms) HEIGHT (mm) UP1B-100 10 1.9 0.111 5.0 UP1B-220 22 1.2 0.254 5.0 UP2B-220 22 2.0 0.062 6.0 UP2B-330 33 1.7 0.092 6.0 UP1B-150 15 1.5 0.175 5.0 D01813P-153HC 15 1.5 0.170 5.0 D01813P-103HC 10 1.9 0.111 5.0 D53316P-223 22 1.6 0.207 5.1 D53316P-333 33 1.4 0.334 5.1 LP025060B-682 6.8 1.3 0.165 1.
PAGE 15
LT1977 U W U U APPLICATIO S I FOR ATIO low value such that the current doesn’t continue to ratchet higher. When the FB pin voltage is abnormally low thereby indicating some sort of short-circuit condition, the oscillator frequency will be reduced. Oscillator frequency is reduced by a factor of 4 when the FB pin voltage is below 0.4V and increases linearly to its typical value of 500kHz at a FB voltage of 0.95V (see Typical Performance Characteristics).
PAGE 16
LT1977 U W U U APPLICATIO S I FOR ATIO Example: For VOUT = 3.3V, VIN = 12V ⎛ 3.3 ⎞ (125µA + 12.5µA + 0.5µA ) IIN(AVG) = 45µA + 5µA + ⎜ ⎟ ⎝ 12 ⎠ (0.8) VOUT 50mV/DIV = 45µA + 5µA + 44µA = 99µA 150 VSHDN 2V/DIV VOUT = 3.3V TA = 25°C SUPPLY CURRENT (µA) 125 ISW 500mA/DIV 100 75 VIN = 12V VOUT = 3.3V IQ = 15µA 50 Figure 6. Burst Mode with Shutdown Pin 25 To maximize high and low load current efficiency a fast switching diode with low forward drop and low reverse leakage should be used.
PAGE 17
LT1977 U W U U APPLICATIO S I FOR ATIO OPTIONAL must weigh the importance of each specification in choosing the best diode for the application. The use of so-called “ultrafast” recovery diodes is generally not recommended. When operating in continuous mode, the reverse recovery time exhibited by “ultrafast” diodes will result in a slingshot type effect. The power internal switch will ramp up VIN current into the diode in an attempt to get it to recover.
PAGE 18
LT1977 U W U U APPLICATIO S I FOR ATIO In addition to the shutdown feature, the LT1977 has an undervoltage lockout function. When the input voltage is below 2.4V, switching will be disabled. The undervoltage lockout threshold doesn’t have any hysteresis and is mainly used to insure that all internal voltages are at the correct level before switching is enabled.
PAGE 19
LT1977 U W U U APPLICATIO S I FOR ATIO the user to generate a delayed signal after the power good threshold is exceeded. Referring to Figure 2, the PGFB pin is the positive input to a comparator whose negative input is set at VPGFB. When PGFB is taken above VPGFB, current (ICSS) is sourced into the CT pin starting the delay period. When the voltage on the PGFB pin drops below VPGFB the CT pin is rapidly VOUT 500mV/DIV PG 100k TO VIN VCT 500mV/DIV VSHDN 2V/DIV TIME (10ms/DIV) 1977 F09 Figure 9.
PAGE 20
LT1977 U W U U APPLICATIO S I FOR ATIO a limited amount of drive capability and is susceptible to noise during start-up and Burst Mode operation. If erratic operation occurs during these conditions a small filter capacitor from the PGOOD pin to ground will ensure proper operation. Figure 10 shows several different configurations for the LT1977 Power Good circuitry.
PAGE 21
LT1977 U W U U APPLICATIO S I FOR ATIO Boost current loss: PBOOST Input Voltage vs Operating Frequency Considerations 2 VOUT ) (IOUT / 32) ( = VIN Quiescent current loss: PQ = VIN (0.0015) + VOUT (0.003) RSW = switch resistance (≈0.3 when hot ) tEFF = effective switch current/voltage overlap time (tr + tf + tIR + tIF) tr = (VIN/1.1)ns tf = (VIN/1.8)ns tIR = tIF = (IOUT/0.05)ns f = switch frequency Example: with VIN = 12V, VOUT = 5V and IOUT = 1A: 2 0.3)(1) (5) ( = + ( ) 57.
PAGE 22
LT1977 U W U U APPLICATIO S I FOR ATIO power. In questionable cases a prototype supply should be built and exercised to verify acceptable operation. 2. The simultaneous requirements of high VIN, low VOUT and high fOSC can result in an unacceptably short minimum switch on time. Cycle skipping and/or Burst Mode behavior will result causing an increase in output voltage ripple while maintaining the correct output voltage.
PAGE 23
LT1977 U W U U APPLICATIO S I FOR ATIO First, the combination of output capacitor ESR and a large RC may stop loop gain rolling off altogether. Second, if the loop gain is not rolled off sufficiently at the switching frequency output ripple will perturb the VC pin enough to cause unstable duty cycle switching similar to subharmonic oscillation. This may not be apparent at the output. Smallsignal analysis will not show this since a continuous time system is assumed.
PAGE 24
LT1977 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1074/LT1074HV 4.4A (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converters VIN: 7.3V to 45V/64V, VOUT(MIN) = 2.21V, IQ = 8.5mA, ISD < 10µA, DD5/7, TO220-5/7 LT1076/LT1076HV 1.6A (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converters VIN: 7.3V to 45V/64V, VOUT(MIN) = 2.21V, IQ = 8.5mA, ISD < 10µA, DD5/7, TO220-5/7 LT1676 60V, 700mA (ISW), 100kHz, High Efficiency Step-Down DC/DC Converter VIN: 7.4V to 60V, VOUT(MIN) = 1.24V, IQ = 3.