LM25115AMTX [NSC]
Secondary Side Post Regulator/DC-DC Converter with Power-Up/Power-Down Tracking; 次级侧后稳压器/ DC -DC转换器上电/断电跟踪型号: | LM25115AMTX |
厂家: | National Semiconductor |
描述: | Secondary Side Post Regulator/DC-DC Converter with Power-Up/Power-Down Tracking |
文件: | 总20页 (文件大小:403K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
February 2007
LM25115A
Secondary Side Post Regulator/DC-DC Converter with
Power-Up/Power-Down Tracking
General Description
Features
The LM25115A controller contains all of the features neces-
sary to produce multiple tracking outputs using the Secondary
Side Post Regulation (SSPR) technique. The SSPR tech-
nique develops a highly efficient and well regulated auxiliary
output from the secondary side switching waveform of an iso-
lated power converter. LM25115A can be also used as a
standalone DC/DC synchronous buck controller (Refer to
Synchronous Buck section). Regulation of the auxiliary
output voltage is achieved by leading edge pulse width mod-
ulation (PWM) of the main channel duty cycle. Leading edge
modulation is compatible with either current mode or voltage
mode control of the main output. The LM25115A drives ex-
ternal high-side and low-side NMOS power switches config-
ured as a synchronous buck regulator. A current sense
amplifier provides overload protection and operates over a
wide common mode input range. Additional features include
a low dropout (LDO) bias regulator, error amplifier, precision
reference, adaptive dead time control of the gate signals and
thermal shutdown.
Power-up/Power-down Tracking
■
■
■
■
■
■
■
■
■
■
■
■
Self-synchronization to main channel output
Leading edge pulse width modulation
Valley current Mode control
Standalone DC/DC synchronous buck mode
Operates from AC or DC input up to 42V
Wide 4.5V to 30V bias supply range
Wide 0.75V to 13.5V output range.
Top and bottom gate drivers sink 2.5A peak
Adaptive gate driver dead-time control
Wide bandwidth error amplifier (4MHz)
Programmable soft-start
Thermal shutdown protection
■
TSSOP-16 package
■
Typical Application Circuit
30008301
FIGURE 1. Simplified Multiple Output Power Converter Utilizing SSPR Technique
© 2007 National Semiconductor Corporation
300083
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Connection Diagram
16-Lead TSSO3P0008302
See NS Package Numbers MTC16
Ordering Information
Ordering Number
LM25115AMT
Package Type
Nsc Package Drawing
MTC16
Supplied As
TSSOP-16
TSSOP-16
92 Units Per Anti-Static Tube
2500 units shipped as Tape & Reel
LM25115AMTX
MTC16
Pin Descriptions
Pin
Name
Description
Application Information
1
CS
Current Sense amplifier positive input
A low inductance current sense resistor is connected between CS
and VOUT. Current limiting occurs when the differential voltage
between CS and VOUT exceeds 45mV (typical).
2
VOUT
Current sense amplifier negative input
Connected directly to the output voltage. The current sense
amplifier operates over a voltage range from 0V to 13.5V at the
VOUT pin.
3
4
AGND
CO
Analog ground
Connect directly to the power ground pin (PGND).
Current limit output
For normal current limit operation, connect the CO pin to the
COMP pin through a diode. CO pin is connected to ground through
a resistor in series with a capacitor to provide adequate control
loop compensation for the current limit gm amplifier. Leave this
pin open to disable the current limit function.
5
6
COMP
FB
Compensation. Error amplifier output
Feedback. Error amplifier inverting input
COMP pin pull-up is provided by an internal 300uA current source.
Connected to the regulated output through the feedback resistor
divider and compensation components. The non-inverting input of
the error amplifier is internally connected to the SS pin.
7
8
TRK/SS
RAMP
Tracking/Soft-start control
PWM Ramp signal
Non-inverting input to error amp with 15 µA pull-up current source.
Can be used with capacitor for soft-start or tied to external divider
of a master output for tracking. TRK/SS is the reference input to
the amplifier when the voltage applied to the pin is < 0.75V. For
higher inputs, the internal reference controls the amplifier.
An external capacitor connected to this pin sets the ramp slope
for the voltage mode PWM. The RAMP capacitor is charged with
a current that is proportional to current into the SYNC pin. The
capacitor is discharged at the end of every cycle by an internal
MOSFET.
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2
Pin
Name
Description
Synchronization input
Application Information
9
SYNC
A low impedance current input pin. The current into this pin sets
the RAMP capacitor charge current and the frequency of an
internal oscillator that provides a clock for the free-run (DC input)
mode .
10
11
PGND
LO
Power Ground
Connect directly to the analog ground pin (AGND).
Low-side gate driver output
Connect to the gate of the low-side synchronous MOSFET
through a short low inductance path.
12
VCC
Output of bias regulator
Nominal 7V output from the internal LDO bias regulator. Locally
decouple to PGND using a low ESR/ESL capacitor located as
close to controller as possible.
13
14
15
HS
HO
HB
High-side MOSFET source connection
High-side gate driver output
Connect to negative terminal of the bootstrap capacitor and the
source terminal of the high-side MOSFET.
Connect to the gate of high-side MOSFET through a short low
inductance path.
High-side gate driver bootstrap rail
Connect to the cathode of the bootstrap diode and the positive
terminal of the bootstrap capacitor. The bootstrap capacitor
supplies current to charge the high-side MOSFET gate and should
be placed as close to controller as possible.
16
VBIAS
Supply Bias Input
Input to the LDO bias regulator and current sense amplifier that
powers internal blocks. Input range of VBIAS is 4.5V to 30V.
3
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Junction Temperature
ESD Rating
HBM (Note 2)
+150°C
2 kV
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Operating Ratings
VBIAS supply voltage
VCC supply voltage
HS voltage
VBIAS to GND
VCC to GND
–0.3V to 32V
–0.3V to 9V
5V to 30V
5V to 7.5V
0V to 42V
HS to GND
–1V to 45V
VOUT, CS to GND
All other inputs to GND
Storage Temperature Range
– 0.3V to 15V
−0.3V to 7.0V
–55°C to +150°C
HB voltage
Operating Junction Temperature
VCC + HS
–40°C to +125°C
Typical Operating Conditions
Min
Parameter
Typ
Max
30
7
Units
V
Supply Voltage, VBIAS
4.5
4.5
Supply Voltage, VCC
V
Supply voltage bypass, CVBIAS
Reference bypass capacitor, CVCC
HB-HS bootstrap capacitor
0.1
1
1
µF
µF
µF
µA
V
0.1
10
0.047
50
SYNC Current Range (VCC = 4.5V)
RAMP Saw Tooth Amplitude
150
1.75
13.5
1
VOUT regulation voltage (VBIAS min = 3V + VOUT)
0.75
V
Electrical Characteristics (Note 3) Unless otherwise specified, TJ = –40°C to +125°C, VBIAS = 12V, No Load
on LO or HO.
Symbol
Parameter
Conditions
FSYNC = 200kHz
Min
Typ
Max
4
Units
VBIAS SUPPLY
Ibias
VBIAS Supply Current
mA
VCC LOW DROPOUT BIAS REGULATOR
VccReg
VCC Regulation
VCC open circuit. Outputs not switching
(Note 4)
6.65
7
7.15
V
mA
V
VCC Current Limit
40
VCC Under-voltage Lockout Voltage Positive going VCC
VCC Under-voltage Hysteresis
4
4.5
0.3
0.2
0.25
V
TRACK / SOFT-START
SS Pull-up Source
10
15
20
µA
SS Discharge Impedance
140
Ω
ERROR AMPLIFIER and FEEDBACK REFERENCE
VREF
FB Reference Voltage
FB Input Bias Current
COMP Source Current
Open Loop Voltage Gain
Gain Bandwidth Product
Input Offset Voltage
COMP Offset
Measured at FB pin
FB = 2V
.737
.750
0.2
300
60
4
.763
0.5
V
µA
µA
dB
MHz
mV
V
GBW
Vio
22
2
Threshold for VHO = high RAMP = CS =
VOUT = 0V
RAMP Offset
Threshold for VHO = high COMP = 1.5V,
CS = VOUT = 0V
1.0
V
V
CURRENT SENSE AMPLIFIER
Current Sense Amplifier Headroom Headroom = Vbias – Vout
Vbias= 4.5 V and Vout= 1.5 V
3
Current Sense Amplifier Gain
16
V/V
V
Output DC Offset
1.27
500
Amplifier Bandwidth
kHz
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4
Symbol
Parameter
Conditions
Min
Typ
Max
Units
CURRENT LIMIT
Slow ILIMIT Amp Transconductance
Overall Transconductance
Slow ILimit Threshold
5
mA / V
mA / V
mV
90
45
VCL = VCS - VVOUT
39
34
51
46
VOUT = 6V and CO/COMP = 1.5V
VCL = VCS - VVOUT
Slow ILimit Foldback
39
mV
VOUT = 0V and CO/COMP = 1.5V
Vds = 2V
Fast ILimit Pull-Down Current
Fast ILimit Threshold
45
60
mA
mV
mV
VCLNEG
Negative Current Limit
VOUT = 6V
-17
VCL = VCS - VVOUT to cause LO to shutoff
CO Clamp Voltage
ICO Pull-Up Current
5.5
6
6.5
V
15
µA
RAMP GENERATOR
SYNC Input Impedance
2.5
20
kΩ
µA
V
SYNC Threshold
End of cycle detection threshold
Free Run Mode Peak Threshold
RAMP peak voltage with dc current
applied to SYNC.
2.35
3.3
Current Mirror Gain
Ratio of RAMP charge current to SYNC
input current.
2.7
A/A
Discharge Impedance
100
Ω
LOW-SIDE GATE DRIVER
VOLL
VOHL
LO Low-state Output Voltage
LO High-state Output Voltage
LO Rise Time
ILO = 100mA
0.15
0.35
15
0.5
0.8
V
V
ILO = -100mA, VOHL = VCC -VLO
CLOAD = 1000pF
CLOAD = 1000pF
VLO = 0V
ns
ns
A
LO Fall Time
12
IOHL
IOLL
Peak LO Source Current
Peak LO Sink Current
2
VLO = 12V
2.5
A
HIGH-SIDE GATE DRIVER
VOLH
VOHH
HO Low-state Output Voltage
IHO = 100mA
0.15
0.35
15
0.5
0.8
V
V
HO High-state Output Voltage
HO Rise Time
IHO = -100mA, VOHH = VHB –VHO
CLOAD = 1000pF
CLOAD = 1000pF
VHO = 0V
ns
ns
A
HO High-side Fall Time
Peak HO Source Current
Peak HO Sink Current
12
IOHH
IOLH
2
VHO = 12V
2.5
A
SWITCHING CHARACTERISITCS
LO Fall to HO Rise Delay
CLOAD = 0
CLOAD = 0
CLOAD = 0
CLOAD = 0
40
50
ns
ns
ns
ns
HO Fall to LO Rise Delay
SYNC Fall to HO Fall Delay
SYNC Rise to LO Fall Delay
120
80
5
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Symbol
Parameter
Conditions
Min
150
Typ
Max
Units
THERMAL SHUTDOWN
TSD
Thermal Shutdown Temp.
165
25
°C
°C
Thermal Shutdown Hysteresis
THERMAL RESISTANCE
Junction to Ambient
Junction to Ambient
MTC Package
SDA Package
125
32
°C/W
°C/W
θJA
θJA
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation
of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions,
see the Electrical Characteristics tables.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 3: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical
Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL).
Note 4: Device thermal limitations may limit usable range.
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6
Typical Performance Characteristics
Efficiency vs. Load Current and Vphase
(VOUT = 2.5V)
VCC Regulator Start-up Characteristics, VCC vs. VBIAS
30008322
30008304
Current Value (CV) vs. Current Limit (VCL
)
Current Sense Amplifier Gain and Phase vs. Frequency
30008307
30008306
Current Error Amplifier Transconductance
Overall Current Amplifier Transconductance
30008309
30008308
7
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Common Mode Output Voltage vs. Positive Current Limit Common Mode Output Voltage vs. Negative Current Limit
(Room Temp)
30008310
30008311
VCC Load Regulation to Current Limit
30008305
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8
Block Diagram
30008303
9
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Detailed Operating Description
Synchronization (SYNC) and Feed-
Forward (RAMP)
The LM25115A controller contains all of the features neces-
sary to implement multiple output power converters utilizing
the Secondary Side Post Regulation (SSPR) technique. The
SSPR technique develops a highly efficient and well regulat-
ed auxiliary output from the secondary side switching wave-
form of an isolated power converter. Regulation of the
auxiliary output voltage is achieved by leading edge pulse
width modulation (PWM) of the main channel duty cycle.
Leading edge modulation is compatible with either current
mode or voltage mode control of the main output. The
LM25115A drives external high-side and low-side NMOS
power switches configured as a synchronous buck regulator.
A current sense amplifier provides overload protection and
operates over a wide common mode input range from 0V to
13.5V. Additional features include a low dropout (LDO) bias
regulator, error amplifier, precision reference, adaptive dead
time control of the gate driver signals and thermal shutdown.
The pulsing “phase signal” from the main converter synchro-
nizes the PWM ramp and gate drive outputs of the LM25115A.
The phase signal is the square wave output from the trans-
former secondary winding before rectification (Figure 1). A
resistor connected from the phase signal to the low
impedance SYNC pin produces a square wave current
(ISYNC) as shown in Figure 2. A current comparator at the
SYNC input monitors ISYNC relative to an internal 15µA refer-
ence. When ISYNC exceeds 15µA, the internal clock signal
(CLK) is reset and the capacitor connected to the RAMP be-
gins to charge. The current source that charges the RAMP
capacitor is equal to 3 times the ISYNC current. The falling edge
of the phase signal sets the CLK signal and discharges the
RAMP capacitor until the next rising edge of the phase signal.
The RAMP capacitor is discharged to ground by a low
impedance (100Ω) n-channel MOSFET. The input
impedance at SYNC pin is 2.5kΩ which is normally much
smaller than the external SYNC pin resistance.
Low Drop-Out Bias Regulator (VCC)
The LM25115A contains an internal LDO regulator that op-
erates over an input supply range from 4.5V to 30V. The
output of the regulator at the VCC pin is nominally regulated
at 7V and is internally current limited to 40mA. VCC is the
main supply to the internal logic, PWM controller, and gate
driver circuits. When power is applied to the VBIAS pin, the
regulator is enabled and sources current into an external ca-
pacitor connected to the VCC pin. The recommended output
capacitor range for the VCC regulator is 0.1uF to 100uF.
When the voltage at the VCC pin reaches the VCC under-
voltage lockout threshold of 4.25V, the controller is enabled.
The controller is disabled if VCC falls below 4.0V (250mV
hysteresis). In applications where an appropriate regulated dc
bias supply is available, the LM25115A controller can be
powered directly through the VCC pin instead of the VBIAS
pin. In this configuration, it is recommended that the VCC and
the VBIAS pins be connected together such that the external
bias voltage is applied to both pins. The allowable VCC range
when biased from an external supply is 4.5V to 7V.
The RAMP and SYNC functions illustrated in Figure 2 provide
line voltage feed-forward to improve the regulation of the aux-
iliary output when the input voltage of the main converter
changes. Varying the input voltage to the main converter pro-
duces proportional variations in amplitude of the phase signal.
The main channel PWM controller adjusts the pulse width of
the phase signal to maintain constant volt*seconds and a
regulated main output as shown in Figure 3. The variation of
the phase signal amplitude and duration are reflected in the
slope and duty cycle of the RAMP signal of the LM25115A
(ISYNC α phase signal amplitude). As a result, the duty cycle
of the LM25115A is automatically adjusted to regulate the
auxiliary output voltage with virtually no change in the PWM
threshold voltage. Transient line regulation is improved be-
cause the PWM duty cycle of the auxiliary converter is imme-
diately corrected, independent of the delays of the voltage
regulation loop.
30008312
FIGURE 2. Line Feed-Forward Diagram
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10
30008313
FIGURE 3. Line Feed-Forward Waveforms
The recommended SYNC input current range is 50µA to
VOUT(final) = regulator output set point
150µA. The SYNC pin resistor (RSYNC) should be selected to
set the SYNC current (ISYNC) to 150µA with the maximum
phase signal amplitude, VPHASE(max). This will guarantee that
ISYNC stays within the recommended range over a 3:1 change
in phase signal amplitude. The SYNC pin resistor is therefore:
Pull-up current for the error amplifier output is provided by an
internal 300µA current source. The PWM threshold signal at
the COMP pin can be controlled by either the open drain error
amplifier or the open drain current amplifier connected
through the CO pin to COMP. Since the internal error amplifier
is configured as an open drain output it can be disabled by
connecting FB to ground. The current sense amplifier and
current limiting function will be described in a later section.
RSYNC = (VPHASE(max) / 150µA) - 2.5kΩ
Once ISYNC has been established by selecting RSYNC, the
RAMP signal slope/amplitude may be programmed by se-
lecting the proper RAMP pin capacitor value. The RAMP
signal slope should be selected to provide adequate slope
compensation for the Valley current mode control scheme
(Please refer to the Valley current mode section). The rec-
ommended peak amplitude of the ramp waveform is 1.75V.
Power-Up/Power-Down Tracking
The LM25115A can track the output of a master power supply
during soft start by connecting a resistor divider to the TRACK
pin (Figure 4). Therefore, the output voltage slew rate of the
LM25115A will be controlled by the master supply for loads
that require precise sequencing. In order to track properly the
output voltage of the LM25115A must be lower than the output
voltage of the master supply.
Error Amplifier and Soft-Start (FB,
CO, COMP & TRK/SS)
One way to use the tracking feature is to design the tracking
resistor divider so that the master supply output voltage
(VOUT1) and the LM25115A output voltage (VOUT2) both
rise together and reach their target values at the same time.
For this case, the equation governing the values of the track-
ing divider resistors RT1 and RT2 is:
An internal wide bandwidth error amplifier is provided within
the LM25115A for voltage feedback to the PWM controller.
The amplifier’s inverting input is connected to the FB pin. The
output of the auxiliary converter is regulated by connecting a
voltage setting resistor divider between the output and the FB
pin. Loop compensation networks are connected between the
FB pin and the error amplifier output (COMP). The amplifier
has two non-inverting inputs. The first non-inverting input
connects to a 0.75V bandgap reference while the second non-
inverting input connects to the TRK/SS pin and it has 15 µA
pull-up current source. The TRK/SS pin can be tied to an ex-
ternal resistor divider from the master output for tracking, or
it can be tied to a capacitor for soft-start . TRK/SS is the ref-
erence input to the amplifier when the voltage applied to the
pin is < 0.75V. For higher inputs, the internal reference con-
trols the amplifier. When the VCC voltage is below the UVLO
threshold, the TRK/SS pin is discharged to ground. When
VCC rises and exceeds the positive going UVLO threshold
(4.25V), the TRK/SS pin is released and allowed to rise. If an
external capacitor is connected to the TRK/SS pin, it will be
charged by the internal 15uA pull-up current source to grad-
ually increase the non-inverting input of the error amplifier to
0.75V. During start-up, the output of the LM25115A converter
will follow the following equation:
A value of 10kΩ (1%) is recommended for RT2 as a good
compromise between high precision and low quiescent cur-
rent through the divider. If the master supply voltage was 3.3V
and the LM25115A output voltage was 2.5 V, then the value
of RT1 needed to give the two supplies identical soft start
times would be 2.94 kΩ (1%). The timing diagram and wave-
forms for the equal soft start time configuration are shown in
Figure 5 and Figure 6.
VOUT(t) = VOUT(final) x15 µA x t /(.75 Vx Css )
Where
Css = external Soft-Start capacitor
11
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Alternatively, the tracking feature can be used to create equal
slew rates between the output voltages of the master supply
and the LM25115A. This method ensures that the output volt-
age of the LM25115A always reaches regulation before the
output voltage of the master supply. In this case, the tracking
resistors can be determined based on the following equation:
Again, a value of 10kΩ 1% is recommended for RT2. For the
case of VOUT1 = 3.3V and VOUT2 = 2.5V, RT1 should be
4.32 kΩ 1%. The timing diagram and the waveforms for equal
slew rates configuration are shown in Figure 7 and Figure 8.
30008329
FIGURE 4.
30008330
FIGURE 7.
30008331
FIGURE 5.
30008328
Vphase = 10V
CH1 = Master output, 1V/Div
CH2 = COMP, 5/Div
CH3 = Iout, 1A/Div
CH4 = SSPR Output (Slave), 1V/Div
Horizontal Resolution= 200 µs/Div
30008327
Vphase = 10V
CH1 = Master output, 1V/Div
CH2 = COMP, 5/Div
CH3 = Iout, 1A/Div
FIGURE 8. Tracking with Equal Slew Rate
CH4 = SSPR Output (Slave), 1V/Div
Horizontal Resolution= 200 µs/Div
FIGURE 6. Tracking with Equal Soft Start Time
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12
Leading edge modulation of the auxiliary PWM controller is
required if the main converter uses peak current mode con-
trol. If trailing edge modulation were used, the additional load
on the transformer secondary from the auxiliary channel
would be drawn only during the first portion of the phase signal
pulse. Referring to Figure 10, the turn-off of the high- side
MOSFET of the auxiliary regulator would create a non-mono-
tonic negative step in the transformer current. This negative
current step would produce instability in a peak current mode
controller. With leading edge modulation, the additional load
presented by the auxiliary regulator on the transformer sec-
ondary will be present during the latter portion of the phase
signal. This positive step in the phase signal current can be
accommodated by a peak current mode controller without in-
stability.
Leading Edge Pulse Width
Modulation
Unlike conventional voltage mode controllers, the LM25115A
implements leading edge pulse width modulation. A current
source equal to 3 times the ISYNC current is used to charge
the capacitor connected to the RAMP pin as shown in Figure
9. The ramp signal and the output of the error amplifier
(COMP) are combined through a resistor network to produce
a voltage ramp with variable dc offset (CRMIX in Figure 9).
The high-side MOSFET which drives the HS pin is held in the
off state at the beginning of the phase signal. When the volt-
age of CRMIX exceeds the internal threshold voltage CV, the
PWM comparator turns on the high-side MOSFET. The HS
pin rises and the MOSFET delivers current from the main
converter phase signal to the output of the auxiliary regulator.
The PWM cycle ends when the phase signal falls and power
is no longer supplied to the drain of the high-side MOSFET.
30008314
FIGURE 9. Synchronization and Leading Edge Modulation
30008320
FIGURE 10. Leading versus Trailing Edge Modulation
13
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old (CV), the PWM comparator turns on the high-side MOS-
FET. Insure that the Vbias voltage is at least 3V above the
regulated output voltage (VOUT) to provide enough head-
room for the current sense amplifier.
Valley Current Mode Control
The LM25115A controller uniquely utilizes the elements and
benefits of valley current mode control in conjunction with
leading edge modulation to correct changes in output voltage
due to line and load transients. Contrary to peak current mode
control, valley current mode control turns on the high-side
MOSFET when the inductor valley current reaches a pro-
grammable threshold. This programmable threshold (CRMIX)
is the sum of the output of voltage error amplifier and the
RAMP signal generated at the RAMP pin. Valley current
mode control experiences sub-harmonic oscillation when the
duty ratio, D, is less than or equal to 50%. Therefore, ade-
quate slope compensation is needed for the proper operation
across the full range of the duty ratio. The RAMP signal is
proportional to the input voltage and it provides the required
slope compensation for the valley current mode scheme. The
desired RAMP pin capacitance can be calculated from the
following equation:
Valley current mode control improves the control loop stability
and bandwidth. It also eliminates the R-C lead network in the
feedback path that is normally required with voltage mode
control (Figure 12). Eliminating the lead network not only sim-
plifies the compensation, but also reduces sensitivity to output
noise that could pass through the lead network to the error
amplifier.
The design of the voltage feedback path through the error
amp begins with the selection of R1 and R2 in Figure 12 to
set the regulated output voltage. The steady state output volt-
age after soft-start is determined by the following equation:
VOUT(final) = 0.75V x (1+R1/R2)
The parallel impedance of the R1, R2 resistor divider should
be approximately 2kΩ (between 0.5kΩ and 5kΩ). Lower re-
sistance values may not be properly driven by the error am-
plifier output and higher feedback resistances can introduce
noise sensitivity. The next step in the design process is se-
lection of R3, which sets the ac gain of the error amplifier.
CRAMP = (0.05 x L) /(RSYNC x RSENSE
)
Where L is the power inductor, RSYNC is the SYNC pin resistor
and RSENSE is the current sense resistor.
The current sense amplifier shown in Figure 11 monitors the
inductor current as it flows through a sense resistor connected
between CS and VOUT. The voltage gain of the sense am-
plifier is nominally equal to 16. The current sense output
signal is shifted by 1.27V to produce the internal CV reference
signal. The CV signal is applied to the negative input of the
PWM comparator and compared to CRMIX as illustrated in
Figure 11. Therefore when CRMIX exceeds the PWM thresh-
The capacitor C1 is connected in series with R3 to increase
the dc gain of the voltage regulation loop and improve output
voltage accuracy. The corner frequency set by R3 x C1 should
be less than 1/10th of the cross-over frequency of the overall
converter such that capacitor C1 does not add phase lag at
the crossover frequency.
30008315
FIGURE 11. Current Sensing and Limiting
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14
30008316
FIGURE 12. Voltage Sensing and Feedback
comparator will inhibit output pulses. Once the fault condition
is removed, the fast current limit amplifier will release COMP.
Therefore, the CRMIX signal will increase to a normal oper-
ating threshold and the switching will resume (Figure 14). A
current limit fold-back feature is provided by the LM25115A
to reduce the peak output current delivered to a shorted load.
When the common mode input voltage to the current sense
amplifier (CS and VOUT pins) falls below 2V, the current limit
threshold is reduced from the normal level. At common mode
voltages > 2V, the current limit threshold is nominally 45mV.
When VOUT is reduced to 0 V the current limit threshold drops
to 39mV to reduce stress on the inductor and power MOS-
FETs.
Current Limiting (CS, CO and VOUT)
Current limiting is implemented through the current sense
amplifier as illustrated in Figure 11. The current sense ampli-
fier monitors the inductor current that flows through a sense
resistor connected between CS and VOUT. The voltage gain
of the current sense amplifier is nominally equal to 16. The
output of current sense signal is shifted by 1.27V to produce
the internal CV reference signal. The CV signal drives two
current limit amplifiers. Both of the current limit amplifiers
have open drain (sink only) output stages which are connect-
ed to the CO pin. The CO pin is typically connected to the
COMP pin through a diode (the cathode is connected to the
CO pin and the anode is connected to the COMP pin). The
slow current limit amplifier has a nominal transconductance
of 5 mA/V and provides constant current mode operation at
the desired current limit set point. The fast current limit am-
plifier has nominal current pull-down capability of 100mA and
provides protection against fast over-current conditions. Dur-
ing normal operation, the voltage error amplifier controls the
COMP pin voltage which adjusts the PWM duty cycle by vary-
ing the internal CRMIX level. However when the current
sense input voltage, VCL, exceeds 45mV, the slow current
limit amplifier gradually pulls down on COMP through the CO
pin. Pulling COMP low reduces the CRMIX signal and thereby
reducing the operating duty cycle. By controlling the operating
duty cycle, the slow current limit amplifier will force a constant
current mode of operation at the desired current limit set point
(Figure 13). A resistor in series with a capacitor are connected
from the CO Pin to ground to provide adequate control loop
compensation for the slow current limit (Figure 11). The de-
sired current limit set point, ILimit, can be programmed by
selecting the proper current sense resistor, RSENSE,using the
following equation:
30008325
Vphase=10V
CH1 = CO, 5V/Div
CH2 = COMP, 5/Div
CH3 = Iout, 5A/Div
CH4 = SSPR Switch Signal, 5V/Div
Horizontal Resolution= 2 µs/Div
RSENSE = 0.045 V/ ILimit
In the event that the current sense input voltage, VCL, exceeds
60mV, the fast current limit amplifier will pull down hard on
COMP through the CO pin. This will reduce the CRMIX signal
to a voltage below the CV signal level. Therefore, the PWM
FIGURE 13. SSPR Steady State Current Limit (Output
Shorted)
15
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dead-time control to avoid shoot through currents. The adap-
tive dead-time control circuit monitors the state of each driver
to ensure that one MOSFET is turned off before the other is
turned on. The HB and VCC capacitors should be placed
close to the pins of the LM25115A to minimize voltage tran-
sients due to parasitic inductances and the high peak output
currents of the drivers. The recommended range of the HB
capacitor is 0.047µF to 0.22µF.
Both drivers are controlled by the PWM logic signal from the
PWM latch. When the phase signal is low, the outputs are
held in the reset state with the low-side MOSFET on and the
high-side MOSFET off. When the phase signal switches to
the high state, the PWM latch reset signal is de-asserted. The
high-side MOSFET remains off until the PWM latch is set by
the PWM comparator (CRMIX > CV as shown in Figure 9).
When the PWM latch is set, the LO driver turns off the low-
side MOSFET and the HO driver turns on the high-side MOS-
FET. The high-side pulse is terminated when the phase signal
falls and SYNC input comparator resets the PWM latch.
30008326
Vphase=10V
CH1 = CO, 5V/Div
CH2 = COMP, 5/Div
CH3 = Iout, 10A/Div
CH4 = SSPR Switch Signal, 4V/Div
Horizontal Resolution= 20 µs/Div
Thermal Protection
Internal thermal shutdown circuitry is provided to protect the
integrated circuit in the event the maximum junction temper-
ature limit is exceeded. When activated, typically at 165 de-
grees Celsius, the controller is forced into a low power
standby state with the output drivers and the bias regulator
disabled. The device will restart when the junction tempera-
ture falls below the thermal shutdown hysteresis, which is
typically 25 degrees. The thermal protection feature is pro-
vided to prevent catastrophic failures from accidental device
overheating.
FIGURE 14. SSPR Short Circuit Transient (No-Load to
Short-Circuit)
Negative Current Limit
Under certain conditions synchronous buck regulators are
capable of sinking current from the output capacitors. This
energy is stored in the inductor and returned to the input
source. The LM25115A detects this current reversal by de-
tecting a negative voltage being developed across the current
sense resistor. The intent of this negative current comparator
is to protect the low-side MOSFET from excessive currents.
Excessive negative current can also lead to a large positive
voltage spike on the HS pin at the turn-off of the low-side
MOSFET. This voltage spike may damage the chip if its mag-
nitude exceeds the maximum voltage rating of the part. The
negative current comparator threshold is sufficiently negative
to allow inductor current to reverse at no load or light load
conditions. It is not intended to support discontinuous con-
duction mode with diode emulation by the low-side MOSFET.
The negative current comparator shown in Figure 11 monitors
the CV signal and compares this signal to a fixed 1V thresh-
old. This corresponds to a negative VCL voltage between CS
and VOUT of -17mV. The negative current limit comparator
turns off the low-side MOSFET for the remainder of the cycle
when the VCL input falls below this threshold.
Standalone DC/DC Synchronous
Buck Mode
The LM25115A can be configured as a standalone DC/DC
synchronous buck controller. In this mode the LM25115A us-
es leading edge modulation in conjunction with valley current
mode control to control the synchronous buck power stage.
The internal oscillator within the LM25115A sets the clock
frequency for the high and low-side drivers of the external
synchronous buck power MOSFETs . The clock frequency in
the synchronous buck mode is programmed by the SYNC pin
resistor and RAMP pin capacitor. Connecting a resistor be-
tween a dc bias supply and the SYNC pin produces a current,
ISYNC, which sets the charging current of the RAMP pin ca-
pacitor. The RAMP capacitor is charged until its voltage
reaches the peak ramp threshold of 2.25V. The RAMP ca-
pacitor is then discharged for 300ns before beginning a new
PWM cycle. The 300ns reset time of the RAMP pin sets the
minimum off-time of the PWM controller in this mode. The in-
ternal clock frequency in the synchronous buck mode is set
by ISYNC, the ramp capacitor, the peak ramp threshold, and
the 300ns deadtime.
Gate Driver Outputs (HO & LO)
The LM25115A provides two gate driver outputs, the floating
high-side gate driver HO and the synchronous rectifier low-
side driver LO. The low-side driver is powered directly by the
VCC regulator. The high-side gate driver is powered from a
bootstrap capacitor connected between HB and HS. An ex-
ternal diode connected between VCC and HB charges the
bootstrap capacitor when the HS is low. When the high-side
MOSFET is turned on, HB rises with HS to a peak voltage
equal to VCC + VHS - VD where VD is the forward drop of the
external bootstrap diode. Both output drivers have adaptive
FCLK ≊ 1 / ((CRAMP x 2.25V) / (ISYNC x 3) + 300ns)
See the LM5115 dc evaluation board application note
(AN-1367) for more details on the synchronous buck mode.
Please note that LM25115A is similar to LM5115 except for
the tracking feature.
www.national.com
16
Application Circuit
17
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Physical Dimensions inches (millimeters) unless otherwise noted
TSSOP-16 Outline Drawing
NS Package Number MTC16
www.national.com
18
Notes
19
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Notes
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COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
Life support devices or systems are devices which (a) are intended for surgical implant into the body, or (b) support or sustain life and
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Copyright© 2007 National Semiconductor Corporation
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