TI LM2694 Lm2694 30v, 600 ma step down switching regulator Datasheet

LM2694
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LM2694 30V, 600 mA Step Down Switching Regulator
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FEATURES
DESCRIPTION
•
•
•
•
•
•
The LM2694 Step Down Switching Regulator features
all of the functions needed to implement a low cost,
efficient, buck bias regulator capable of supplying
0.6A to the load. This buck regulator contains an NChannel Buck Switch, and is available in the 3 x 3
thermally enhanced WSON-10 package and a
TSSOP-14 package. The feedback regulation
scheme requires no loop compensation, results in
fast load transient response, and simplifies circuit
implementation. The operating frequency remains
constant with line and load variations due to the
inverse relationship between the input voltage and
the on-time. The valley current limit results in a
smooth transition from constant voltage to constant
current mode when current limit is detected, reducing
the frequency and output voltage, without the use of
foldback. Additional features include: VCC undervoltage lockout, thermal shutdown, gate drive undervoltage lockout, and maximum duty cycle limiter.
1
2
•
•
•
•
•
•
•
•
Integrated N-Channel Buck Switch
Integrated Start-Up Regulator
Input Voltage Range: 8V to 30V
No Loop Compensation Required
Ultra-Fast Transient Response
Operating Frequency Remains Constant with
Load Current and Input Voltage Variations
Maximum Duty Cycle Limited During Start-Up
Adjustable Output Voltage
Valley Current Limit At 0.6A
Maximum Switching Frequency: 1 MHz
Precision Internal Reference
Low Bias Current
Highly Efficient Operation
Thermal Shutdown
Package
TYPICAL APPLICATIONS
•
•
•
High Efficiency Point-Of-Load (POL) Regulator
Non-Isolated Telecommunication Buck
Regulator
Secondary High Voltage Post Regulator
•
•
WSON-10 (3 mm x 3 mm) w/Exposed Pad
TSSOP-14
Basic Step Down Regulator
8V - 30V
Input
VIN
VCC
C3
C1
LM2694
RON
BST
C4
L1
RON/SD
SHUTDOWN
VOUT
SW
D1
SS
R1
R3
ISEN
C2
C6
FB
RTN
SGND
R2
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
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LM2694
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Connection Diagrams
1
2
3
4
5
6
7
14
NC
NC
SW
VIN
BST
VCC
ISEN
RON/SD
SGND
SS
RTN
FB
NC
NC
1
13
2
12
3
11
4
10
5
SW
VIN
BST
VCC
ISEN
RON/SD
SGND
SS
RTN
FB
10
9
8
7
6
9
8
Figure 1. 14-Lead TSSOP Package
See Package Number PW0014A
Figure 2. 10-Lead WSON Package
See Package Number DSC0010A
Pin Descriptions
PIN NUMBER
DESCRIPTION
APPLICATION INFORMATION
TSSOP-14
1
2
SW
Switching Node
Internally connected to the buck switch source.
Connect to the inductor, free-wheeling diode, and
bootstrap capacitor.
2
3
BST
Boost pin for bootstrap capacitor
Connect a 0.022 µF capacitor from SW to the BST
pin. The capacitor is charged from VCC via an
internal diode during the buck switch off-time.
3
4
ISEN
Current sense
During the buck switch off-time, the inductor current
flows through the internal sense resistor, and out of
the ISEN pin to the free-wheeling diode. The current
limit is nominally set at 0.62A.
4
5
SGND
Current Sense Ground
Re-circulating current flows into this pin to the current
sense resistor.
5
6
RTN
Circuit Ground
Ground return for all internal circuitry other than the
current sense resistor.
6
9
FB
Voltage feedback input from the
regulated output
Input to both the regulation and over-voltage
comparators. The FB pin regulation level is 2.5V.
7
10
SS
Softstart
An internal current source charges the SS pin
capacitor to 2.5V to soft-start the reference input of
the regulation comparator.
8
11
RON/SD
On-time control and shutdown
An external resistor from VIN to the RON/SD pin sets
the buck switch on-time. Grounding this pin shuts
down the regulator.
9
12
VCC
Output of the startup regulator
The voltage at VCC is nominally regulated at 7V.
Connect a 0.1 µF, or larger capacitor from VCC to
ground, as close as possible to the pins. An external
voltage can be applied to this pin to reduce internal
dissipation. MOSFET body diodes clamp VCC to VIN
if VCC > VIN.
10
13
VIN
Input supply voltage
Nominal input range is 8V to 30V. Input bypass
capacitors should be located as close as possible to
the VIN pin and RTN pins.
1,7,8,14
NC
No connection.
No internal connection. Can be connected to ground
plane to improve heat dissipation.
EP
Exposed Pad
Exposed metal pad on the underside of the WSON
package. It is recommended to connect this pad to
the PC board ground plane to aid in heat dissipation.
EP
2
NAME
WSON-10
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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.
Absolute Maximum Ratings (1) (2) (3)
VIN to RTN
33V
BST to RTN
47V
SW to RTN (Steady State)
ESD Rating (4)
-1.5V
Human Body Model
2kV
BST to VCC
33V
VIN to SW
33V
BST to SW
14V
VCC to RTN
14V
SGND to RTN
-0.3V to +0.3V
SS to RTN
-0.3V to 4V
All Other Inputs to RTN
-0.3 to 7V
Storage Temperature Range
-65°C to +150°C
Junction Temperature
(1)
(2)
(3)
(4)
150°C
Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which
operation of the device is intended to be functional. For ensured specifications and test conditions, see the Electrical Characteristics.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
For detailed information on soldering plastic TSSOP and WSON packages, refer to the Packaging Data Book available from TI.
The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
Operating Ratings (1)
VIN
8.0V to 30V
−40°C to + 125°C
Junction Temperature
(1)
Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which
operation of the device is intended to be functional. For ensured specifications and test conditions, see the Electrical Characteristics.
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Electrical Characteristics
Specifications with standard type are for TJ = 25°C only; limits in boldface type apply over the full Operating Junction
Temperature (TJ) range. Minimum and Maximum limits are ensured through test, design, or statistical correlation. Typical
values represent the most likely parametric norm at TJ = 25°C, and are provided for reference purposes only. Unless
otherwise stated the following conditions apply: VIN = 24V, RON = 200kΩ (1)
Symbol
Parameter
Conditions
Min
Typ
Max
Units
6.6
7
7.4
V
Start-Up Regulator, VCC
VCCReg
UVLOVCC
VCC regulated output
VIN-VCC dropout voltage
ICC = 0 mA,
VCC = UVLOVCC + 250 mV
1.3
VCC output impedance
0 mA ≤ ICC ≤ 5 mA, VIN = 8V
175
Ω
VCC current limit (2)
VCC = 0V
9
mA
VCC under-voltage lockout
threshold
VCC increasing
5.7
V
UVLOVCC hysteresis
VCC decreasing
150
mV
UVLOVCC filter delay
100 mV overdrive
3
µs
IIN operating current
Non-switching, FB = 3V
0.5
0.8
mA
IIN shutdown current
RON/SD = 0V
90
180
µA
0.5
1.0
Ω
4.4
5.5
V
Switch Characteristics
Rds(on)
Buck Switch Rds(on)
ITEST = 200 mA
UVLOGD
Gate Drive UVLO
VBST - VSW Increasing
3.0
V
UVLOGD hysteresis
490
mV
Pull-up voltage
2.5
V
Internal current source
12
µA
Softstart Pin
Current Limit
ILIM
Threshold
Current out of ISEN
0.5
0.62
0.74
A
Resistance from ISEN to SGND
180
mΩ
Response time
150
ns
On Timer
tON - 1
On-time
VIN = 10V, RON = 200 kΩ
tON - 2
On-time
VIN = 30V, RON = 200 kΩ
Shutdown threshold
Voltage at RON/SD rising
Threshold hysteresis
Voltage at RON/SD falling
2.1
2.8
3.6
900
0.45
0.8
µs
ns
1.2
V
35
mV
265
ns
Off Timer
tOFF
Minimum Off-time
Regulation and Over-Voltage Comparators (FB Pin)
VREF
FB regulation threshold
SS pin = steady state
FB over-voltage threshold
2.440
2.5
2.550
V
2.9
V
1
nA
Thermal shutdown temperature
175
°C
Thermal shutdown hysteresis
20
°C
FB bias current
Thermal Shutdown
TSD
Thermal Resistance
θJA
θJC
(1)
(2)
4
Junction to Ambient
0 LFPM Air Flow
Junction to Case
WSON Package
33
TSSOP Package
40
WSON Package
8.8
TSSOP Package
5.2
°C/W
°C/W
Typical specifications represent the most likely parametric norm at 25°C operation.
VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading
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Typical Performance Characteristics
7.5
8
VIN = 9V
6
FS = 100 kHz
FS = 620 kHz
6.5
5
VCC (V)
VCC (V)
VIN t 10V
7
7.0
6.0
FS = 200 kHz
VIN = 8V
4
3
2
5.5
VCC Externally Loaded
Load Current = 300 mA
ICC = 0 mA
5.0
6.5
7.0
7.5
8.0
1
8.5
9.0
FS = 200 kHz
0
10
9.5
0
2
4
6
8
10
VIN (V)
ICC (mA)
Figure 3. VCC vs VIN
Figure 4. VCC vs ICC
10
8.0
7.0
RON = 500k
3.0
ON-TIME (Ps)
ICC INPUT CURRENT(mA)
FS = 550 kHz
6.0
5.0
4.0
FS = 200 kHz
3.0
100k
300k
1.0
50k
0.3
2.0
FS = 100 kHz
1.0
0.1
0
7
8
9
10
11
12
13
8 10
5
14
15
20
25
30
VIN (V)
EXTERNALLY APPLIED VCC (V)
Figure 5. ICC vs Externally Applied VCC
Figure 6. ON-Time vs VIN and RON
800
3.0
600
Operating Current (FB = 3V)
2.0
500
100k
IIN (PA)
RON/SD PIN VOLTAGE (V)
700
RON = 50k
500k
400
300
1.0
200
Shutdown Current (RON/SD = 0V)
100
0
0
5
8 10
15
20
25
30
5
8 10
15
20
25
30
VIN (V)
VIN (V)
Figure 7. Voltage at RON/SD Pin
Figure 8. IIN vs VIN
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Typical Application Circuit and Block Diagram
7V SERIES
REGULATOR
8V-30V
Input
LM2694
VIN
VCC
C3
VCC
UVLO
THERMAL
SHUTDOWN
C5
C1
RON
ON TIMER
RON
/SD
RON
START
COMPLETE
+
OFF TIMER
0.8V
START
COMPLETE
BST
GATE DRIVE
UVLO
C4
VIN
2.5V
12 PA
SS
C6
DRIVER
FB +
REGULATION
COMPARATOR
+
OVER2.9V VOLTAGE
COMPARATOR
RTN
6
DRIVER
LOGIC
L1
LEVEL
SHIFT
SW
VOUT1
D1
CURRENT LIMIT
COMPARATOR
R3
+
62 mV
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+
ISEN
R1
RSENSE
100 m:
SGND
R2
C2
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VIN
7.0V
UVLO
VCC
SW Pin
Inductor
Current
2.5V
SS Pin
VOUT
t1
t2
Figure 9. Startup Sequence
Functional Description
The LM2694 Step Down Switching Regulator features all the functions needed to implement a low cost, efficient
buck bias power converter capable of supplying at least 0.6A to the load. This high voltage regulator contains a
30V N-Channel buck switch, is easy to implement, and is available in the TSSOP-14 and the thermally enhanced
WSON-10 packages. The regulator’s operation is based on a constant on-time control scheme, where the ontime is determined by VIN. This feature allows the operating frequency to remain relatively constant with load and
input voltage variations. The feedback control requires no loop compensation resulting in very fast load transient
response. The valley current limit detection circuit, internally set at 0.62A, holds the buck switch off until the high
current level subsides. This scheme protects against excessively high currents if the output is short-circuited
when VIN is high. The functional block diagram is shown in Typical Application Circuit and Block Diagram.
The LM2694 can be applied in numerous applications to efficiently regulate down higher voltages. Additional
features include: Thermal shutdown, VCC under-voltage lockout, gate drive under-voltage lockout, and maximum
duty cycle limiter.
Control Circuit Overview
The LM2694 buck DC-DC regulator employs a control scheme based on a comparator and a one-shot on-timer,
with the output voltage feedback (FB) compared to an internal reference (2.5V). If the FB voltage is below the
reference the buck switch is turned on for a time period determined by the input voltage and a programming
resistor (RON). Following the on-time the switch remains off for a minimum of 265 ns, and until the FB voltage
falls below the reference. The buck switch then turns on for another on-time period. Typically, during start-up, or
when the load current increases suddenly, the off-times are at the minimum of 265 ns. Once regulation is
established, the off-times are longer.
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When in regulation, the LM2694 operates in continuous conduction mode at heavy load currents and
discontinuous conduction mode at light load currents. In continuous conduction mode current always flows
through the inductor, never reaching zero during the off-time. In this mode the operating frequency remains
relatively constant with load and line variations. The minimum load current for continuous conduction mode is
one-half the inductor’s ripple current amplitude. The operating frequency is approximately:
FS =
VOUT x (VIN ± 1.5V)
1.14 x 10-10 x (RON + 1.4 k:) x VIN
(1)
The buck switch duty cycle is equal to:
VOUT
tON
=
= tON x FS
VIN
tON + tOFF
DC =
(2)
In discontinuous conduction mode current through the inductor ramps up from zero to a peak during the on-time,
then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB
falls below the reference - until then the inductor current remains zero, and the load current is supplied by the
output capacitor (C2). In this mode the operating frequency is lower than in continuous conduction mode, and
varies with load current. Conversion efficiency is maintained at light loads since the switching losses reduce with
the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as
follows:
FS =
VOUT2 x L1 x 1.54 x 1020
RL x (RON)2
where
•
RL = the load resistance
(3)
The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as
follows:
VOUT = 2.5 x (R1 + R2) / R2
(4)
Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR
for the output capacitor C2. The LM2694 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases
where the capacitor’s ESR is insufficient additional series resistance may be required (R3 in Typical Application
Circuit and Block Diagram).
Start-Up Regulator, VCC
The start-up regulator is integral to the LM2694. The input pin (VIN) can be connected directly to line voltage up
to 30V, with transient capability to 33V. The VCC output regulates at 7.0V, and is current limited at 9 mA. Upon
power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC
pin reaches the under-voltage lockout threshold of 5.7V, the buck switch is enabled and the Softstart pin is
released to allow the Softstart capacitor (C6) to charge up.
The minimum input voltage is determined by the regulator’s dropout voltage, the VCC UVLO falling threshold
(≊5.5V), and the frequency. When VCC falls below the falling threshold the VCC UVLO activates to shut off the
output. If VCC is externally loaded, the minimum input voltage increases.
To reduce power dissipation in the start-up regulator, an auxiliary voltage can be diode connected to the VCC pin.
Setting the auxiliary voltage to between 8V and 14V shuts off the internal regulator, reducing internal power
dissipation. The sum of the auxiliary voltage and the input voltage (VCC + VIN) cannot exceed 47V. Internally, a
diode connects VCC to VIN. See Figure 10.
8
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VCC
C3
BST
C4
L1
LM2694
D2
SW
VOUT1
D1
ISEN
R1
R3
SGND
R2
C2
FB
Figure 10. Self Biased Configuration
Regulation Comparator
The feedback voltage at FB is compared to the voltage at the Softstart pin (2.5V). In normal operation (the output
voltage is regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch
stays on for the programmed on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the
buck switch stays off until the FB voltage falls below 2.5V. Input bias current at the FB pin is less than 100 nA
over temperature.
Over-Voltage Comparator
The voltage at FB is compared to an internal 2.9V reference. If the voltage at FB rises above 2.9V the on-time
pulse is immediately terminated. This condition can occur if the input voltage or the output load changes
suddenly, or if the inductor (L1) saturates. The buck switch remains off until the voltage at FB falls below 2.5V.
ON-Time Timer, and Shutdown
The on-time for the LM2694 is determined by the RON resistor and the input voltage (VIN), and is calculated from:
tON =
1.14 x 10-10 x (RON + 1.4 k:)
VIN ± 1.5V
+ 95 ns
(5)
See Figure 6. The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a
specific continuous conduction mode switching frequency (FS), the RON resistor is determined from the following:
RON =
VOUT x (VIN ± 1.5V)
FS x 1.14 x 10-10 x VIN
- 1.4 k:
(6)
In high frequency applications the minimum value for tON is limited by the maximum duty cycle required for
regulation and the minimum off-time of 265 ns, ±15%. The minimum off-time limits the maximum duty cycle
achievable with a low voltage at VIN. The minimum allowed on-time to regulate the desired VOUT at the minimum
VIN is determined from the following:
tON(min) =
VOUT x 305 ns
(VIN(min) - VOUT)
(7)
The LM2694 can be remotely shut down by taking the RON/SD pin below 0.8V. See Figure 11. In this mode the
SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the RON/SD pin
allows normal operation to resume. The voltage at the RON/SD pin is normally between 1.5V and 3.0V,
depending on VIN and the RON resistor.
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VIN
Input
Voltage
RON
LM2694
RON/SD
STOP
RUN
Figure 11. Shutdown Implementation
Current Limit
Current limit detection occurs during the off-time by monitoring the recirculating current through the free-wheeling
diode (D1). Referring to Typical Application Circuit and Block Diagram, when the buck switch is turned off the
inductor current flows through the load, into SGND, through the sense resistor, out of ISEN and through D1. If
that current exceeds 0.62A the current limit comparator output switches to delay the start of the next on-time
period if the voltage at FB is below 2.5V. The next on-time starts when the current out of ISEN is below 0.62A
and the voltage at FB is below 2.5V. If the overload condition persists causing the inductor current to exceed
0.62A during each on-time, that is detected at the beginning of each off-time. The operating frequency is lower
due to longer-than-normal off-times.
Figure 12 illustrates the inductor current waveform. During normal operation the load current is Io, the average of
the ripple waveform. When the load resistance decreases the current ratchets up until the lower peak reaches
0.62A. During the Current Limited portion of Figure 12, the current ramps down to 0.62A during each off-time,
initiating the next on-time (assuming the voltage at FB is <2.5V). During each on-time the current ramps up an
amount equal to:
ΔI = (VIN - VOUT) x tON / L1
(8)
During this time the LM2694 is in a constant current mode, with an average load current (IOCL) equal to 0.62A +
ΔI/2.
IPK
'I
IOCL
Inductor Current
0.62A
IO
Normal Operation
Load Current
Increases
Current Limited
Figure 12. Inductor Current - Current Limit Operation
N - Channel Buck Switch and Driver
The LM2694 integrates an N-Channel buck switch and associated floating high voltage gate driver. The peak
current allowed through the buck switch is 1.5A, and the maximum allowed average current is 1A. The gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.022
µF capacitor (C4) connected between BST and SW provides the voltage to the driver during the on-time. During
each off-time, the SW pin is at approximately -1V, and C4 charges from VCC through the internal diode. The
minimum off-time of 265 ns ensures a minimum time each cycle to recharge the bootstrap capacitor.
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Softstart
The softstart feature allows the converter to gradually reach a steady state operating point, thereby reducing
start-up stresses and current surges. Upon turn-on, after VCC reaches the under-voltage threshold, an internal 12
µA current source charges up the external capacitor at the SS pin to 2.5V. The ramping voltage at SS (and the
non-inverting input of the regulation comparator) ramps up the output voltage in a controlled manner.
An internal switch grounds the SS pin if VCC is below the under-voltage lockout threshold, if a thermal shutdown
occurs, or if the RON/SD pin is grounded.
Thermal Shutdown
The LM2694 should be operated so the junction temperature does not exceed 125°C. If the junction temperature
increases, an internal Thermal Shutdown circuit, which activates (typically) at 175°C, takes the controller to a low
power reset state by disabling the buck switch and the on-timer, and grounding the Softstart pin. This feature
helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces
below 155°C (typical hysteresis = 20°C), the Softstart pin is released and normal operation resumes.
APPLICATIONS INFORMATION
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with a design example. Referring to the
Block Diagram, the circuit is to be configured for the following specifications:
• VOUT = 5V
• VIN = 8V to 30V
• FS = 250 kHz
• Minimum load current = 100 mA
• Maximum load current = 600 mA
• Softstart time = 5 ms.
R1 and R2: These resistors set the output voltage, and their ratio is calculated from:
R1/R2 = (VOUT/2.5V) - 1
(9)
R1/R2 calculates to 1.0. The resistors should be chosen from standard value resistors in the range of 1.0 kΩ - 10
kΩ. A value of 2.5 kΩ will be used for R1 and for R2.
RON, FS: RON can be chosen using Equation 6 to set the nominal frequency, or from Equation 5 if the on-time at
a particular VIN is important. A higher frequency generally means a smaller inductor and capacitors (value, size
and cost), but higher switching losses. A lower frequency means a higher efficiency, but with larger components.
Generally, if PC board space is tight, a higher frequency is better. The resulting on-time and frequency have a
±25% tolerance. Using Equation 6 at a VIN of 8V,
RON =
5V x (8V ± 1.5V)
8V x 250 kHz x 1.14 x 10-10
- 1.4 k: = 141 k:
(10)
A value of 140 kΩ will be used for RON, yielding a nominal frequency of 252 kHz.
L1: The guideline for choosing the inductor value in this example is that it must keep the circuit’s operation in
continuous conduction mode at minimum load current. This is not a strict requirement since the LM2694
regulates correctly when in discontinuous conduction mode, although at a lower frequency. However, to provide
an initial value for L1 the above guideline will be used.
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L1 Current
IPK+
IO
IOR
IPK-
0 mA
1/Fs
Figure 13. Inductor Current
To keep the circuit in continuous conduction mode, the maximum allowed ripple current is twice the minimum
load current, or 200 mAp-p. Using this value of ripple current, the inductor (L1) is calculated using the following:
VOUT x (VIN(max) - VOUT)
IOR x FS(min) x VIN(max)
L1 =
where
•
FS(min) is the minimum frequency of 189 kHz (252 kHz - 25%)
5V x (30V - 5V)
L1 =
0.2A x 189 kHz x 30V
(11)
= 110 PH
(12)
This provides a minimum value for L1 - the next higher standard value (150 µH) will be used. To prevent
saturation, and possible destructive current levels, L1 must be rated for the peak current which occurs if the
current limit and maximum ripple current are reached simultaneously. The maximum ripple amplitude is
calculated by re-arranging Equation 11 using VIN(max), FS(min), and the minimum inductor value, based on the
manufacturer’s tolerance. Assume, for this exercise, the inductor’s tolerance is ±20%.
IOR(max) =
VOUT x (VIN(max) - VOUT)
L1MIN x FS(min) x VIN(max)
(13)
IOR(max) =
5V x (30V - 5V)
= 184 mAp-p
120 PH x 189 kHz x 30V
(14)
IPK = ILIM + IOR(max) = 0.74A + 0.18A = 0.92A
where
•
ILIM is the maximum specified current limit threshold
(15)
At the nominal maximum load current of 0.6A, the peak inductor current is 692 mA.
C1: This capacitor limits the ripple voltage at VIN resulting from the source impedance of the supply feeding this
circuit, and the on/off nature of the switch current into VIN. At maximum load current, when the buck switch turns
on, the current into VIN steps up from zero to the lower peak of the inductor current waveform (IPK-in Figure 13),
ramps up to the peak value (IPK+), then drops to zero at turn-off. The average current into VIN during this on-time
is the load current. For a worst case calculation, C1 must supply this average current during the maximum ontime. The maximum on-time is calculated at VIN = 8V using Equation 5, with a 25% tolerance added:
tON(max) =
1.14 x 10-10 x (140k + 1.4k)
8V - 1.5V
+ 95 ns x 1.25 = 3.22 Ps
(16)
The voltage at VIN should not be allowed to drop below 7.5V in order to maintain VCC above its UVLO.
C1 =
12
IO x tON 0.6A x 3.22 Ps
= 3.8 PF
=
'V
0.5V
(17)
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Normally a lower value can be used for C1 since the above calculation is a worst case calculation which
assumes the power source has a high source impedance. A quality ceramic capacitor with a low ESR should be
used for C1.
C2 and R3: Since the LM2694 requires a minimum of 25 mVp-p of ripple at the FB pin for proper operation, the
required ripple at VOUT is increased by R1 and R2, and is equal to:
VRIPPLE = 25 mVp-p x (R1 + R2)/R2 = 50 mVp-p
(18)
This necessary ripple voltage is created by the inductor ripple current acting on C2’s ESR + R3. First, the
minimum ripple current, which occurs at minimum VIN, maximum inductor value, and maximum frequency, is
determined.
VOUT x (VIN(min) - VOUT)
L1max x FS(max) x VIN(min)
IOR(min) =
=
5V x (8V - 5V)
= 33 mAp-p
180 PH x 315 kHz x 8V
(19)
The minimum ESR for C2 is then equal to:
ESR(min) =
50 mV
= 1.5:
33 mA
(20)
If the capacitor used for C2 does not have sufficient ESR, R3 is added in series as shown in Typical Application
Circuit and Block Diagram. The value chosen for C2 is application dependent, and it is recommended that it be
no smaller than 3.3 µF. C2 affects the ripple at VOUT, and transient response. Experimentation is usually
necessary to determine the optimum value for C2.
C3: The capacitor at the VCC pin provides noise filtering and stability, prevents false triggering of the VCC UVLO
at the buck switch on/off transitions, and limits the peak voltage at VCC when a high voltage with a short rise time
is initially applied at VIN. C3 should be no smaller than 0.1 µF, and should be a good quality, low ESR, ceramic
capacitor, physically close to the IC pins.
C4: The recommended value for C4 is 0.022 µF. A high quality ceramic capacitor with low ESR is recommended
as C4 supplies the surge current to charge the buck switch gate at each turn-on. A low ESR also ensures a
complete recharge during each off-time.
C5: This capacitor suppresses transients and ringing due to lead inductance at VIN. A low ESR, 0.1 µF ceramic
chip capacitor is recommended, located physically close to the LM2694.
C6: The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage at the
regulation comparator, and the output voltage, to reach their final value. The capacitor value is determined from
the following:
tSS x 12 PA
C6 =
2.5V
(21)
For a 5 ms softstart time, C6 calculates to 0.024 µF.
D1: A Schottky diode is recommended. Ultra-fast recovery diodes are not recommended as the high speed
transitions at the SW pin may inadvertently affect the IC’s operation through external or internal EMI. The diode
should be rated for the maximum VIN (30V), the maximum load current (0.6A), and the peak current which occurs
when current limit and maximum ripple current are reached simultaneously (IPK in Figure 12), previously
calculated to be 0.92A. The diode’s forward voltage drop affects efficiency due to the power dissipated during the
off-time. The average power dissipation in D1 is calculated from:
PD1 = VF x IO x (1 - D)
where
•
IO is the load current, and D is the duty cycle
(22)
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LM2694
SNVS444A – MAY 2006 – REVISED APRIL 2013
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FINAL CIRCUIT
The final circuit is shown in Figure 14, and its performance is shown in Figure 15 and Figure 16. Current limit
measured approximately 0.64A.
8V - 30V
Input
VIN
VCC
C5
0.1 PF
C1
3.3 PF
C3
0.1 PF
LM2694
RON
BST
140k
0.022 PF
C4
L1 150 PH
RON/SD
SW
5V
VOUT
D1
SS
ISEN
R1
2.5k
C6
0.022 PF
R3
1.5
SGND
FB
C2
22 PF
R2
2.5k
RTN
GND
Figure 14. Example Circuit
Item
Description
Value
C1
Ceramic Capacitor
3.3 µF, 50V
C2
Ceramic Capacitor
22 µF, 16V
C4, C6
Ceramic Capacitor
0.022 µF, 16V
C3, C5
Ceramic Capacitor
0.1 µF, 50V
D1
Schottky Diode
60V, 1A
L1
Inductor
150 µH
R1
Resistor
2.5 kΩ
R2
Resistor
2.5 kΩ
R3
Resistor
1.5 Ω
RON
Resistor
140 kΩ
U1
TI Semi LM2694
100
350
VIN = 8V
90
80
300
FREQUENCY (kHz)
EFFICIENCY (%)
12V
30V
70
60
200
Load Curent = 400 mA
50
150
0
100
200
300
400
500
5
600
LOAD CURRENT (mA)
8 10
15
20
25
30
VIN (V)
Figure 15. Efficiency vs Load Current and VIN
Circuit of Figure 14
14
250
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Figure 16. Frequency vs VIN
Circuit of Figure 14
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SNVS444A – MAY 2006 – REVISED APRIL 2013
MINIMUM LOAD CURRENT
The LM2694 requires a minimum load current of 500 µA. If the load current falls below that level, the bootstrap
capacitor (C4) may discharge during the long off-time, and the circuit will either shutdown, or cycle on and off at
a low frequency. If the load current is expected to drop below 500 µA in the application, R1 and R2 should be
chosen low enough in value so they provide the minimum required current at nominal VOUT.
LOW OUTPUT RIPPLE CONFIGURATIONS
For applications where low output voltage ripple is required the output can be taken directly from the low ESR
output capacitor (C2) as shown in Figure 17. However, R3 slightly degrades the load regulation. The specific
component values, and the application determine if this is suitable.
L1
SW
LM2694
R3
R1
FB
VOUT
R2
C2
Figure 17. Low Ripple Output
Where the circuit of Figure 17 is not suitable for reducing output ripple, the circuits of Figure 18 or Figure 19 can
be used.
SW
L1
VOUT
LM2694
Cff
R1
R3
FB
R2
C2
Figure 18. Low Output Ripple Using a Feedforward Capacitor
In Figure 18, Cff is added across R1 to AC-couple the ripple at VOUT directly to the FB pin. This allows the ripple
at VOUT to be reduced, in some cases considerably, by reducing R3. In the circuit of Figure 14, the ripple at VOUT
ranged from 50 mVp-p at VIN = 8V to 100 mVp-p at VIN = 30V. By adding a 2700 pF capacitor at Cff and
reducing R3 to 0.75Ω, the VOUT ripple is reduced by 50%.
SW
LM2694
FB
L1
VOUT
RA
CB
C2
CA
R1
R2
Figure 19. Minimum Output Ripple Using Ripple Injection
To reduce VOUT ripple further, the circuit of Figure 19 can be used. R3 has been removed, and the output ripple
amplitude is determined by C2’s ESR and the inductor ripple current. RA and CA are chosen to generate a 40-50
mVp-p sawtooth at their junction, and that voltage is AC-coupled to the FB pin via CB. In selecting RA and CA,
VOUT is considered a virtual ground as the SW pin switches between VIN and -1V. Since the on-time at SW varies
inversely with VIN, the waveform amplitude at the RA/CA junction is relatively constant. Typical values for the
additional components are RA = 110k, CA = 2700 pF, and CB = 0.01 µF.
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LM2694
SNVS444A – MAY 2006 – REVISED APRIL 2013
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PC BOARD LAYOUT and THERMAL CONSIDERATIONS
The LM2694 regulation, over-voltage, and current limit comparators are very fast, and will respond to short
duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be
as neat and compact as possible, and all the components must be as close as possible to their associated pins.
The two major current loops have currents which switch very fast, and so the loops should be as small as
possible to minimize conducted and radiated EMI. The first loop is that formed by C1, through the VIN to SW
pins, L1, C2, and back to C1. The second loop is that formed by D1, L1, C2, and the SGND and ISEN pins. The
ground connection from C2 to C1 should be as short and direct as possible, preferably without going through
vias. Directly connect the SGND and RTN pin to each other, and they should be connected as directly as
possible to the C1/C2 ground line without going through vias. The power dissipation within the IC can be
approximated by determining the total conversion loss (PIN - POUT), and then subtracting the power losses in the
free-wheeling diode and the inductor. The power loss in the diode is approximately:
PD1 = IO x VF x (1-D)
where
•
•
•
Io is the load current
VF is the diode’s forward voltage drop
D is the duty cycle
(23)
The power loss in the inductor is approximately:
PL1 = IO2 x RL x 1.1
where
•
•
RL is the inductor’s DC resistance
1.1 factor is an approximation for the AC losses
(24)
If it is expected that the internal dissipation of the LM2694 will produce high junction temperatures during normal
operation, good use of the PC board’s ground plane can help considerably to dissipate heat. The exposed pad
on the WSON package bottom should be soldered to a ground plane, and that plane should both extend from
beneath the IC, and be connected to exposed ground plane on the board’s other side using as many vias as
possible. The exposed pad is internally connected to the IC substrate. The use of wide PC board traces at the
pins, where possible, can help conduct heat away from the IC. The four No Connect pins on the TSSOP package
are not electrically connected to any part of the IC, and may be connected to ground plane to help dissipate heat
from the package. Judicious positioning of the PC board within the end product, along with the use of any
available air flow (forced or natural convection) can help reduce the junction temperature.
16
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SNVS444A – MAY 2006 – REVISED APRIL 2013
REVISION HISTORY
Changes from Original (April 2013) to Revision A
•
Page
Changed layout of National Data Sheet to TI format .......................................................................................................... 16
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17
PACKAGE OPTION ADDENDUM
www.ti.com
1-Nov-2015
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
94
Eco Plan
Lead/Ball Finish
MSL Peak Temp
(2)
(6)
(3)
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
L2694
MT
TBD
Call TI
Call TI
L2694
MT
L2694
MT
LM2694MT/NOPB
ACTIVE
TSSOP
PW
14
LM2694MTX
NRND
TSSOP
PW
14
LM2694MTX/NOPB
ACTIVE
TSSOP
PW
14
2500
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
LM2694SD/NOPB
ACTIVE
WSON
DSC
10
1000
Green (RoHS
& no Sb/Br)
CU SN
Level-1-260C-UNLIM
Op Temp (°C)
Device Marking
(4/5)
-40 to 125
L2694
(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)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3)
MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4)
There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.
(5)
Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation
of the previous line and the two combined represent the entire Device Marking for that device.
(6)
Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish
value exceeds the maximum column width.
Addendum-Page 1
Samples
PACKAGE OPTION ADDENDUM
www.ti.com
1-Nov-2015
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
Addendum-Page 2
PACKAGE MATERIALS INFORMATION
www.ti.com
6-Nov-2015
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device
Package Package Pins
Type Drawing
SPQ
Reel
Reel
A0
Diameter Width (mm)
(mm) W1 (mm)
B0
(mm)
K0
(mm)
P1
(mm)
W
Pin1
(mm) Quadrant
LM2694MTX/NOPB
TSSOP
PW
14
2500
330.0
12.4
6.95
5.6
1.6
8.0
12.0
Q1
LM2694SD/NOPB
WSON
DSC
10
1000
178.0
12.4
3.3
3.3
1.0
8.0
12.0
Q1
Pack Materials-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
6-Nov-2015
*All dimensions are nominal
Device
Package Type
Package Drawing
Pins
SPQ
Length (mm)
Width (mm)
Height (mm)
LM2694MTX/NOPB
LM2694SD/NOPB
TSSOP
PW
14
2500
367.0
367.0
35.0
WSON
DSC
10
1000
210.0
185.0
35.0
Pack Materials-Page 2
PACKAGE OUTLINE
DSC0010B
WSON - 0.8 mm max height
SCALE 4.000
PLASTIC SMALL OUTLINE - NO LEAD
3.1
2.9
B
A
PIN 1 INDEX AREA
3.1
2.9
C
0.8 MAX
0.08
SEATING PLANE
0.05
0.00
1.2±0.1
(0.2) TYP
6
5
8X 0.5
2X
2
2±0.1
1
10
10X
PIN 1 ID
(OPTIONAL)
10X
0.5
0.4
0.3
0.2
0.1
0.05
C A
C
B
4214926/A 07/2014
NOTES:
1. All linear dimensions are in millimeters. Any dimensions in parenthesis are for reference only. Dimensioning and tolerancing
per ASME Y14.5M.
2. This drawing is subject to change without notice.
3. The package thermal pad must be soldered to the printed circuit board for thermal and mechanical performance.
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EXAMPLE BOARD LAYOUT
DSC0010B
WSON - 0.8 mm max height
PLASTIC SMALL OUTLINE - NO LEAD
(1.2)
10X (0.65)
SYMM
10
1
10X (0.25)
SYMM
(2)
(0.75) TYP
8X (0.5)
5
( 0.2) TYP
VIA
6
(0.35) TYP
(2.75)
LAND PATTERN EXAMPLE
SCALE:20X
0.07 MIN
ALL AROUND
0.07 MAX
ALL AROUND
SOLDER MASK
OPENING
METAL
SOLDER MASK
OPENING
NON SOLDER MASK
DEFINED
(PREFERRED)
METAL
UNDER
SOLDER MASK
SOLDER MASK
DEFINED
SOLDER MASK DETAILS
4214926/A 07/2014
NOTES: (continued)
4. This package is designed to be soldered to a thermal pad on the board. For more information, see Texas Instruments literature
number SLUA271 (www.ti.com/lit/slua271).
www.ti.com
EXAMPLE STENCIL DESIGN
DSC0010B
WSON - 0.8 mm max height
PLASTIC SMALL OUTLINE - NO LEAD
10X (0.65)
SYMM
METAL
TYP
10X (0.25)
(0.55)
SYMM
(0.89)
8X (0.5)
(1.13)
(2.75)
SOLDER PASTE EXAMPLE
BASED ON 0.125 mm THICK STENCIL
EXPOSED PAD
84% PRINTED SOLDER COVERAGE BY AREA
SCALE:25X
4214926/A 07/2014
NOTES: (continued)
5. Laser cutting apertures with trapezoidal walls and rounded corners may offer better paste release. IPC-7525 may have alternate
design recommendations.
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