LT1977
High Voltage 1.5A, 500kHz
Step-Down Switching Regulator
with 100µA Quiescent Current
FEATURES
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DESCRIPTIO
Wide Input Range: 3.3V to 60V
1.5A Peak Switch Current
Burst Mode
®
Operation: 100µA Quiescent Current**
Low Shutdown Current: I
Q
< 1µA
Power Good Flag with Programmable Threshold
Load Dump Protection to 60V
500kHz Switching Frequency
Saturating Switch Design: 0.2Ω On-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
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
SS
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.
Efficiency and Power Loss
vs Load Current
100
V
IN
= 12V
V
OUT
= 5V
APPLICATIO S
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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.
*Protected by U.S. Patents including 6498466, 6531909.
**See Burst Mode Operation section for conditions
TYPICAL APPLICATIO
Supply Current vs Input Voltage
150
125
V
OUT
= 3.3V
T
A
= 25°C
14V to 3.3V Step-Down Converter with
100µA No Load Quiescent Current
V
IN
V
IN
SHDN
LT1977
V
C
1500pF
26k
1µF
C
T
SYNC
GND
330pF
C
SS
V
BIAS
FB
PGFB
PG
100k
1%
10pF
165k
1%
+
100µF
6.3V
TANT
BOOST
0.1µF
SW
0.1µF
10µH
1N4148
V
OUT
3.3V
1A
SUPPLY CURRENT (µA)
2.2µF
100V
CER
EFFICIENCY (%)
100
75
50
25
0
0
10
30
40
20
INPUT VOLTAGE (V)
50
60
1977 F05
10MQ100N
1977 TA01
U
10
U
U
75
V
OUT
= 3.3V
1
POWER LOSS (W)
50
TYPICAL
POWER LOSS
0.1
25
0.01
0
0.0001
0.001
0.001
0.1
0.01
LOAD CURRENT (A)
1
10
1977 TA02
1977fa
1
LT1977
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
NC
SW
NC
V
IN
NC
BOOST
C
T
GND
V
IN
, SHDN, BIAS, SW, PG....................................... 60V
BOOST Pin Above SW ............................................ 35V
BOOST Pin Voltage ................................................. 68V
SYNC, C
SS
, 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
1
2
3
4
5
6
7
8
17
16 PG
15 SHDN
14 SYNC
13 PGFB
12 FB
11 V
C
10 BIAS
9
C
SS
ORDER PART
NUMBER
LT1977EFE
LT1977IFE
FE PART
MARKING
1977EFE
1977IFE
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 45°C/W,
θ
JC(PAD)
= 10°C/W
EXPOSED PAD IS GND (PIN 17)
MUST BE SOLDERED TO GND (PIN 8)
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
J
= 25°C. V
IN
= 12V, SHDN = 12V, BIAS = 5V, FB/PGFB = 1.25V, C
SS
/SYNC = 0V
unless otherwise noted.
SYMBOL
V
SHDN
I
SHDN
I
VINS
PARAMETER
SHDN Threshold
SHDN Input Current
Minimum Input Voltage (Note 3)
Supply Shutdown Current
Supply Sleep Current (Note 4)
I
VIN
Supply Quiescent Current
Minimum BIAS Voltage (Note 5)
I
BIASS
I
BIAS
BIAS Sleep Current (Note 4)
BIAS Quiescent Current
Minimum Boost Voltage (Note 6)
Input Boost Current (Note 7)
V
REF
I
FB
Reference Voltage (V
REF
)
FB Input Bias Current
EA Voltage Gain (Note 8)
EA Voltage g
m
EA Source Current
EA Sink Current
V
C
to SW g
m
V
C
High Clamp
I
PK
SW Current Limit
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
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
FB = 1.15V
SYNC = 3.3V
I
SW
= 1.5A
I
SW
= 1.5A
3.3V < V
VIN
< 60V
●
●
●
●
●
●
●
MIN
1.15
TYP
1.3
5
2.4
0.1
170
45
MAX
1.45
20
3
2
250
75
4.10
3.25
UNITS
V
µA
V
µA
µA
µA
mA
mA
V
µA
µA
V
mA
2.7
110
700
1.8
40
1.225
1.25
75
900
3
180
900
1.275
200
900
55
40
2.4
3.5
dI(V
C
)=
±10µA
FB = 1.15V
FB = 1.35V
450
20
15
2.1
1.5
650
40
30
3
2.2
2.4
µMho
µA
µA
A/V
V
A
1977fa
2
U
V
nA
V/V
W
U
U
W W
W
LT1977
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
J
= 25°C. V
IN
= 12V, SHDN = 12V, BIAS = 5V, FB/PGFB = 1.25V, C
SS
/SYNC = 0V
unless otherwise noted.
SYMBOL
PARAMETER
Switch On Resistance (Note 9)
Switching Frequency
Maximum Duty Cycle
Minimum SYNC Amplitude
SYNC Frequency Range
SYNC Input Impedance
I
CSS
I
PGFB
V
PGFB
I
CT
V
CT
C
SS
Current Threshold (Note 10)
PGFB Input Current
PGFB Voltage Threshold (Note 11)
C
T
Source Current (Note 11)
C
T
Sink Current (Note 11)
C
T
Voltage Threshold (Note 11)
PG Leakage (Note 11)
PG Sink Current (Note 11)
V
PG
= 12V
PGFB = 1V, PG = 400mV
100
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
●
●
MIN
425
86
575
TYP
0.2
500
92
1.5
85
MAX
0.4
575
2.0
700
UNITS
Ω
kHz
%
V
kHz
kΩ
µA
nA
%
µA
mA
V
µA
µA
FB = 0V
7
88
2
1
1.16
13
25
90
3.6
2
1.2
0.1
200
20
100
92
5.5
1.26
1
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 V
IN
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 I
BIAS
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 V
C
swing from 1.15V to 750mV.
Note 9:
Switch on resistance is calculated by dividing V
IN
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 C
SS
threshold is defined as the value of current sourced into the
C
SS
pin which results in an increase in sink current from the V
C
pin. See the
Soft-Start section in Applications Information.
Note 11:
The PGFB threshold is defined as the percentage of V
REF
voltage
which causes the current source output of the C
T
pin to change from
sinking (below threshold) to sourcing current (above threshold). When
sourcing current, the voltage on the C
T
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 C
T
clamp is inactive the PG pin will
be set active low with a current sink capability of 200µA.
1977fa
3
LT1977
TYPICAL PERFOR A CE CHARACTERISTICS
FB Voltage
1.30
1.29
1.28
FREQUENCY (kHz)
1.27
VOLTAGE (V)
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
510
500
490
480
470
460
450
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
VOLTAGE (V)
SHDN Pin Current
5.5
5.0
4.5
4.0
CURRENT (µA)
T
J
= 25°C
CURRENT (µA)
3.5
3.0
2.5
2.0
1.5
1.0
0
0
10
30
40
20
SHDN VOLTAGE (V)
50
60
1977 G04
15
CURRENT (µA)
Bias Sleep Current
200
180
160
140
1.20
1.18
1.16
1.14
1.12
1.10
1.08
1.06
1.04
1.02
50
25
0
75
TEMPERATURE (°C)
100
125
CURRENT (µA)
120
100
80
60
40
20
0
–50 –25
CURRENT (µA)
VOLTAGE (V)
4
U W
1977 G01
1977 G07
Oscillator Frequency
550
540
530
520
SHDN Threshold
1.40
1.35
1.30
1.25
1.20
0.15
1.10
1.05
1.00
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
1977 G02
1977 G03
Shutdown Supply Current
25
200
180
20
V
IN
= 60V
160
140
120
100
80
60
5
V
IN
= 42V
0
–50 –25
V
IN
= 12V
100
125
40
20
Sleep Mode Supply Current
V
BIAS
= 0V
10
V
BIAS
= 5V
50
25
0
75
TEMPERATURE (°C)
0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
1977 G05
1977 G06
PGFB Threshold
250
PG Sink Current
200
150
100
50
1.00
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
1977 G08
1977 G09
1977fa
LT1977
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Peak Current Limit
3.5
50
45
PEAK SWITCH CURRENT (A)
3.0
CURRENT (µA)
40
35
30
25
20
15
10
5
1.5
–50 –25
75
–0
25
50
TEMPERATURE (°C)
100
125
0
0
0.2
0.6
0.8
0.4
FB VOLTAGE (V)
1.0
1.2
1977 G11
FOLDBACK PERCENTAGE (%)
2.5
2.0
Switch On Voltage (V
CESAT
)
500
450
400
SUPPLY CURRENT (µA)
350
VOLTAGE (mV)
100
75
50
25
INPUT VOLTAGE (V)
300
250
200
150
100
50
0
–0.1 0.1
0.3
T
J
= –50°C
T
J
= 125°C
T
J
= 25°C
0.5 0.7 0.9 1.1
LOAD CURRENT (A)
Burst Mode Threshold
vs Input Voltage
500
V
OUT
= 3.3V
450 L = 10µH
= 100µF
C
400
OUT
LOAD CURRENT (mA)
350
ON TIME (ns)
350
Burst Mode EXIT
(INCREASING LOAD)
LOAD CURRENT = 0.5A
BOOST CURRENT (mA)
300
250
200
150
100
50
0
Burst Mode ENTER
(DECREASING LOAD)
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
INPUT VOLTAGE (V)
1977 G20
U W
1977 G10
Soft-Start Current Threshold
vs FB Voltage
100
Frequency Foldback Percentage
90
80
70
60
50
40
30
20
10
0
0
0.25
0.75
0.5
FB PIN VOLTAGE (V)
1
1.25
1977 G12
T
J
= 25°C
SOFT-START
DEFEATED
Supply Current vs Input Voltage
150
125
V
OUT
= 3.3V
T
A
= 25°C
Minimum Input Voltage
7.5
7.0
6.5
6.0
5V START
5.5
5.0
4.5
4.0
3.5
3.3V RUNNING
3.3V START
5V RUNNING
0
1.3
1.5
3.0
0
10
30
40
20
INPUT VOLTAGE (V)
50
60
1977 F05
0
0.2
0.4
0.6 0.8 1 1.2
LOAD CURRENT (A)
1.4
1.6
1977 G13
1977 G19
Minimum On Time
500
450
400
45
40
35
30
25
20
15
10
5
0
50
25
0
75
TEMPERATURE (°C)
100
125
Boost Current vs Switch Current
300
250
200
150
100
50
0
–50 –25
LOAD CURRENT = 1A
0
0.25
0.75
0.5
1
SWITCH CURRENT (A)
1.25
1.5
1977 G21
1977 G22
1977fa
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