LT1977 Datasheet PDF - Linear Technology

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LT1977
Linear Technology

Part Number LT1977
Description Step-Down Switching Regulator
Page 24 Pages


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LT1977
High Voltage 1.5A, 500kHz
Step-Down Switching Regulator
with 100µA Quiescent Current
FEATURES
DESCRIPTIO
Wide Input Range: 3.3V to 60V
1.5A Peak Switch Current
Burst Mode® Operation: 100µA Quiescent Current**
Low Shutdown Current: IQ < 1µA
Power Good Flag with Programmable Threshold
Load Dump Protection to 60V
500kHz Switching Frequency
Saturating Switch Design: 0.2On-Resistance
Peak Switch Current Maintained Over
Full Duty Cycle Range*
1.25V Feedback Reference Voltage
Easily Synchronizable
Soft-Start Capability
Small 16-Pin Thermally Enhanced TSSOP Package
U
APPLICATIO S
High Voltage Power Conversion
14V and 42V Automotive Systems
Industrial Power Systems
Distributed Power Systems
Battery-Powered Systems
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a registered trademark of Linear Technology Corporation.
*U.S. Patent 6,498,466 and 6,531,909 **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, 0.2switch 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 CSS 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.
TYPICAL APPLICATIO
14V to 3.3V Step-Down Converter with
100µA No Load Quiescent Current
VIN
1500pF
26k
2.2µF
100V
CER
330pF
1µF
VIN BOOST
SHDN
SW
0.1µF 10µH 1N4148
LT1977
0.1µF
10MQ100N
VC CSS
CT
SYNC
GND
VBIAS
FB
PGFB
PG
10pF
165k
1% +
100k
1%
VOUT
3.3V
1A
100µF
6.3V
TANT
1977 TA01
Supply Current vs Input Voltage
150
VOUT = 3.3V
TA = 25°C
125
100
75
50
25
0
0 10 20 30 40 50 60
INPUT VOLTAGE (V)
1977 F05
Efficiency and Power Loss
vs Load Current
100 VIN = 12V
EFFICIENCY
5V
75
3.3V
10
1
50 0.1
TYPICAL
POWER LOSS
25 0.01
0
0.0001 0.001 0.01
0.1
LOAD CURRENT (A)
0.001
1 10
1977 TA02
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LT1977
ABSOLUTE AXI U RATI GS
(Note 1)
VIN, SHDN, BIAS ..................................................... 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) ......................... – 40°C to 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
UW U
PACKAGE/ORDER I FOR ATIO
NC 1
SW 2
NC 3
VIN 4
NC 5
BOOST 6
CT 7
GND 8
TOP VIEW
16 PG
15 SHDN
14 SYNC
13 PGFB
17
12 FB
11 VC
10 BIAS
9 CSS
ORDER PART
NUMBER
LT1977EFE
LT1977IFE
FE PART
MARKING
FE PACKAGE
16-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 45°C/W, θJC(PAD) = 10°C/W
EXPOSED PAD IS GND (PIN 17)
MUST BE SOLDERED TO GND (PIN 8)
1977EFE
1977IFE
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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.
SYMBOL PARAMETER
CONDITIONS
MIN TYP MAX UNITS
VSHDN
ISHDN
IVINS
IVIN
SHDN Threshold
SHDN Input Current
Minimum Input Voltage (Note 3)
Supply Shutdown Current
Supply Sleep Current (Note 4)
Supply Quiescent Current
SHDN = 12V
SHDN = 0V, BOOST = 0V, FB/PGFB = 0V
BIAS = 0V, FB = 1.35V
FB = 1.35V
BIAS = 0V, FB = 1.15V
BIAS = 5V, FB = 1.15V
1.15 1.3 1.45
5 20
2.4 3
0.1 2
170 250
45 75
4.10
3.25
V
µA
V
µA
µA
µA
mA
mA
IBIASS
IBIAS
VREF
IFB
Minimum BIAS Voltage (Note 5)
BIAS Sleep Current (Note 4)
BIAS Quiescent Current
Minimum Boost Voltage (Note 6)
Input Boost Current (Note 7)
Reference Voltage (VREF)
FB Input Bias Current
EA Voltage Gain (Note 8)
SYNC = 3.3V
ISW = 1.5A
ISW = 1.5A
3.3V < VVIN < 60V
2.7 3
110 180
700 900
1.8
40
1.225 1.25 1.275
75 200
900
V
µA
µA
V
mA
V
nA
V/V
EA Voltage gm
EA Source Current
EA Sink Current
VC to SW gm
VC High Clamp
IPK SW Current Limit
dI(VC)= ±10µA
FB = 1.15V
FB = 1.35V
450 650 900 µMho
20 40 55
µA
15 30 40
µA
3 A/V
2.1 2.2 2.4
V
1.5 2.4 3.5
A
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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.
SYMBOL
PARAMETER
Switch On Resistance (Note 9)
Switching Frequency
Maximum Duty Cycle
CONDITIONS
MIN TYP MAX UNITS
0.2 0.4
425 500 575
kHz
86 92
%
ICSS
IPGFB
VPGFB
ICT
VCT
Minimum SYNC Amplitude
SYNC Frequency Range
SYNC Input Impedance
CSS Current Threshold (Note 10)
PGFB Input Current
PGFB Voltage Threshold (Note 11)
CT Source Current (Note 11)
CT Sink Current (Note 11)
CT Voltage Threshold (Note 11)
PG Leakage (Note 11)
PG Sink Current (Note 11)
VPG = 12V
PGFB = 1V, PG = 400mV
1.5 2.0
575 700
85
7 13 20
25 100
88 90 92
2 3.6 5.5
12
1.16 1.2 1.26
0.1 1
100 200
V
kHz
k
µA
nA
%
µA
mA
V
µA
µA
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LT1977EFE is guaranteed to meet performance specifications
from 0°C to 125°C junction temperature. Specifications over the –40°C to
125°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LT1977IFE is guaranteed and tested over the full –40°C to 125°C
operating junction temperature range.
Note 3: Minimum input voltage is defined as the voltage where switching
starts. Actual minimum input voltage to maintain a regulated output will
depend upon output voltage and load current. See Applications
Information.
Note 4: Supply input current is the quiescent current drawn by the input
pin. Its typical value depends on the voltage on the BIAS pin and operating
state of the LT1977. With the BIAS pin at 0V, all of the quiescent current
required to operate the LT1977 will be provided by the VIN pin. With the
BIAS voltage above its minimum input voltage, a portion of the total
quiescent current will be supplied by the BIAS pin. Supply sleep current is
defined as the quiescent current during the “sleep” portion of Burst Mode
operation. See Applications Information for determining application supply
currents.
Note 5: Minimum BIAS voltage is the voltage on the BIAS pin when IBIAS is
sourced into the pin.
Note 6: This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
Note 7: Boost current is the current flowing into the BOOST pin with the pin
held 3.3V above input voltage. It flows only during switch on time.
Note 8: Gain is measured with a VC swing from 1.15V to 750mV.
Note 9: Switch on resistance is calculated by dividing VIN to SW voltage by the
forced current (1.5A). See Typical Performance Characteristics for the graph
of switch voltage at other currents.
Note 10: The CSS threshold is defined as the value of current sourced into the
CSS pin which results in an increase in sink current from the VC pin. See the
Soft-Start section in Applications Information.
Note 11: The PGFB threshold is defined as the percentage of VREF voltage
which causes the current source output of the CT pin to change from
sinking (below threshold) to sourcing current (above threshold). When
sourcing current, the voltage on the CT pin rises until it is clamped
internally. When the clamp is activated, the output of the PG pin will be set
to a high impedance state. When the CT clamp is inactive the PG pin will
be set active low with a current sink capability of 200µA.
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LT1977
TYPICAL PERFOR A CE CHARACTERISTICS
FB Voltage
1.30
1.29
1.28
1.27
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G01
SHDN Pin Current
5.5
TJ = 25°C
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0
0 10 20 30 40
SHDN VOLTAGE (V)
50 60
1977 G04
Bias Sleep Current
200
180
160
140
120
100
80
60
40
20
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G07
Oscillator Frequency
550
540
530
520
510
500
490
480
470
460
450
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G02
Shutdown Supply Current
25
20
VIN = 60V
15
10
5
VIN = 42V VIN = 12V
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G05
PGFB Threshold
1.20
1.18
1.16
1.14
1.12
1.10
1.08
1.06
1.04
1.02
1.00
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G08
SHDN Threshold
1.40
1.35
1.30
1.25
1.20
0.15
1.10
1.05
1.00
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G03
Sleep Mode Supply Current
200
180
160 VBIAS = 0V
140
120
100
80
60 VBIAS = 5V
40
20
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G06
PG Sink Current
250
200
150
100
50
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1977 G09
1977f
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