LTC1871-7
High Input Voltage,
Current Mode Boost,
Flyback and SEPIC Controller
Description
Features
Optimized for High Input Voltage Applications
n Wide Chip Supply Voltage Range: 6V to 36V
n Internal 7V Low Dropout Voltage Regulator
Optimized for 6V-Rated MOSFETs
n Current Mode Control Provides Excellent
Transient Response
n High Maximum Duty Cycle (92% Typ)
n ±2% RUN Pin Threshold with 100mV Hysteresis
n ±1% Internal Voltage Reference
n Micropower Shutdown: I = 10µA
Q
n Programmable Operating Frequency
(50kHz to 1MHz) with One External Resistor
n Synchronizable to an External Clock Up to 1.3 × f
OSC
n User-Controlled Pulse Skip or Burst Mode® Operation
n Output Overvoltage Protection
n Can be Used in a No R
SENSE™ Mode for VDS < 36V
n Small 10-Lead MSOP Package
The LTC®1871-7 is a current mode, boost, flyback and
SEPIC controller optimized for driving 6V-rated MOSFETs
in high voltage applications. The LTC1871-7 works equally
well in low or high power applications and requires few
components to provide a complete power supply solution.
The switching frequency can be set with an external resistor
over a 50kHz to 1MHz range, and can be synchronized to
an external clock using the MODE/SYNC pin. Burst Mode
operation at light loads, a low minimum operating supply
voltage of 6V and a low shutdown quiescent current of
10µA make the LTC1871-7 well suited for battery-operated
systems. For applications requiring constant frequency
operation, Burst Mode operation can be defeated using
the MODE/SYNC pin. The LTC1871-7 is available in the
10-lead MSOP package.
n
PARAMETER
n
n
n
LTC1871
7.0V
5.2V
UV +
5.6V
2.1V
INTVCC UV–
4.6V
1.9V
INTVCC
Applications
n
LTC1871-7
INTVCC
Telecom Power Supplies
42V Automotive Systems
24V Industrial Controls
IP Phone Power Supplies
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
of Linear Technology Corporation. No RSENSE is a trademark of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Typical Application
D3
10BQ060
VIN
36V TO 72V
D2
4148
LTC1871-7
FB
FREQ
MODE/SYNC
110k
120k
47µF
16V
X5R
10Ω
VIN
ITH
12.4k
T1
VP1-0076
SENSE
RUN
3.4k
Q1
FMMT625
•
D1
9.1V
2.2nF
3:1
100k
•
2.2µF
100V
X7R
604k
26.7k
VOUT
12V
0.4A
INTVCC
M1
FDC2512
GATE
GND
4.7µF
X5R
0.1µF
X5R
0.12Ω
18717 F01
Figure 1. Small, Nonisolated 12V Flyback Telecom Housekeeping Supply
18717fd
1
LTC1871-7
Absolute Maximum Ratings
(Note 1)
Pin Configuration
VIN Voltage ................................................ – 0.3V to 36V
INTVCC Voltage............................................ –0.3V to 9V
INTVCC Output Current...........................................50mA
GATE Voltage............................ –0.3V to VINTVCC + 0.3V
ITH, FB Voltages........................................ –0.3V to 2.7V
RUN Voltage................................................ –0.3V to 7V
MODE/SYNC Voltage.................................... –0.3V to 9V
FREQ Voltage............................................. –0.3V to 1.5V
SENSE Pin Voltage..................................... –0.3V to 36V
Operating Temperature Range (Note 2)
LTC1871E-7...........................................–40°C to 85°C
LTC1871I-7.......................................... –40°C to 125°C
Junction Temperature (Note 3).............................. 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec).................... 300°C
TOP VIEW
RUN
ITH
FB
FREQ
MODE/
SYNC
10
9
8
7
6
1
2
3
4
5
SENSE
VIN
INTVCC
GATE
GND
MS PACKAGE
10-LEAD PLASTIC MSOP
TJMAX = 125°C, θJA = 120°C/W
Order Information
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC1871EMS-7#PBF
LTC1871EMS-7#TRPBF
LTG4
10-Lead Plastic MSOP
–40°C to 85°C
LTC1871IMS-7#PBF
LTC1871IMS-7#TRPBF
LTBTR
10-Lead Plastic MSOP
–40°C to 125°C
LEAD BASED FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC1871EMS-7
LTC1871EMS-7#TR
LTG4
10-Lead Plastic MSOP
–40°C to 85°C
LTC1871IMS-7
LTC1871IMS-7#TR
LTBTR
10-Lead Plastic MSOP
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
electrical characteristics
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 8V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loop
VIN(MIN)
Minimum Input Voltage
I-Grade (Note 2)
IQ
Input Voltage Supply Current
Continuous Mode
VMODE/SYNC = 5V, VFB = 1.4V, VITH = 0.75V
●
VMODE/SYNC = 0V, VITH = 0.2V (Note 5)
VMODE/SYNC = 0V, VITH = 0.2V (Note 5),
I-Grade (Note 2)
Shutdown Mode
6
V
6
V
(Note 4)
VMODE/SYNC = 5V, VFB = 1.4V, VITH = 0.75V,
I-Grade (Note 2)
Burst Mode Operation, No Load
●
●
VRUN = 0V
VRUN = 0V, I-Grade (Note 2)
●
550
1000
µA
600
1100
µA
280
500
µA
280
600
µA
12
25
µA
12
25
µA
18717fd
2
LTC1871-7
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 8V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified.
SYMBOL
PARAMETER
VRUN+
Rising RUN Input Threshold Voltage
VRUN
–
Falling RUN Input Threshold Voltage
VRUN(HYST)
RUN Pin Input Threshold Hysteresis
IRUN
RUN Input Current
VFB
Feedback Voltage
CONDITIONS
●
VITH = 0.2V (Note 5)
VITH = 0.2V (Note 5), I-Grade (Note 2)
IFB
FB Pin Input Current
VITH = 0.2V (Note 5)
∆VFB
Line Regulation
6V ≤ VIN ≤ 30V
∆VIN
Load Regulation
∆VITH
TYP
MAX
1.348
I-Grade (Note 2)
∆VFB
MIN
●
UNITS
V
1.223
1.198
1.248
1.273
1.298
50
100
150
mV
35
100
175
mV
5
60
nA
1.230
1.242
1.248
V
V
1.255
V
●
1.218
1.212
●
1.205
V
V
18
60
nA
0.002
0.02
%/V
0.002
0.02
%/V
6V ≤ VIN ≤ 30V, I-Grade (Note 2)
●
VMODE/SYNC = 0V, VITH = 0.5V to 0.9V (Note 5)
●
–1
–0.1
%
VMODE/SYNC = 0V, VITH = 0.5V to 0.9V (Note 5)
I-Grade (Note 2)
●
–1
–0.1
%
2.5
6
∆VFB(OV)
∆FB Pin, Overvoltage Lockout
VFB(OV) – VFB(NOM) in Percent
gm
Error Amplifier Transconductance
ITH Pin Load = ±5µA (Note 5)
600
VITH(BURST)
Burst Mode Operation ITH Pin Voltage
Falling ITH Voltage (Note 5)
VSENSE(MAX)
Maximum Current Sense Input Threshold
Duty Cycle < 20%
ISENSE(ON)
SENSE Pin Current (GATE High)
VSENSE = 0V
35
ISENSE(OFF)
SENSE Pin Current (GATE Low)
VSENSE = 30V
Oscillator Frequency
RFREQ = 80k
Duty Cycle < 20%, I-Grade (Note 2)
10
µmho
0.3
120
●
150
%
V
180
mV
200
mV
70
µA
0.1
5
µA
250
300
350
kHz
300
350
kHz
100
Oscillator
fOSC
RFREQ = 80k, I-Grade (Note 2)
●
250
50
1000
kHz
I-Grade (Note 2)
●
50
1000
kHz
I-Grade (Note 2)
●
Oscillator Frequency Range
DMAX
Maximum Duty Cycle
87
92
97
%
87
92
98.5
%
1.25
1.30
1.25
1.30
fSYNC/fOSC
Recommended Maximum Synchronized
Frequency Ratio
fOSC = 300kHz (Note 6)
tSYNC(MIN)
MODE/SYNC Minimum Input Pulse Width
VSYNC = 0V to 5V
25
ns
tSYNC(MAX)
MODE/SYNC Maximum Input Pulse Width
VSYNC = 0V to 5V
0.8/fOSC
ns
VIL(MODE)
Low Level MODE/SYNC Input Voltage
VIH(MODE)
fOSC = 300kHz (Note 6), I-Grade (Note 2)
●
I-Grade (Note 2)
●
I-Grade (Note 2)
●
High Level MODE/SYNC Input Voltage
RMODE/SYNC
MODE/SYNC Input Pull-Down Resistance
VFREQ
Nominal FREQ Pin Voltage
0.3
V
0.3
V
1.2
V
1.2
V
50
kΩ
0.62
V
Low Dropout Regulator
VINTVCC
INTVCC Regulator Output Voltage
VIN = 8V
VIN = 8V, I-Grade (Note 2)
●
6.5
7
7.5
V
6.5
7
7.5
V
18717fd
3
LTC1871-7
Electrical
Characteristics
The
l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 8V, VRUN = 1.5V, RFREQ = 80k, VMODE/SYNC = 0V, unless otherwise specified.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
UVLO
INTVCC Undervoltage Lockout Threshold
Rising INTVCC
Falling INTVCC
UVLO Hysteresis
∆VINTVCC
INTVCC Regulator Line Regulation
8V ≤ VIN ≤ 15V
8
25
mV
∆VIN1
∆VINTVCC
INTVCC Regulator Line Regulation
15V ≤ VIN ≤ 30V
70
200
mV
VLDO(LOAD)
INTVCC Load Regulation
0 ≤ IINTVCC ≤ 20mA, VIN = 8V
VDROPOUT
INTVCC Regulator Dropout Voltage
tr
tf
5.6
4.6
1.0
V
V
V
∆VIN2
–2
–0.2
%
VIN = 6V, INTVCC Load = 20mA
280
mV
GATE Driver Output Rise Time
CL = 3300pF (Note 7)
17
100
ns
GATE Driver Output Fall Time
CL = 3300pF (Note 7)
8
100
ns
GATE Driver
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: The LTC1871E-7 is guaranteed to meet performance specifications
from 0°C to 70°C junction temperature. Specifications over the – 40°C
to 85°C operating junction temperature range are assured by design,
characterization and correlation with statistical process controls. The
LTC1871I-7 is guaranteed over the full –40°C to 125°C operating junction
temperature range.
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
TJ = TA + (PD • 120°C/W)
Note 4: The dynamic input supply current is higher due to power MOSFET
gate charging (QG • fOSC). See Applications Information.
Note 5: The LTC1871-7 is tested in a feedback loop that servos VFB to the
reference voltage with the ITH pin forced to a voltage between 0V and 1.4V
(the no load to full load operating voltage range for the ITH pin is 0.3V to
1.23V).
Note 6: In a synchronized application, the internal slope compensation
gain is increased by 25%. Synchronizing to a significantly higher ratio will
reduce the effective amount of slope compensation, which could result in
subharmonic oscillation for duty cycles greater than 50%.
Note 7: Rise and fall times are measured at 10% and 90% levels.
typical performance characteristics
FB Voltage vs Temp
FB Voltage Line Regulation
1.25
FB Pin Current vs Temperature
60
1.231
50
1.23
FB PIN CURRENT (nA)
FB VOLTAGE (V)
FB VOLTAGE (V)
1.24
1.230
1.22
40
30
20
10
1.21
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
18717 G01
1.229
0
5
10
15
20
VIN (V)
25
30
35
18717 G02
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
18717 G03
18717fd
4
LTC1871-7
Typical Performance Characteristics
Shutdown Mode IQ vs VIN
Shutdown Mode IQ vs Temperature
30
20
600
VIN = 8V
Burst Mode IQ vs VIN
20
10
BURST MODE IQ (µA)
SHUTDOWN MODE IQ (µA)
SHUTDOWN MODE IQ (µA)
500
15
10
5
400
300
200
100
0
0
10
20
VIN (V)
30
0
–50 –25
40
0
18717 G04
18
500
TIME (ns)
IQ (mA)
40
10
8
6
FALL TIME
10
2
0
25 50 75 100 125 150
TEMPERATURE (°C)
0
200
400
600
800
FREQUENCY (kHz)
RUN Thresholds vs VIN
10
20
VIN (V)
30
40
18717 G10
2000
4000
6000 8000
CL (pF)
10000 12000
18717 G09
RT vs Frequency
1.35
RT (kΩ)
RUN THRESHOLDS (V)
1.3
0
1000
1.40
0
0
1200
RUN Thresholds vs Temperature
1.5
RUN THRESHOLDS (V)
1000
18717 G08
18717 G07
1.2
RISE TIME
30
20
4
100
1.4
40
50
12
0
30
60
14
0
–50 –25
20
VIN (V)
Gate Drive Rise and
Fall Time vs CL
CL = 3300pF
IQ(TOT) = 600µA + Qg • f
16
400
200
10
18717 G06
Dynamic IQ vs Frequency
300
0
18717 G05
Burst Mode IQ vs Temperature
Burst Mode IQ (µA)
0
25 50 75 100 125 150
TEMPERATURE (°C)
1.30
100
1.25
1.20
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
18717 G11
10
0 100 200 300 400 500 600 700 800 900 1000
FREQUENCY (kHz)
18717 G12
18717fd
5
LTC1871-7
Typical Performance Characteristics
Maximum Sense Threshold
vs Temperature
Frequency vs Temperature
325
SENSE Pin Current vs Temperature
35
160
GATE FREQUENCY (kHz)
315
310
305
300
295
290
285
155
SENSE PIN CURRENT (µA)
MAX SENSE THRESHOLD (mV)
320
150
145
GATE HIGH
VSENSE = 0V
30
280
275
–50 –25
0
140
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
0
0
25 50 75 100 125 150
TEMPERATURE (°C)
18717 G14
18717 G13
INTVCC Load Regulation
18717 G15
INTVCC Dropout Voltage
vs Current, Temperature
INTVCC Line Regulation
500
7.2
VIN = 8V
450
6.9
DROPOUT VOLTAGE (mV)
INTVCC VOLTAGE (V)
7.0
INTVCC VOLTAGE (V)
25
–50 –25
25 50 75 100 125 150
TEMPERATURE (°C)
7.1
7.0
150°C
400
125°C
350
75°C
300
25°C
250
200
0°C
150
–50°C
100
50
6.8
0
10
20
30
40
50
60
70
80
6.9
0
5
10
15
18717 G16
20 25
VIN (V)
30
35
40
18717 G17
0
0
5
10
15
INTVCC LOAD (mA)
20
18717 G18
pin functions
RUN (Pin 1): The RUN pin provides the user with an
accurate means for sensing the input voltage and programming the start-up threshold for the converter. The
falling RUN pin threshold is nominally 1.248V and the
comparator has 100mV of hysteresis for noise immunity.
When the RUN pin is below this input threshold, the IC
is shut down and the VIN supply current is kept to a low
value (typ 10µA). The Absolute Maximum Rating for the
voltage on this pin is 7V.
ITH (Pin 2): Error Amplifier Compensation Pin. The current
comparator input threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 1.40V.
FB (Pin 3): Receives the feedback voltage from the external
resistor divider across the output. Nominal voltage for this
pin in regulation is 1.230V.
FREQ (Pin 4): A resistor from the FREQ pin to ground
programs the operating frequency of the chip. The nominal
voltage at the FREQ pin is 0.6V.
18717fd
6
LTC1871-7
Pin Functions
MODE/SYNC (Pin 5): This input controls the operating
mode of the converter and allows for synchronizing the
operating frequency to an external clock. If the MODE/
SYNC pin is connected to ground, Burst Mode operation
is enabled. If the MODE/SYNC pin is connected to INTVCC,
or if an external logic-level synchronization signal is applied to this input, Burst Mode operation is disabled and
the IC operates in a continuous mode.
GND (Pin 6): Ground Pin.
GATE (Pin 7): Gate Driver Output.
INTVCC (Pin 8): The Internal 7V Regulator Output. The
gate driver and control circuits are powered from this
voltage. Decouple this pin locally to the IC ground with a
minimum of 4.7µF low ESR tantalum or ceramic capacitor.
This 7V regulator has an undervoltage lockout circuit with
5.6V and 4.6V rising and falling thresholds, respectively.
VIN (Pin 9): Main Supply Pin. Must be closely decoupled
to ground.
SENSE (Pin 10): The Current Sense Input for the Control
Loop. Connect this pin to a resistor in the source of the
power MOSFET. Alternatively, the SENSE pin may be connected to the drain of the power MOSFET, in applications
where the maximum VDS is less than 36V. Internal leading
edge blanking is provided for both sensing methods.
Block Diagram
BIAS AND
START-UP
CONTROL
SLOPE
COMPENSATION
RUN
+
1
C2
–
1.248V
VIN
FREQ
4
0.6V
V-TO-I
PWM LATCH
85mV
1.230V
+
–
S
50k
OV
R
+
0.30V
FB
3
1.230V
9
INTVCC
IOSC
MODE/SYNC
5
OSC
–
+
EA
+
BURST
COMPARATOR
GATE
7
LOGIC
Q
GND
CURRENT
COMPARATOR
+
SENSE
10
C1
–
–
gm
ITH
V-TO-I
2
INTVCC
8
7V
–
5.6V UP
4.6V DOWN
+
ILOOP
LDO
1.230V
SLOPE
1.230V
UV
TO
START-UP
CONTROL
RLOOP
GND
BIAS
VREF
6
18717 BD
VIN
18717fd
7
LTC1871-7
operation
Main Control Loop
The LTC1871-7 is a constant frequency, current mode
controller for DC/DC boost, SEPIC and flyback converter
applications. With the LTC1871-7 the current control loop
can be closed by sensing the voltage drop either across
the power MOSFET switch or across a discrete sense
resistor, as shown in Figure 2.
L
VIN
D
VOUT
VIN
+
SENSE
VSW
COUT
GATE
GND
GND
2a. SENSE Pin Connection for
Maximum Efficiency (VSW < 36V)
L
VIN
VIN
D
VOUT
VSW
GATE
SENSE
GND
+
COUT
RS
GND
18717 F02
2b. SENSE Pin Connection for Precise
Control of Peak Current or for VSW > 36V
Figure 2. Using the SENSE Pin On the LTC1871-7
For circuit operation, please refer to the Block Diagram of
the IC and Figure 1. In normal operation, the power MOSFET
is turned on when the oscillator sets the PWM latch and
is turned off when the current comparator C1 resets the
latch. The divided-down output voltage is compared to an
internal 1.230V reference by the error amplifier EA, which
outputs an error signal at the ITH pin. The voltage on the
ITH pin sets the current comparator C1 input threshold.
When the load current increases, a fall in the FB voltage
relative to the reference voltage causes the ITH pin to rise,
which causes the current comparator C1 to trip at a higher
peak inductor current value. The average inductor current
will therefore rise until it equals the load current, thereby
maintaining output regulation.
The nominal operating frequency of the LTC1871-7 is
programmed using a resistor from the FREQ pin to ground
and can be controlled over a 50kHz to 1000kHz range. In
addition, the internal oscillator can be synchronized to
an external clock applied to the MODE/SYNC pin and can
be locked to a frequency between 100% and 130% of its
nominal value. When the MODE/SYNC pin is left open, it
is pulled low by an internal 50k resistor and Burst Mode
operation is enabled. If this pin is taken above 2V or an
external clock is applied, Burst Mode operation is disabled
and the IC operates in continuous mode. With no load (or
an extremely light load), the controller will skip pulses
in order to maintain regulation and prevent excessive
output ripple.
The RUN pin controls whether the IC is enabled or is in a low
current shutdown state. A micropower 1.248V reference
and comparator C2 allow the user to program the supply
voltage at which the IC turns on and off (comparator C2
has 100mV of hysteresis for noise immunity). With the
RUN pin below 1.248V, the chip is off and the input supply
current is typically only 10µA.
An overvoltage comparator OV senses when the FB pin
exceeds the reference voltage by 6.5% and provides a
reset pulse to the main RS latch. Because this RS latch is
reset-dominant, the power MOSFET is actively held off for
the duration of an output overvoltage condition.
The LTC1871-7 can be used either by sensing the voltage drop across the power MOSFET or by connecting the
SENSE pin to a conventional shunt resistor in the source
of the power MOSFET, as shown in Figure 2. Sensing the
voltage across the power MOSFET maximizes converter
efficiency and minimizes the component count, but limits
the output voltage to the maximum rating for this pin (36V).
By connecting the SENSE pin to a resistor in the source
of the power MOSFET, the user is able to program output
voltages significantly greater than 36V.
Programming the Operating Mode
For applications where maximizing the efficiency at very
light loads (e.g.,