LM2694
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SNVS444A – MAY 2006 – REVISED APRIL 2013
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.
Copyright © 2006–2013, Texas Instruments Incorporated
LM2694
SNVS444A – MAY 2006 – REVISED APRIL 2013
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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