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LM25037, LM25037-Q1
SNVS572E – JULY 2008 – REVISED JANUARY 2016
LM25037/-Q1 Dual-Mode PWM Controller With Alternating Outputs
1 Features
2 Applications
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Qualified for Automotive Applications
AEC-Q100 Qualified With the Following Results:
– Device Temperature Grade 1: –40°C to
+125°C Operating Junction Temperature
– Device HBM ESD Classification Level 2
– Device CDM ESD Classification Level C4B
Alternating Outputs for Double-Ended Topologies
Ultra Wide Input Operating Range from 5.5 V to
75 V
Current-Mode or Feed-Forward Voltage-Mode
Control
Programmable Maximum Duty Cycle Limit
2% Feedback Reference Accuracy
High Gain-Bandwidth Error Amplifier
Programmable Line Undervoltage Lockout (UVLO)
With Adjustable Hysteresis
Versatile Dual Mode Overcurrent Protection With
Hiccup Delay Timer
Programmable Soft-Start
Precision 5-V Reference Output
Current Sense Leading Edge Blanking
Resistor Programmed 2-MHz Capable Oscillator
Oscillator Synchronization Capability With LowFrequency Lockout Protection
16-Pin TSSOP
Telecom Power Converters
Industrial Power Converters
Automotive Power Converters (Q1 Version)
3 Description
The LM25037 PWM controller contains all the
features necessary to implement balanced doubleended power converter topologies, such as push-pull,
half-bridge and full-bridge. These double-ended
topologies allow for higher efficiencies and greater
power densities compared to common single-ended
topologies such as the flyback and forward. The
LM25037 can be configured for either voltage mode
or current mode control with minimum external
components. Two alternating gate drive outputs are
provided, each capable of 1.2-A peak output current.
The LM25037 can be configured to operate directly
from the input voltage rail over an ultra-wide range of
5.5 V to 75 V. Additional features include
programmable maximum duty cycle limit, line
undervoltage lockout, cycle-by-cycle current limit and
a hiccup mode fault protection with adjustable timeout
delay, soft-start and a 2-MHz capable oscillator with
synchronization capability, precision reference and
thermal shutdown.
Device Information(1)
PART NUMBER
LM25037
LM25037-Q1
PACKAGE
TSSOP (16)
BODY SIZE (NOM)
5.00 mm × 4.40 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Simplified Push-Pull Power Converter
VIN
VOUT
LM25037
VIN
VCC
UVLO
OUTA
REF
OUTB
RT1
RAMP
RT2
CS
RES
COMP
SS
PGND
Isolated
Feedback
FB
AGND
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM25037, LM25037-Q1
SNVS572E – JULY 2008 – REVISED JANUARY 2016
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
6.7
4
4
4
4
5
5
7
Absolute Maximum Ratings ......................................
ESD Ratings: LM25037 ............................................
ESD Ratings: LM25037-Q1 ......................................
Recommended Operating Conditions ......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Detailed Description .............................................. 9
7.1 Overview ................................................................... 9
7.2 Functional Block Diagram ......................................... 9
7.3 Feature Description................................................. 10
7.4 Device Functional Modes........................................ 13
8
Application and Implementation ........................ 17
8.1 Application Information............................................ 17
8.2 Typical Application .................................................. 24
9 Power Supply Recommendations...................... 30
10 Layout................................................................... 30
10.1 Layout Guidelines ................................................. 30
10.2 Layout Example .................................................... 30
11 Device and Documentation Support ................. 31
11.1
11.2
11.3
11.4
11.5
11.6
Documentation Support ........................................
Related Links ........................................................
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
31
31
31
31
31
31
12 Mechanical, Packaging, and Orderable
Information ........................................................... 31
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision D (March 2013) to Revision E
Page
•
Added ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section ................................................................................................. 1
•
Deleted Typical Application Circuit Efficiency (previously Figure 1.) graph from Typical Characteristics ............................. 7
Changes from Revision C (March 2013) to Revision D
•
2
Page
Changed layout of National Data Sheet to TI format ........................................................................................................... 24
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SNVS572E – JULY 2008 – REVISED JANUARY 2016
5 Pin Configuration and Functions
PW Package
16-Pin TSSOP
Top View
RAMP
1
16
VIN
UVLO
2
15
REF
COMP
3
14
VCC
FB
4
13
OUTA
RT2
5
12
OUTB
AGND
6
11
PGND
RT1
7
10
SS
CS
8
9
RES
LM25037
Pin Functions
PIN
NO.
NAME
1
RAMP
I/O
I
DESCRIPTION
APPLICATION INFORMATION
Pulse width modulator ramp
Modulation ramp for the PWM comparator. This ramp can be a
representative of the primary current (current mode) or proportional
to input voltage (feed-forward voltage mode). This pin is reset to
ground at the conclusion of every cycle by an internal FET.
Line undervoltage lockout
An external voltage divider from the power source sets the shutdown
and standby comparator threshold levels. When UVLO exceeds the
0.45V shutdown threshold, the VCC and REF regulators are
enabled. When UVLO exceeds the 1.25V standby threshold, the SS
pin is released and the device enters the active mode.
Input to the pulse width modulator
Output of the error amplifier and input to the PWM comparator.
2
UVLO
I
3
COMP
I/O
4
FB
I
Feedback
Connected to inverting input of the error amplifier. An internal 1.25-V
reference is connected to the noninverting input of the error
amplifier. In isolated applications using an external error amplifier,
this pin should be connected to the AGND pin.
5
RT2
I
Oscillator dead-time control
The resistance connected between RT2 and AGND sets the forced
dead-time between switching periods of the alternating outputs.
6
AGND
—
Analog ground
Connect directly to Power Ground.
7
RT1
I
Oscillator maximum on-time control
The resistance connected between RT1 and AGND sets the
oscillator maximum on-time. The sum of this maximum on-time and
the forced dead-time (set by RT2) sets the oscillator period.
8
CS
I
Current sense input
If CS exceeds 250 mV the output pulse will be terminated, entering
cycle-by-cycle current limit. An internal switch holds CS low for 65
nS after either output switches high to blank leading edge transients.
Restart timer
If cycle-by-cycle current limit is reached during any cycle, a 18-µA
current is sourced to the external RES pin capacitor. If the RES
capacitor voltage reaches 2 V, the soft-start capacitor will be fully
discharged and then released with a pullup current of 1 uA. After the
first output pulse (when SS = 1V), the SS pin charging current will
increase to the normal level of 100 µA.
Soft-start
An external capacitor and an internal 100uA current source set the
soft-start ramp. The SS current source is reduced to 1 µA following a
restart event (RES pin high).
9
RES
I/O
10
SS
I
11
PGND
—
Power ground
Connect directly to Analog Ground
12
OUTB
O
Output driver
Alternating gate drive output of the pulse width modulator. Capable
of 1.2-A peak source and sink current.
13
OUTA
O
Output driver
Alternating gate drive output of the pulse width modulator. Capable
of 1.2-A peak source and sink current.
14
VCC
I/O
Output of the high voltage start-up
regulator. The VCC voltage is
regulated to 7.7 V.
If an auxiliary winding raises the voltage on this pin above the
regulation set point, the internal start-up regulator will shutdown thus
reducing the IC power dissipation. Locally decouple VCC with a 0.47
µF or greater capacitor.
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SNVS572E – JULY 2008 – REVISED JANUARY 2016
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Pin Functions (continued)
PIN
NO.
NAME
I/O
DESCRIPTION
APPLICATION INFORMATION
15
REF
O
Output of a 5-V reference
Locally decouple with a 0.1 µF or greater capacitor. Maximum output
current is 10 mA (typical).
16
VIN
I
Input voltage source
Input to the VCC Start-up regulator. Operating input range is 5.5 V to
75 V. For power sources outside of this range, the LM25037 can be
biased directly at VCC by an external regulator.
6 Specifications
6.1 Absolute Maximum Ratings
over operating junction temperature range (unless otherwise noted) (1) (2)
MIN
MAX
UNIT
VIN to GND
–0.3
76
V
VCC, RAMP , OUTA, OUTB to GND
–0.3
16
V
CS to GND
–0.3
1
V
UVLO, FB, RT2, RT1, SS, REF to GND
–0.3
7
V
150
°C
150
°C
COMP, RES (3)
Junction temperature
Storage temperature
(1)
(2)
(3)
–65
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
If Military/Aerospace specified devices are required, contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
COMP, RES are output pins. As such, TI does not recommend connecting external power sources to these pins.
6.2 ESD Ratings: LM25037
VALUE
V(ESD)
(1)
(2)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1)
±2000
Charged-device model (CDM), per JEDEC specification JESD22C101 (2)
±750
UNIT
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
6.3 ESD Ratings: LM25037-Q1
VALUE
Human-body model (HBM), per AEC Q100-002 (1)
V(ESD)
(1)
Electrostatic
discharge
Charged-device model (CDM), per AEC Q100-011
UNIT
±2000
All pins
±750
Corner pins (1, 8, 9, 16)
±750
V
AEC Q100-002 indicates that HBM stressing shall be in accordance with the ANSI/ESDA/JEDEC JS-001 specification.
6.4 Recommended Operating Conditions
over operating junction temperature range (unless otherwise noted)
MIN
VIN voltage
External voltage applied to VCC
Operating junction temperature
4
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NOM
MAX
UNIT
5.5
75
V
8
14
V
−40
125
°C
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SNVS572E – JULY 2008 – REVISED JANUARY 2016
6.5 Thermal Information
LM25037/LM25037-Q1
THERMAL METRIC (1)
PW (TSSOP)
UNIT
16 PINS
RθJA
Junction-to-ambient thermal resistance
99.9
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
32.6
°C/W
RθJB
Junction-to-board thermal resistance
45.8
°C/W
ψJT
Junction-to-top characterization parameter
2
°C/W
ψJB
Junction-to-board characterization parameter
45.1
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
N/A
°C/W
(1)
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
6.6 Electrical Characteristics
VVIN = 12V, VVCC = 10V, RRT1 = 30.1 kΩ, RRT2 = 30.1 kΩ, VUVLO = 3 V, TJ =−40°C to +125° unless otherwise stated. (1) (2)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
7.7
8.1
UNIT
START-UP REGULATOR (VCC PIN)
VVCC
VCC voltage
IVCC = 10 mA
7.2
IVCC(Lim)
VCC current limit
VVCC = 7 V
20
VVCC(UV)
IVIN
VCC Undervoltage threshold
4.6
Hysteresis
Start-up regulator current
V
mA
5
5.4
V
0.5
V
VVIN = 20 V, VUVLO = 0 V
35
58
µA
VVIN = 75 V, VUVLO = 0 V
45
80
µA
Supply current into VCC from Outputs and COMP open, VVCC = 10 V, Outputs
external source
switching
4
mA
VOLTAGE REFERENCE REGULATOR (REF PIN)
VREF
REF Voltage
IREF = 0 mA
REF Voltage Regulation
IREF = 0 to 2.5 mA
REF Current Limit
VREF = 4.5 V
IREF(Lim)
VREF Undervoltage
threshold
VREF(UV)
Hysteresis
4.75
5
5.15
7
25
5
10
3.7
4
V
mV
mA
4.3
0.4
V
V
UNDERVOLTAGE LOCKOUT AND SHUTDOWN (UVLO PIN)
VUVLO
IUVLO
Undervoltage threshold
Hysteresis current
UVLO pin sinking
Undervoltage shutdown
threshold
UVLO voltage rising
1.20
1.25
1.295
V
17
22
26
µA
0.35
0.45
0.6
V
Hysteresis
0.1
V
CURRENT SENSE INPUT (CS PIN)
Current limit threshold
VCS
(1)
(2)
CS delay to output
0.22
CS from 0 V to 1 V. Time for OUTA and OUTB to
fall to 90% of VCC. Output load = 0 pF.
0.255
0.29
V
27
ns
Leading edge blanking time
at CS
65
ns
CS sink impedance (clocked) Internal FET sink impedance
21
45
Ω
All limits are ensured. All electrical characteristics having room temperature limits are tested during production at TA = 25°C. All hot and
cold limits are ensured by correlating the electrical characteristics to process and temperature variations and applying statistical process
control.
Typical specifications represent the most likely parametric norm at 25°C operation.
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Electrical Characteristics (continued)
VVIN = 12V, VVCC = 10V, RRT1 = 30.1 kΩ, RRT2 = 30.1 kΩ, VUVLO = 3 V, TJ =−40°C to +125° unless otherwise stated.(1)(2)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
1.9
2
2.2
V
14
18
21
µA
5
8
11
µA
CURRENT LIMIT RESTART (RES PIN)
VRES
RES threshold
Charge source current
VRES = 1.5 V
Discharge sink current
VRES = 1 V
SOFT-START (SS PIN)
ISS
Charging current in normal
operation
VSS = 0
70
100
130
µA
Charging current during a
hiccup mode restart
VSS = 0
0.6
1
1.4
µA
Soft-stop current sink
VSS = 2 V
70
100
130
µA
40
75
105
ns
OSCILLATOR (RT1 AND RT2 PINS)
DT1
Dead-time 1
RRT2 = 15 kΩ
DT2
Dead-time 2
RRT2 = 50 kΩ
FSW1
Frequency 1 (at OUTA, half
oscillator frequency)
RRT1 = 30.1 kΩ, RRT2 = 30.1 kΩ,
178
200
222
kHz
FSW2
Frequency 2 (at OUTA, half
oscillator frequency)
RRT1 = 11 kΩ, RRT2 = 30.1 kΩ,
448
515
578
kHz
250
DC level
ns
2
Input sync threshold
2.5
3
V
3.4
V
PWM CONTROLLER (COMP PIN)
Delay to output
VPWM-OS
65
SS to RAMP offset
0.7
1
TJ = 25°C
ns
1.2
V
Minimum duty cycle
VSS = 0 V
0%
COMP open-circuit voltage
VFB = 0 V
4.5
4.75
5
COMP short-circuit current
VFB = 0 V, COMP = 0 V
0.5
1
1.5
mA
5
20
Ω
V
VOLTAGE FEED-FORWARD (RAMP PIN)
RAMP sink impedance
(clocked)
ERROR AMPLIFIER
GBW
Gain bandwidth
DC gain
Input voltage
VFB = COMP
COMP sink capability
VFB = 1.5 V COMP = 1 V
TJ = 25°C
4
MHz
75
dB
1.22
1.245
5
13
mA
10
nA
FB bias current
1.27
V
MAIN OUTPUT DRIVERS (OUTA and OUTB Pins)
Output high voltage
IOUT = 50 mA, (source)
Output low voltage
IOUT = 100 mA (sink)
Vcc-0.5
Vcc-0.25
0.2
V
Rise time
CLOAD = 1 nF
17
ns
Fall time
CLOAD = 1 nF
18
ns
Peak source current
VVCC = 10 V
1.2
A
Peak sink current
VVCC = 10 V
1.2
A
Thermal shutdown threshold
165
°C
Thermal shutdown hysteresis
25
°C
0.5
V
THERMAL SHUTDOWN
TSD
6
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SNVS572E – JULY 2008 – REVISED JANUARY 2016
6.7 Typical Characteristics
Figure 1. VVCC and VREF vs VVIN
Figure 2. Start-Up Regulator Current (UVLO = 0)
Figure 3. VVCC vs IVCC
Figure 4. VREF vs IREF
180
50
150
40
120
30
90
20
60
10
30
0
-10
0
-30
-20
-60
-30
-90
-40
-120
-50
-150
-60
10k
PHASE (o)
GAIN (dB)
60
-180
10M
100k
1M
FREQUENCY
Figure 5. Feedback Amplifier Gain/Phase
Figure 6. Oscillator Frequency vs RT1
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Typical Characteristics (continued)
Figure 7. Dead-Time vs RT2
Figure 8. VFB vs Temperature
Figure 9. Oscillator Frequency vs Temperature
Figure 10. Dead-Time vs Temperature
Figure 11. Soft-Start and Restart Current vs Temperature
8
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7 Detailed Description
7.1 Overview
The LM25037 PWM controller contains all the features necessary to implement double-ended power converter
topologies such as push-pull, half-bridge and full-bridge. The unique architecture allows the modulator to be
configured for either voltage-mode or current-mode control. The LM25037 provides two alternating gate driver
outputs to drive the primary side power MOSFETs with programmable forced dead-time. The LM25037 can be
configured to operate with bias voltages ranging from 5.5 V to 75 V. Additional features include line undervoltage
lockout, cycle-by-cycle current limit, voltage feed-forward compensation, and hiccup mode fault protection with
adjustable delays, soft-start, and a 2-MHz capable oscillator with synchronization capability, precision reference,
and thermal shutdown. These rich set of features simplify the design of double ended topologies. The functional
block diagram is show in the Functional Block Diagram section.
7.2 Functional Block Diagram
7.7V LDO
VCC
VIN
SHUTDOWN
0.45V
UVLO
5V
Reference
STANDBY
1.25V
REF
VCC/REF
UVLO
MODE
CONTROL
LOGIC
THERMAL LIMIT
( 165°C)
UVLO HYSTERESIS (22 PA)
VCC
RT1/SYNC
CLK
J
SET
Q
K
CLR
Q
DRIVER
OSCILLATOR
RT2
S
OUTA
Q
VCC
R
RAMP
1.25V
DRIVER
OUTB
ERROR
AMP
FB
+5V
PGND
5k
PWM
COMP
AGND
PWM
LOGIC
1V
SS
SS Buffer
2.0V
+5V
CS
0.25V
Hiccup
CLK + LEB
Restart
Current
Source
Logic
18 PA
RES
+5V
+5V
SS
CLK
RESTART DELAY
SOFT-START
100 PA
1 PA
8 PA
SS
Shutdown
100 PA
Standby
SOFT-STOP
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7.3 Feature Description
7.3.1 High-Voltage Start-Up Regulator
The LM25037 contains an internal high voltage, low drop-out start-up regulator that allows the input pin (VIN) to
be connected directly to the supply voltage over a range of 5.5 V to a maximum of 75 V. The regulator output at
VCC (7.7 V) is internally current limited with a specified minimum of 20 mA. When the UVLO pin potential is
greater than 0.45 V, the VCC regulator is enabled to charge an external capacitor connected to the VCC pin. The
VCC regulator provides power to the voltage reference (REF) and the gate drivers (OUTA and OUTB). When the
voltage on the VCC pin exceeds its undervoltage (VCC UV) threshold of 5-V nominal, the internal voltage
reference (REF) reaches its regulation set point of 5 V and the UVLO voltage is greater than 1.25 V, the
controller outputs are enabled. The value selected for the VCC capacitor depends on the total system design,
and its start-up characteristics. The recommended range of values for the VCC capacitor is 0.47 µF to 10 µF.
The internal power dissipation of the LM25037 can be reduced by powering VCC from an external supply. In
typical applications, an auxiliary transformer winding is connected through a diode to the VCC pin. This winding
must raise the VCC voltage above 8.2 V to shut off the internal start-up regulator. Powering VCC from an
auxiliary winding improves efficiency while reducing the controller’s power dissipation. The VCC UV circuit will
still function in this mode, requiring that VCC never falls below 5-V nominal during the start-up sequence. The
VCC regulator series pass transistor includes a diode between VCC and VIN that should not be forward biased
in normal operation. Therefore the auxiliary VCC voltage should never exceed the VIN voltage.
An external DC bias voltage can be used instead of the internal regulator by connecting the external bias voltage
to both the VCC and the VIN pins. In this particular case, the external bias must be greater than max VCC UV of
5.4 V and less than the VCC maximum operating voltage rating (14 V).
7.3.2 Line Undervoltage Detector
The LM25037 contains a dual level line Undervoltage Lock Out (UVLO) circuit. When the UVLO pin voltage is
less than 0.45 V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than
0.45 V but less than 1.25 V, the controller is in standby mode. In standby mode the VCC and REF bias
regulators are active while the controller outputs are disabled. When the VCC and REF outputs exceed their
respective undervoltage thresholds and the UVLO pin voltage is greater than 1.25 V, the outputs are enabled
and normal operation begins. An external set-point voltage divider from VIN to GND can be used to set the
minimum operating voltage of the converter. The divider must be designed such that the voltage at the UVLO pin
will be greater than 1.25 V when VIN enters the desired operating range. UVLO hysteresis is accomplished with
an internal 22-µA current source that is switched on or off into the impedance of the set-point divider. When the
UVLO pin voltage exceeds 1.25-V threshold, the current source is activated to quickly raise the voltage at the
UVLO pin. When the UVLO pin voltage falls below the 1.25-V threshold, the current source is disabled causing
the voltage at the UVLO pin to quickly fall. The hysteresis of the 0.45-V shutdown comparator is internally fixed
at 100 mV.
The UVLO pin can also be used to implement various remote enable/disable functions. Turning off the converter
by forcing the UVLO pin to standby condition provides a controlled soft-stop. See the Soft-Start section for more
details.
7.3.3 Reference
The REF pin is the output of a 5-V linear regulator that can be used to bias an opto-coupler transistor and
external housekeeping circuits. The regulator output is internally current limited to 10 mA (typical).
7.3.4 Error Amplifier
An internal high gain error amplifier is provided within the LM25037. The amplifier’s noninverting reference is tied
to a 1.25-V reference. In non-isolated applications the power converter output is connected to the FB pin through
the voltage setting resistors and loop compensation is connected between the COMP and FB pins. A typical
gain/phase plot is shown in Typical Characteristics.
For most isolated applications the error amplifier function is implemented on the secondary side. Because the
internal error amplifier is configured as an open-drain output, it can be disabled by connecting FB to ground. The
internal 5-K pullup resistor connected between the COMP pin and the 5-V reference can be used as the pullup
for an opto-coupler or other isolation device.
10
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Feature Description (continued)
7.3.5 Cycle-By-Cycle Current Limit
The CS pin is to be driven by a signal representative of the transformer primary current. The current sense signal
can be generated by using a sense resistor or a current sense transformer. If the voltage sensed at the CS pin
exceeds 0.255 V, the current sense comparator terminates the output driver pulse. If the high current condition
persists, the controller operates in a cycle-by-cycle current limit mode with duty cycle determined by the current
sense comparator instead of the PWM comparator. Cycle-by-cycle current limiting may eventually trigger the
hiccup mode restart cycle; depending on the configuration of the RES pin (see Overload Protection Timer). To
suppress noise, TI recommends connecting a small R-C filter to the CS pin and placing it near the controller. An
internal 21-Ω MOSFET discharges the external current sense filter capacitor at the conclusion of every cycle.
The discharge MOSFET remains on for an additional 65 ns after either OUTA or OUTB driver switches high to
blank leading edge transients in the current sensing circuit. Discharging the CS pin filter each cycle and blanking
leading edge spikes reduces the filtering requirements and improves the current sense response time. The
current sense comparator is very fast and may respond to short duration noise pulses. Layout considerations are
critical for the current sense filter and sense resistor. The capacitor associated with the CS filter must be placed
very close to the device and connected directly to the CS and AGND pins. If a sense resistor located in the
source of the main MOSFET switch is used for current sensing, a low inductance type of resistor is required.
When designing with a current sense resistor, all the noise sensitive, low power ground connections should be
connected together near the AGND pin, and a single connection should be made to the power ground (sense
resistor ground point).
7.3.6 Overload Protection Timer
The LM25037 provides a current limit restart timer to disable the outputs and force a delayed restart (hiccup
mode) if a current limit condition is repeatedly sensed. The number of cycle-by-cycle current limit events required
to trigger the restart is programmed by the external capacitor at the RES pin. During each PWM cycle, the
LM25037 either sources to or sinks current from the RES pin capacitor. If no current limit is detected during a
cycle, a 8-µA discharge current sink is enabled to pull the RES pin towards ground. If a current limit is detected,
the 8 µA sink current is disabled and an 18-µA current source causes the voltage at the RES pin to gradually
increase. The LM25037 protects the converter with cycle-by-cycle current limiting while the voltage at RES pin
increases. If the RES voltage reaches the 2-V threshold, the following restart sequence occurs (also see
Figure 12):
• The RES capacitor and SS capacitors are fully discharged.
• The soft-start current source is reduced from 100 µA to 1 µA.
• The SS capacitor voltage slowly increases. When the SS voltage reaches ≊1 V, the PWM comparator will
produce the first narrow output pulse. After the first pulse occurs, the SS source current reverts to the normal
100-µA level. The SS voltage increases at its normal rate, gradually increasing the duty cycle of the output
drivers.
• If the overload condition persists after restart, cycle-by-cycle current limiting will begin to increase the voltage
on the RES capacitor again, repeating the hiccup mode sequence.
• If the overload condition no longer exists after restart, the RES pin will be held at ground by the 8-µA current
sink and normal operation resumes.
The overload timer function is very versatile and can be configured for the following modes of protection:
1. Cycle-by-cycle only: The hiccup mode can be completely disabled by connecting a 0-kΩ to 50-kΩ resistor
from the RES pin to AGND. In this configuration, the cycle-by-cycle protection will limit the output current
indefinitely and no hiccup sequences will occur.
2. Hiccup only: The timer can be configured for immediate activation of a hiccup sequence upon detection of
an overload by leaving the RES pin open circuit. In this configuration, the first detection of current limit
condition by the CS pin comparator will initiate a hiccup cycle with SS capacitor fully discharged and a
delayed restart.
3. Delayed Hiccup: Connecting a capacitor to the RES pin provides a programmed interval of cycle-by-cycle
limiting before initiating a hiccup mode restart, as previously described. The dual advantages of this
configuration are that a short term overload will not cause a hiccup mode restart but during extended
overload conditions, the average dissipation of the power converter will be very low.
4. Externally Controlled Hiccup: The RES pin can also be used as an input. By externally driving the pin to a
level greater than the 2-V hiccup threshold, the controller will be forced into the delayed restart sequence.
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Feature Description (continued)
For example, the external trigger for a delayed restart sequence could come from an overtemperature
protection circuit or an output overvoltage sensor.
Current
Limit
CS
Current
Sense Circuit
5V
Restart
Current
Source Logic
0.25V
CLK
18 PA
RES
C RES
8 PA
SS
Voltage
Feedback
COMP
2.0V
To Output
Drivers
PWM
S
R Q
Restart
Latch
Drivers Off
+5V
Restart
Comparator
+5V
100 PA
CSS
SS
1PA
SS
Logic
100 mV
Drivers Off
Soft-start
LM25037
Figure 12. Current Limit Restart Circuit
Current Limit Detected
at CS
Current Limit Persists
2.0V
RES
0V
100 PA
5V
# 1V
SS
1 PA
OUTA
OUTB
t1
t3
t2
Figure 13. Current Limit Restart Timing
7.3.7 Soft-Start
The soft-start circuit allows the regulator to gradually reach a steady-state operating point, thereby reducing startup stresses and current surges. When bias is supplied to the LM25037, the SS pin capacitor is discharged by an
internal MOSFET. When the UVLO, VCC and REF pins reach their operating thresholds, the SS capacitor is
released and charged with a 100-µA current source. The PWM comparator control voltage at the COMP pin is
clamped to the SS pin voltage by an internal amplifier. When the PWM comparator input reaches 1 V, output
pulses commence with slowly increasing duty cycle. The voltage at the SS pin eventually increases to 5 V, while
the voltage at the PWM comparator increases to the value required for regulation as determined by the voltage
feedback loop.
One method to disable the regulator is to ground the SS pin. This forces the internal PWM control signal to
ground, reducing the output duty cycle quickly to zero. Releasing the SS pin initiates a soft-start sequence and
normal operation resumes. A second shutdown method is discussed in UVLO Divider Selection.
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Feature Description (continued)
7.3.8 PWM Comparator
The pulse width modulation (PWM) comparator compares the voltage ramp signal at the RAMP pin to the loop
error signal. The loop error signal is derived from the internal error amplifier (COMP pin). The resulting control
voltage passes through a 1-V level shift before being applied to the PWM comparator. This comparator is
optimized for speed to achieve minimum controllable duty cycles. The common mode input voltage range of the
PWM comparator is from 0 V to 4.3 V.
7.3.9 RAMP Pin
The voltage at the RAMP pin provides the modulation ramp for the PWM comparator. The PWM comparator
compares the modulation ramp signal at the RAMP pin to the loop error signal to control the output duty cycle.
The modulation ramp can be implemented either as a ramp proportional to input voltage, known as feed-forward
voltage mode control, or as a ramp proportional to the primary current, known as current mode control. The
RAMP pin is reset by an internal FET with an RDS(ON) of 5 Ω (typical) at the end of every cycle. The ability to
configure the RAMP pin for either voltage mode or current mode allows the controller to be implemented for the
optimum control method for the selected power stage topology. Configuring RAMP pin is explained below and
the differences between voltage mode control and current mode control in various double-ended topologies is
explained in Application and Implementation.
7.4 Device Functional Modes
7.4.1 Feed-Forward Voltage Mode
An external resistor (RFF) and capacitor (CFF) connected to VIN, AGND, and the RAMP pins is required to create
the PWM ramp signal as shown in Figure 14. It can be seen that the slope of the signal at RAMP will vary in
proportion to the input line voltage. This varying slope provides line feed-forward information necessary to
improve line transient response with voltage mode control. The RAMP signal is compared to the error signal by
the pulse width modulator comparator to control the duty cycle of the outputs. With a constant error signal, the
on-time (tON) varies inversely with the input voltage (VIN) to stabilize the Volt • Second product of the transformer
primary. At the end of clock period, an internal FET will be enabled to reset the CFF capacitor. The formulae for
RFF and CFF and component selection criteria are explained in the Application and Implementation section. The
amplitude of the signal driving RAMP pin must not exceed the common mode input voltage range of the PWM
comparator (3.3 V) while in normal operation.
SLOPE
PROPORTIONAL
TO Vin
VIN
Vin
R FF
COMP
1V
Gate
Drive
RAMP
C FF
CLK
LM25037
Figure 14. Feed-Forward Voltage Mode Configuration
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Device Functional Modes (continued)
7.4.2 Current Mode
The LM25037 can be configured for current mode control by injecting a signal representative of primary current
into the RAMP pin. One way to achieve this is shown in Figure 15. Filter components Rfilter and Cfilter are used to
filter leading edge noise spikes. The signal at the CS pin is thus a ramp on a pedestal. The pedestal corresponds
to the continuous conduction current in the transformer at the beginning of an OUTA or OUTB conduction cycle.
The R-C circuit (RSlope and CSlope), shown in Figure 15, tied to VREF adds an additional ramp to the current sense
signal. This additional ramp signal, known as slope compensation, is required to avoid instabilities at duty cycles
above 50% (25% per phase). The compensated RAMP signal consists of two parts, the primary current signal
and the slope compensation. The compensated RAMP signal is compared to the error signal by the PWM
comparator to control the duty cycle of the outputs. The RAMP capacitor and CS capacitor are reset through
internal discharge FETs. The RDS(ON) of RAMP discharge FET is 5 Ω (typical); this ensures fast discharge of the
RAMP reset capacitor. Any DC voltage source can be used in place of VREF to generate the slope compensation
ramp.
The timing diagram shown in Figure 16 depicts the current mode waveforms and relative timing. When OUTA or
OUTB is enabled, the signal at the RAMP pin consists of the CS pin signal (current ramp on a pedestal) plus the
slope compensation ramp (dotted lines). When OUTA or OUTB is turned off, the primary current component is
absent but the voltage at the RAMP pin continues to rise due to slope compensation component until the end of
the clock period, after which it is reset by the RAMP discharge FET. A component selection example is explained
in detail in Application and Implementation.
V REF (5V)
R slope
RAMP
CLK
C slope
LM 25037
Current
Sense
R filter
cs
CLK + LEB
Rcs
C filter
Figure 15. Current Mode Configuration With Slope Compensation
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Device Functional Modes (continued)
CLK
CS
OUTA
OUTB
COMP
RAMP
Slope
Compensation
OUTPUT
OUTA
OUTB
Figure 16. Timing Diagram for Current Mode Configuration
7.4.3 Oscillator
The LM25037 oscillator frequency and the maximum duty cycle are set by two external resistors connected
between the RT1 and RT2 pins to AGND. The minimum dead-time between OUTA and OUTB pulses is
proportional to the RT2 resistor value and the overall oscillator frequency is inversely proportional to RT1 and
RT2 resistor values. Each output switches at half the oscillator frequency. Initially, RT2 should be selected for the
desired dead-time or for the desired maximum duty cycle (Dmax), as given by Equation 1.
RT2 =
Dead-Time
5.0 x 10
-12
50 ns Maximum VCC UV voltage) yet not exceed 14 V. An auxiliary winding can be used to reduce the power
dissipation in the external regulator once the power converter is active. The NPN base-emitter reverses
breakdown voltage, which can be as low as 5 V for some transistors, should be considered when selecting the
transistor.
5.5V to 14V
V PWR
VIN
from aux winding
VCC
LM25037
9V
Figure 21. Start-Up Regulator for VPWR >75 V
8.1.6 Current Sense
The CS pin should receive an input signal representative of the transformer’s primary current, either from a
current sense transformer or from a resistor in series with the source of the OUTA and OUTB MOSFET switches.
In both cases, the sensed current creates a voltage ramp across R1, and the RF/CF filter suppresses noise and
transients as shown in Figure 22 and Figure 23. R1, RF and CF should be placed as close to the LM25037 as
possible, and the ground connection from the current sense transformer, or R1, should be a dedicated track to
the AGND pin. The current sense components must provide greater than 220 mV at the CS pin when an
overcurrent condition exists.
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Application Information (continued)
V PWR
Current Sense
Power Transformer
Q1
VIN
CS
RF
CF
LM25037
R1
AGND
Level
Shift
OUTA
Q2
OUTB
Figure 22. Current Sense Using Transformer
Power Transformer
VPWR
Vin
OUTA
Q1
Q2
OUTB
RF
CS
LM25037
CF
R1
Figure 23. Current Sense Using Current Sense Resistor (R1)
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Application Information (continued)
8.1.7 UVLO Divider Selection
A dedicated comparator connected to the UVLO pin detects an input undervoltage condition. When the UVLO pin
voltage is less than 0.45 V, the LM25037 controller is in a low current shutdown mode. For a UVLO pin voltage
greater than 0.45 V but less than 1.25 V, the controller is in standby mode with VCC and REF regulators active
but no switching. Once the UVLO pin voltage is greater than 1.25 V, the controller is fully enabled. When the
UVLO pin voltage rises above the 1.25-V threshold, an internal 22-µA current source as shown in Figure 24, is
activated thus providing threshold hysteresis. The 22-µA current source is deactivated when the voltage at the
UVLO pin falls below 1.25 V. Resistance values for R1 and R2 can be determined using Equation 7.
VHYS R1 =
R2 =
20 x 10-3 x VPWR
1.25
22 PA
1.25 x R1
VPWR - 1.25
where
•
VPWR is the desired turnon voltage and VHYS is the desired UVLO hysteresis at VPWR.
LM25037
5.0V
VIN
(7)
22 PA
R1
UVLO
R2
1.25V
STANDBY
0.45V
SHUTDOWN
Figure 24. Basic UVLO Configuration
For example, if the LM25037 is to be enabled when VPWR reaches 33 V, and disabled when VPWR decreases to
30 V, R1 should be 113 kΩ, and R2 should be 4.42 kΩ. The voltage at the UVLO pin should not exceed 7 V at
any time. Be sure to check both the power and voltage rating (0603 resistors can be rated as low as 50 V) for the
selected R1 resistor. To maintain the UVLO threshold accuracy, TI recommends a resistor tolerance of 1% or
better.
Remote control of the LM25037 operational modes can be accomplished with open-drain device(s) connected to
the UVLO pin as shown in Figure 25.
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Application Information (continued)
LM25037
5.0V
VIN
22 PA
R1
1.25V
UVLO
STANDBY
STANDBY
R2
OFF
0.45V
SHUTDOWN
Figure 25. Remote Standby and Disable Control
8.1.8 Hiccup Mode Current Limit Restart (RES)
The basic operation of the hiccup mode current limit is described in the functional description. The delay time to
the initiation of a hiccup cycle is programmed by the selection of the RES pin capacitor CRES as illustrated in
Figure 26.
Current Limit Detected
at CS
Current Limit Persists
2.0V
RES
0V
100 PA
5V
# 1V
SS
1 PA
OUTA
OUTB
t1
t3
t2
Figure 26. Hiccup Over-Load Restart Timing
In the case of continuous cycle-by-cycle current limit detection at the CS pin, the time required for CRES to reach
the 2-V hiccup mode threshold is given by Equation 8:
t1 =
CRES x 2.0V
18 PA
= 111k x CRES
(8)
For example, if CRES = 0.01 µF the time t1 is approximately 2 ms. The cool down time, t2 is set by the soft-start
capacitor (CSS) and the internal 1 µA SS current source, and is equal to Equation 9:
t2 =
CSS x 1V
= 1M x CSS
1 PA
(9)
If CSS = 0.01 µF, t2 is ≊10 ms.
The soft-start time t3 is set by the internal 100-µA current source, and is equal to Equation 10:
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Application Information (continued)
t3 =
CSS x 4V
= 40k x CSS
100 PA
(10)
If CSS = 0.01 µF, t3 is ≊ 400 µs.
The time t2 provides a periodic cool-down time for the power converter in the event of a sustained overload or
short circuit. This off time results in lower average input current and lower power dissipation within the power
components. TI recommends that the ratio of t2 / (t1 + t3) be in the range of 5 to 10 to take advantage of this
feature.
If the application requires no delay from the first detection of a current limit condition to the onset of the hiccup
mode (t1 = 0), the RES pin can be left open (no external capacitor). If it is desired to disable the hiccup mode
entirely, the RES pin should be connected to ground (AGND).
8.2 Typical Application
Figure 27 shows an example of an LM25037-controlled 50-W push-pull converter. The converter provides a
single regulated 5-V output at 10 A, from an input voltage range of 16 V to 32 V. The converter is configured for
current-mode control with external slope compensation. An auxiliary winding on the output filter inductor is used
to supply the VCC voltage externally – the VCC level is chosen higher than the internal regulator level, to reduce
the power dissipation in the LM25037 IC.
Figure 27. Schematic
8.2.1 Design Requirements
The LM25037 evaluation board is designed to provide the design engineer with a fully functional power converter
based on push-pull topology to evaluate the LM25037 controller. Table 1 lists the performance of the evaluation
board.
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Typical Application (continued)
Table 1. Design Parameters
DESIGN PARAMETER
VALUE
Input Voltage Range, VIN
16 V to 32 V
UVLO On/Off Levels
14 V On (rising)/11 V Off (falling)
Output Voltage VOUT
5V
Output Current IOUT
10 A
Oscillator frequency (2x Fsw per phase)
250 kHz
Switching frequency Fsw (per phase)
125 kHz
8.2.2 Detailed Design Procedure
8.2.2.1 Oscillator Frequency and Maximum Duty Cycle
The LM25037 oscillator frequency will be twice the switching frequency of each switch in the push-pull power
stage, that is, Fosc = 2 × Fsw.
First of all the dead-time resistor value RT2 is calculated. The recommended range of dead-time is 50 ns to 250
ns. A value of 200 ns is chosen, which will set a maximum duty cycle of approximately 95%. From Equation 11,
the required value of resistor on the RT2 pin (R6 in Figure 27) is calculated using Equation 11.
RRT2 =
Dead _ time
5.0 ´ 10-12
=
200 ´ 10-9
5.0 ´ 10-12
= 40 kW
(11)
The E96 value of 41.2 kΩ is used.
Next, knowing the resistor value on RT2 pin, the required resistor value for the RT1 pin (R5 in Figure 27) can be
calculated from Equation 12.
æ 1
ö
æ
ö - 200 ´ 10-9
ç F ÷ - Dead _ time ç 1
÷
250 ´ 103 ø
osc ø
è
è
RRT1 =
=
= 23.5 kW
0.162 ´ 10-9
0.162 ´ 10-9
(12)
8.2.2.2 Power Stage Design
Referring to the schematic in Figure 27, the push-pull power stage primarily consists of input capacitors C1, C2,
and C3, power MOSFETs Q1 and Q2, power transformer T1, output rectifier D3, and output filter L3 and C16,
C17, C18. The push-pull transformer has two out-of-phase primary windings, centre-tapped, and two out-ofphase secondary windings, also centre-tapped.
The centre-tap of the primary is connected to the DC input voltage, VIN. The other end of each primary winding
connects to Q1 and Q2 respectively. The push-pull controller alternately drives Q1 and Q2 180° out of phase, so
that the voltage across each winding is approximately equal to Vin when its respective switch (Q1 or Q2) is
turned on.
On the secondary side, the voltage across each secondary winding will be the same as Vin, scaled by the
transformer turns ratio Ns/Np. Double-diode D3 alternately rectifies the positive voltage across each secondary
winding, to generate a full-wave rectified positive-voltage pulse-train at D3 cathode. This pulse train is then
filtered to DC by output filter L3 and C16/C17/C18.
The LM25037 modulates the duty cycle of the pulses to regulate the output voltage to the required level. The
duty cycle is adjusted with input line voltage to regulate VOUT:
Vout ´ Np
D=
Vin ´ Ns
(13)
Choosing a transformer with turns ratio Np/Ns = 2, maximum duty cycle will occur at minimum operating VIN,
which will be approximately 11 V (UVLO turnoff point):
Vout ´ Np 5 ´ 2
=
= 91%
D=
Vin ´ Ns
11´ 1
(14)
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Thus there is sufficient margin to the oscillator 95% Dmax setting (set by choice of RT2 resistor).
Knowing the duty cycle (D), Fosc frequency and turns ratio, the output inductor (LOUT) peak-to-peak ripple current
in Continuous Conduction Mode (CCM) can be estimated. At minimum VIN of 16 V:
DIpp
æ
ö
N
ç Vin ´ s - Vout ÷
ç
÷
Np
ø´ D =
=è
Lout
Fosc
æ
V 2 Np
ç Vout - out ´
ç
Vin
Ns
è
Lout ´ Fosc
ö
÷
÷
ø=
æ
52 2 ö
´ ÷
çç 5 16 1 ÷ø
è
= 1.88 A pp
4 m ´ 250 k
(15)
And at a maximum VIN of 32 V:
DIpp
æ
V 2 Np
ç Vout - out ´
ç
Vin
Ns
=è
Lout ´ Fosc
ö
÷
÷
ø=
æ
52 2 ö
´ ÷
çç 5 32 1 ÷ø
è
= 3.44 A pp
4 m ´ 250 k
(16)
8.2.2.3 UVLO Setting
To ensure start-up at the required minimum system input voltage of 14 V, with the 3 V of hysteresis to the
desired turnoff level, the UVLO divider resistors R3 and R4 are calculated as follows using Equation 17:
æ
20 ´ 10-3 ´ Von
ç Vhys ç
1.25
R3 = è
22 mA
ö
÷
÷
ø=
æ
20 ´ 10-3 ´ 14 ö
çç 3 ÷÷
1.25
è
ø = 126.2 kW
22 mA
(17)
This is rounded down to a convenient value of 100 kΩ. This reduces the effective hysteresis to approximately 2.5
V.
To set the turnon level to 14 V, calculate the lower UVLO divider resistor:
1.25 ´ R3 1.25 ´ 100 k
R4 =
=
= 9.8 kW
Von - 1.25
14 - 1.25
(18)
The nearest E96 value of 9.76 kΩ is used.
8.2.2.4 VIN, VCC, Start-Up
To reduce the power dissipation in the internal start-up regulator on the VIN pin, a separate external VCC supply
is used. This is derived from the auxiliary winding on the output filter inductor L3. The auxiliary to main inductor
winding turns ratio is 2:1, so when VOUT is regulated at 5 V, the auxiliary VCC will be approximately 10 V. This is
sufficiently greater than the maximum internal VCC regulator level of 8 V to back-bias the internal regulator after
start-up.
8.2.2.5 Current Sense Resistor
The CS pin is used to implement cycle-by-cycle current limit, if the peak current exceeds the internal threshold –
255 mV typical, 220 mV minimum. This limit is used to choose the value of the current-sense resistor R14.
Knowing the maximum output inductor peak-peak ripple current in CCM, and the turns ratio the transformer
(Ns/Np), the current-limit point can be set as required. Given that the full load output current is 10 A, the current
limit target is set at 150%, or 15 A. Thus the required value for R14 may be calculated – it must generate a
voltage at the CS pin no higher than the internal cycle-by-cycle limit of minimally 0.22 V at the current limit level
at the output.
0.22 V
0.22
=
= 0.026 W
R14 =
DI ö Ns
3.44 ö 1
æ
æ
ç Ilim + 2 ÷ ´ N
ç 15 + 2 ÷ ´ 2
è
ø
è
ø
p
(19)
The nearest E96 value of 27 mΩ is chosen for R14. A small filter R7, C13 can be optionally added to filter the
leading-edge current spike in the current-sense waveform, if the internal 65-ns leading-edge blanking time is not
sufficient. In this case, a 499-Ω resistor and 470-pF capacitor are used, for approximately 235-ns filter time
constant.
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8.2.2.6 Current-Mode Control
Push-pull power stages always use peak current-mode control rather than voltage-mode, to keep the transformer
balanced, and avoid stair-case saturation. Ideally, the push-pull transformer flux should return to zero at the end
of each switching cycle. However, due to small imbalances between each phase of the primary (for example,
slightly different Rdson between primary FETs Q1 and Q2), the peak current can be slightly different in each
phase. This causes the transformer flux to become slightly biased away from zero after a complete switching.
After several switching cycles, this small net imbalance per cycle can accumulate or stair-case to the point that
the transformer starts to saturate in one or other direction.
By using current-mode control and connecting the sources of both FETs Q1 and Q2 to the same current sense
resistor R14, the LM25037 controller ensures that the same exact peak current flows in each phase of the
primary. This means that the transformer has identical peak flux swing in each direction, so it always returns to
zero at the end of each switching cycle.
The CS pin ramp is capacitively-coupled to the RAMP pin (input to the internal PWM comparator) using C4,
where it is then also summed with the slope compensation ramp (see Slope Compensation Ramp).
8.2.2.7 Slope Compensation Ramp
Because current-mode control is being used, slope compensation is required to prevent instability and subharmonic oscillation at power stage duty cycles above 50%. For the push-pull topology, this means slope
compensation is required when the switch duty cycle is above 25% of the overall switching period. For stability,
the slope compensation ramp should be at least half the output inductor current downslope, or more ideally equal
to it.
The output inductor downslope, scaled by the transformer turns ratio and current-sense resistor R14, can be
expressed as:
dv cs Vout Ns
5
1
=
´
´ R14 =
´ ´ 0.027 = 16.9 mV / ms
dt
Lout Np
4 m 2
(20)
Resistor R2 and capacitor C4 are used to generate the slope compensation ramp and sum it with the currentsense signal at the CS pin. The RC time constant should be quite large compared to the oscillator period to
make the ramp reasonably linear. Assuming that RC >> Tosc, the slope of the ramp is approximately:
dv ramp
Vref
=
dt
R2 ´ C4
(21)
Choosing C4 = 1.5 nF, the value of R2 can be calculated using Equation 22 to achieve the required slope.
Vref
5 V
R2 =
=
= 197 kW
dv cs
16.9 mV / ms ´1.5 nF
´ C4
dt
(22)
A lower value of 150 kΩ is chosen for R2 to allow for tolerances and to ensure a sufficient slope compensation
ramp under all conditions.
8.2.2.8 Soft-start
The soft-start delay to commencement of first PWM switching can be calculated using Equation 23.
1.0 V ´ C9 1.0 ´ 0.68 mF
t ss _ delay =
=
= 6.8 ms
100 mA
100 mA
(23)
Thereafter, the soft-start ramp time depends on the power stage design and the operating conditions (input
voltage and output load).
8.2.2.9 Overload Timer
With a timing capacitor C8 of 2.2 nF on the RES pin, the hiccup-mode timing and duty cycle can be calculated
for a sustained overcurrent condition.
Hiccup-mode current-limit persist time t1:
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t1 =
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2.0 V ´ C9 2.0 ´ 2.2 nF
=
= 0.244 ms
18 mA
18 mA
(24)
Hiccup-mode cool-down off-time t2:
1.0 V ´ C8 1.0 ´ 680 nF
t2 =
=
= 680 ms
1 mA
1 mA
(25)
Thus the hiccup-mode duty cycle is approximately:
t1
0.244
dhiccup =
=
= 0.036%
t1 + t 2 + t ss 0.244 + 680 + 6.8
(26)
8.2.2.10 Output Voltage Feedback
In this example, the output of the DC-DC converter is non-isolated from the input, so resistor divider R15, R10,
R9 are used to scale VOUT down to the required 1.25-V reference level at the FB pin.
Because current-mode control removes the pole due to the output inductor, the loop compensation components
R8, C10, C11 are chosen to implement a simpler Type II compensator. For further details about control loop
design and Type II compensator design, see application notes Seminar 300 Topic 2 - Apdx A - Error Amplifier
and Compensation Network Design (SLUP069) or Seminar 1400 Topic 5 Designing Stable Control Loops
(SLUP173).
8.2.3 Application Curves
Conditions:
Input Voltage = 24 VDC
Trace:
Output Voltage, V/div = 1 V
Figure 28. Efficiency vs Output Current and VIN
28
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Output Current = 10 A
Horizontal Resolution =
0.5 ms/div
Figure 29. Soft Start
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SNVS572E – JULY 2008 – REVISED JANUARY 2016
Conditions:
Input Voltage = 24 VDC
Bandwidth Limit = 20 MHz
Trace:
Output Ripple, V/div = 50 mV
Output Current = 10 A
Horizontal Resolution =
5 µs/div
Conditions:
Input Voltage = 24 VDC
Output Current = 5 to 10 A
Bandwidth Limit = 20 MHz
Traces:
Bottom Trace: Output current
A/div = 5 A
Top Trace: Output voltage response
V/div = 100 mV
Horizontal Resolution = 200 µs/div
Figure 30. Output Ripple
Conditions:
Input Voltage = 24 VDC
Trace:
Q1 drain-to-source voltage,
V/div = 20 V
Output Current = 5 A
Horizontal Resolution =
2 µs/div
Figure 31. Transient Response
Conditions:
Input Voltage = 32 VDC
Trace:
Q1 drain to source voltage,
V/div = 20 V
Figure 32. Drain Waveform of Q1 at 24 V
Output Current = 5 A
Horizontal Resolution =
2 µs/div
Figure 33. Drain Waveform of Q1 at 32 V
Conditions: Input Voltage = 24 VDC
Trace: Output Voltage, V/div = 1 V
Horizontal Resolution = 0.5 ms/div
Figure 34. Soft Stop
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9 Power Supply Recommendations
The VCC pin requires a local decoupling capacitor that is connected to GND. This capacitor ensures stability of
the internal regulator from the VIN pin. The decoupling capacitor also provides the current pulses to drive the
gates of the external MOSFETs through the driver output (OUTA and OUTB) pins. Place the decoupling
capacitor close to the VCC and PGND pins, and track it directly to those pins.
The two ground pins (PGND and AGND) must be connected together with a short direct PCB connection.
10 Layout
10.1 Layout Guidelines
The LM25037 Current Sense and PWM comparators are very fast, and respond to short duration noise pulses.
The components at the CS, COMP, SS, UVLO, RT2 and the RT1 pins should be as physically close as possible
to the IC, thereby minimizing noise pickup on the PC board trace inductances.
Layout considerations are critical for the current sense filter. If a current sense transformer is used, both leads of
the transformer secondary should be routed to the sense filter components and to the IC pins. The ground side
of the transformer should be connected through a dedicated PC board trace to the AGND pin, rather than
through the ground plane.
If the current sense circuit employs a sense resistor in the drive transistor source, low inductance resistors
should be used. In this case, all the noise sensitive, low-current ground trace should be connected in common
near the IC, and then a single connection made to the power ground (sense resistor ground point).
While employing current mode control, RAMP pin capacitor and CS pin capacitor must be placed close to the IC.
Also, a short direct trace should be employed to connect RAMP capacitor to the CS pin.
The gate drive outputs of the LM25037 should have short, direct paths to the power MOSFETs to minimize
inductance in the PC board The two ground pins (AGND, PGND) must be connected together with a short, direct
connection, to avoid jitter due to relative ground bounce.
If the internal dissipation of the LM25037 produces high junction temperatures during normal operation, the use
of multiple vias under the IC to a ground plane can help conduct heat away from the IC. Judicious positioning of
the PC board within the end product, along with use of any available air flow (forced or natural convection) will
help reduce the junction temperatures. If using forced air cooling, avoid placing the LM25037 in the airflow
shadow of tall components, such as input capacitors.
10.2 Layout Example
From VIN
RAMP
VIN
UVLO
REF
COMP
From FB
FB
To GDA
LM25037
RT2
OUTB
AGND
RT1
From CS
VCC
OUTA
CS
To GDB
PGND
SS
RES
Figure 35. Layout Recommendation
30
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SNVS572E – JULY 2008 – REVISED JANUARY 2016
11 Device and Documentation Support
11.1 Documentation Support
11.1.1 Related Documentation
For related documentation see the following:
• Seminar 300 Topic 2 - Apdx A - Error Amplifier and Compensation Network Design (SLUP069)
• Seminar 1400 Topic 5 Designing Stable Control Loops (SLUP173)
• User Guide, AN-1861 LM25037 Evaluation Board (SNVA352)
11.2 Related Links
The following table lists quick access links. Categories include technical documents, support and community
resources, tools and software, and quick access to sample or buy.
Table 2. Related Links
PARTS
PRODUCT FOLDER
SAMPLE & BUY
TECHNICAL
DOCUMENTS
TOOLS &
SOFTWARE
SUPPORT &
COMMUNITY
LM25037
Click here
Click here
Click here
Click here
Click here
LM25037-Q1
Click here
Click here
Click here
Click here
Click here
11.3 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
11.4 Trademarks
E2E is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.5 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
11.6 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM25037MT/NOPB
ACTIVE
TSSOP
PW
16
92
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
L25037
MT
LM25037MTX/NOPB
ACTIVE
TSSOP
PW
16
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
L25037
MT
LM25037QMT/NOPB
ACTIVE
TSSOP
PW
16
92
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
M25037
MT
LM25037QMTX/NOPB
ACTIVE
TSSOP
PW
16
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
M25037
MT
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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