NSC LM2717

May 2005
LM2717
Dual Step-down DC/DC Converter
General Description
Features
The LM2717 is composed of two PWM DC/DC buck (stepdown) converters. The first converter is used to generate a
fixed output voltage of 3.3V. The second converter is used to
generate an adjustable output voltage. Both converters feature low RDSON (0.16Ω) internal switches for maximum efficiency. Operating frequency can be adjusted anywhere between 300kHz and 600kHz allowing the use of small external
components. External soft-start pins for each enables the
user to tailor the soft-start times to a specific application.
Each converter may also be shut down independently with
its own shutdown pin. The LM2717 is available in a low
profile 24-lead TSSOP package ensuring a low profile overall solution.
n Fixed 3.3V output buck converter with a 2.2A, 0.16Ω,
internal switch
n Adjustable buck converter with a 3.2A, 0.16Ω, internal
switch
n Operating input voltage range of 4V to 20V
n Input undervoltage protection
n 300kHz to 600kHz pin adjustable operating frequency
n Over temperature protection
n Small 24-Lead TSSOP package
Applications
n
n
n
n
TFT-LCD Displays
Handheld Devices
Portable Applications
Laptop Computers
Typical Application Circuit
20078501
© 2005 National Semiconductor Corporation
DS200785
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LM2717 Dual Step-down DC/DC Converter
PRELIMINARY
LM2717
Connection Diagram
Top View
20078504
24-Lead TSSOP
Ordering Information
Order Number
Package Type
NSC Package Drawing
LM2717MT
Spec
TSSOP-24
MTC24
61 Units, Rail
Supplied As
LM2717MTX
TSSOP-24
MTC24
2500 Units, Tape and Reel
LM2717MT
NOPB
TSSOP-24
MTC24
61 Units, Rail
LM2717MTX
NOPB
TSSOP-24
MTC24
2500 Units, Tape and Reel
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2
LM2717
Pin Description
Pin
Name
Function
1
PGND
Power ground. PGND and AGND pins must be connected together directly at the part.
2
PGND
Power ground. PGND and AGND pins must be connected together directly at the part.
3
AGND
Analog ground. PGND and AGND pins must be connected together directly at the part.
4
FB1
Fixed buck output voltage feedback input.
5
VC1
Fixed buck compensation network connection. Connected to the output of the voltage
error amplifier.
6
VBG
Bandgap connection.
7
VC2
Adjustable buck compensation network connection. Connected to the output of the voltage
error amplifier.
8
FB2
Adjustable buck output voltage feedback input.
9
AGND
Analog ground. PGND and AGND pins must be connected together directly at the part.
10
AGND
Analog ground. PGND and AGND pins must be connected together directly at the part.
11
PGND
Power ground. PGND and AGND pins must be connected together directly at the part.
12
PGND
Power ground. PGND and AGND pins must be connected together directly at the part.
13
SW2
14
VIN
Adjustable buck power switch input. Switch connected between VIN pins and SW2 pin.
Analog power input. VIN pins should be connected together directly at the part.
15
VIN
Analog power input. VIN pins should be connected together directly at the part.
16
CB2
Adjustable buck converter bootstrap capacitor connection.
17
SHDN2
18
SS2
19
FSLCT
20
SS1
21
SHDN1
22
CB1
23
VIN
24
SW1
Shutdown pin for adjustable buck converter. Active low.
Adjustable buck soft start pin.
Switching frequency select input. Use a resistor to set the frequency anywhere between
300kHz and 600kHz.
Fixed buck soft start pin.
Shutdown pin for fixed buck converter. Active low.
Fixed buck converter bootstrap capacitor connection.
Analog power input. VIN pins should be connected together directly at the part.
Fixed buck power switch input. Switch connected between VIN pins and SW1 pin.
3
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LM2717
Block Diagram
20078503
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4
Power Dissipation(Note 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Lead Temperature
VIN
−0.3V to 22V
SW1 Voltage
−0.3V to 22V
SW2 Voltage
−0.3V to 22V
FB1 Voltage
−0.3V to 7V
FB2 Voltage
−0.3V to 7V
VC1 Voltage
1.75V ≤ VC1 ≤ 2.25V
VC2 Voltage
0.965V ≤ VC2 ≤ 1.565V
SHDN1 Voltage
−0.3V to 7.5V
SHDN2 Voltage
−0.3V to 7.5V
SS1 Voltage
−0.3V to 2.1V
SS2 Voltage
−0.3V to 2.1V
FSLCT Voltage
300˚C
Vapor Phase (60 sec.)
215˚C
Infrared (15 sec.)
220˚C
ESD Susceptibility (Note 3)
Human Body Model
2kV
Operating Conditions
Operating Junction
Temperature Range
(Note 4)
−40˚C to +125˚C
Storage Temperature
−65˚C to +150˚C
Supply Voltage
AGND to 5V
Maximum Junction Temperature
Internally Limited
4V to 20V
SW1 Voltage
20V
SW2 Voltage
20V
150˚C
Electrical Characteristics
Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Temperature Range (TJ = −40˚C to +125˚C). VIN = 5V, IL = 0A, and FSW = 300kHz unless otherwise specified.
Symbol
IQ
Parameter
Conditions
Min
(Note 4)
Total Quiescent Current (both Not Switching
switchers)
Switching, switch open
VSHDN = 0V
VFB1
Fixed Buck Feedback
Voltage
VFB2
Adjustable Buck Feedback
Voltage
ICL1(Note 6) Fixed Buck Switch Current
Limit
VIN = 8V (Note 7)
ICL2(Note 6) Adjustable Buck Switch
Current Limit
VIN = 8V (Note 7)
IB1
IB2
Fixed Buck FB Pin Bias
Current
(Note 8)
VIN = 20V
Adjustable Buck FB Pin Bias
Current
(Note 8)
VIN = 20V
VIN
Input Voltage Range
gm1
Fixed Buck Error Amp
Transconductance
∆I = 20µA
gm2
Adjustable Buck Error Amp
Transconductance
∆I = 20µA
AV1
Typ
(Note 5)
Max
(Note 4)
Units
2.7
6
mA
6
12
mA
9
27
µA
3.3
V
1.267
V
2.2
A
3.2
A
65
µA
65
nA
4
20
V
1340
µmho
1360
µmho
Fixed Buck Error Amp
Voltage Gain
134
V/V
AV2
Adjustable Buck Error Amp
Voltage Gain
136
V/V
DMAX
Maximum Duty Cycle
FSW
Switching Frequency
89
93
RF = 46.4k
200
300
400
kHz
RF = 22.6k
475
600
775
kHz
5
%
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LM2717
Absolute Maximum Ratings (Note 1)
LM2717
Electrical Characteristics
(Continued)
Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Temperature Range (TJ = −40˚C to +125˚C). VIN = 5V, IL = 0A, and FSW = 300kHz unless otherwise specified.
Symbol
Parameter
Conditions
Min
(Note 4)
Typ
(Note 5)
Max
(Note 4)
Units
−5
5
µA
−5
5
µA
0.01
5
µA
0.01
5
µA
ISHDN1
Fixed Buck Shutdown Pin
Current
0V < VSHDN1 < 7.5V
ISHDN2
Adjustable Buck Shutdown
Pin Current
0V < VSHDN2 < 7.5V
IL1
Fixed Buck Switch Leakage
Current
VIN = 20V
IL2
Adjustable Buck Switch
Leakage Current
VIN = 20V
RDSON1
Fixed Buck Switch RDSON
(Note 9)
160
mΩ
RDSON2
Adjustable Buck Switch
RDSON (Note 9)
160
mΩ
ThSHDN1
Fixed Buck SHDN Threshold
Output High
1.8
Output Low
ThSHDN2
Adjustable Buck SHDN
Threshold
1.36
1.33
Output High
1.8
Output Low
0.7
1.36
1.33
0.7
V
V
ISS1
Fixed Buck Soft Start Pin
Current
4
9
15
µA
ISS2
Adjustable Buck Soft Start
Pin Current
4
9
15
µA
On Threshold
4
UVP
Off Threshold
θJA
Thermal Resistance
(Note 10)
3.8
3.6
TSSOP, package only
3.3
115
V
˚C/W
Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to
be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA,
and the ambient temperature, TA. See the Electrical Characteristics table for the thermal resistance. The maximum allowable power dissipation at any ambient
temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the
regulator will go into thermal shutdown.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 4: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% tested
or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods.
All limits are used to calculate Average Outgoing Quality Level (AOQL).
Note 5: Typical numbers are at 25˚C and represent the most likely norm.
Note 6: Duty cycle affects current limit due to ramp generator.
Note 7: Current limit at 0% duty cycle. See TYPICAL PERFORMANCE section for Switch Current Limit vs. VIN
Note 8: Bias current flows into FB pin.
Note 9: Includes the bond wires, RDSON from VIN pin(s) to SW pin.
Note 10: Refer to National’s packaging website for more detailed thermal information and mounting techniques for the TSSOP package.
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6
LM2717
Typical Performance Characteristics
Switching IQ vs. Input Voltage
(FSW = 300kHz)
Shutdown IQ vs. Input Voltage
20078560
20078561
Switching Frequency vs. Input Voltage
(FSW = 300kHz)
Fixed Buck RDS(ON) vs. Input Voltage
20078562
20078563
Adjustable Buck RDS(ON) vs. Input Voltage
Fixed Buck Efficiency vs. Load Current
20078565
20078564
7
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LM2717
Typical Performance Characteristics
(Continued)
Adjustable Buck Efficiency vs. Load Current
(VOUT = 15V)
Adjustable Buck Efficiency vs. Load Current
(VOUT = 5V)
20078566
20078567
Adjustable Buck Switch Current Limt vs. Input Voltage
(VOUT = 5V)
Fixed Buck Switch Current Limt vs. Input Voltage
20078569
20078568
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8
LM2717
Buck Operation
PROTECTION (BOTH REGULATORS)
The LM2717 has dedicated protection circuitry running during normal operation to protect the IC. The Thermal Shutdown circuitry turns off the power devices when the die
temperature reaches excessive levels. The UVP comparator
protects the power devices during supply power startup and
shutdown to prevent operation at voltages less than the
minimum input voltage. The OVP comparator is used to
prevent the output voltage from rising at no loads allowing
full PWM operation over all load conditions. The LM2717
also features a shutdown mode for each converter decreasing the supply current to approximately 10µA (both in shutdown mode).
The RMS current reaches its maximum (IOUT/2) when
VIN equals 2VOUT. This value should be calculated for both
regulators and added to give a total RMS current rating. For
an aluminum or ceramic capacitor, the voltage rating should
be at least 25% higher than the maximum input voltage. If a
tantalum capacitor is used, the voltage rating required is
about twice the maximum input voltage. The tantalum capacitor should be surge current tested by the manufacturer
to prevent being shorted by the inrush current. The minimum
capacitor value should be 47µF for lower output load current
applications and less dynamic (quickly changing) load conditions. For higher output current applications or dynamic
load conditions a 68µF to 100µF low ESR capacitor is recommended. It is also recommended to put a small ceramic
capacitor (0.1µF to 4.7µF) between the input pins and
ground to reduce high frequency spikes.
CONTINUOUS CONDUCTION MODE
The LM2717 contains current-mode, PWM buck regulators.
A buck regulator steps the input voltage down to a lower
output voltage. In continuous conduction mode (when the
inductor current never reaches zero at steady state), the
buck regulator operates in two cycles. The power switch is
connected between VIN and SW1 and SW2.
In the first cycle of operation the transistor is closed and the
diode is reverse biased. Energy is collected in the inductor
and the load current is supplied by COUT and the rising
current through the inductor.
During the second cycle the transistor is open and the diode
is forward biased due to the fact that the inductor current
cannot instantaneously change direction. The energy stored
in the inductor is transferred to the load and output capacitor.
The ratio of these two cycles determines the output voltage.
The output voltage is defined approximately as:
INDUCTOR SELECTION
The most critical parameters for the inductor are the inductance, peak current and the DC resistance. The inductance
is related to the peak-to-peak inductor ripple current, the
input and the output voltages (for 300kHz operation):
A higher value of ripple current reduces inductance, but
increases the conductance loss, core loss, and current
stress for the inductor and switch devices. It also requires a
bigger output capacitor for the same output voltage ripple
requirement. A reasonable value is setting the ripple current
to be 30% of the DC output current. Since the ripple current
increases with the input voltage, the maximum input voltage
is always used to determine the inductance. The DC resistance of the inductor is a key parameter for the efficiency.
Lower DC resistance is available with a bigger winding area.
A good tradeoff between the efficiency and the core size is
letting the inductor copper loss equal 2% of the output
power.
where D is the duty cycle of the switch, D and D' will be
required for design calculations.
DESIGN PROCEDURE
This section presents guidelines for selecting external components.
OUTPUT CAPACITOR
The selection of COUT is driven by the maximum allowable
output voltage ripple. The output ripple in the constant frequency, PWM mode is approximated by:
SETTING THE OUTPUT VOLTAGE (ADJUSTABLE
REGULATOR)
The output voltage is set using the feedback pin and a
resistor divider connected to the output as shown in Figure 1.
The feedback pin voltage is 1.26V, so the ratio of the feedback resistors sets the output voltage according to the following equation:
The ESR term usually plays the dominant role in determining
the voltage ripple. Low ESR ceramic, aluminum electrolytic,
or tantalum capacitors (such as Taiyo Yuden MLCC, Nichicon PL series, Sanyo OS-CON, Sprague 593D, 594D, AVX
TPS, and CDE polymer aluminum) is recommended. An
electrolytic capacitor is not recommended for temperatures
below −25˚C since its ESR rises dramatically at cold temperature. Ceramic or tantalum capacitors have much better
ESR specifications at cold temperature and is preferred for
low temperature applications.
INPUT CAPACITOR
A low ESR aluminum, tantalum, or ceramic capacitor is
needed betwen the input pin and power ground. This capacitor prevents large voltage transients from appearing at the
input. The capacitor is selected based on the RMS current
and voltage requirements. The RMS current is given by:
9
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LM2717
Buck Operation
LAYOUT CONSIDERATIONS
(Continued)
The LM2717 uses two separate ground connections, PGND
for the drivers and boost NMOS power device and AGND for
the sensitive analog control circuitry. The AGND and PGND
pins should be tied directly together at the package. The
feedback and compensation networks should be connected
directly to a dedicated analog ground plane and this ground
plane must connect to the AGND pin. If no analog ground
plane is available then the ground connections of the feedback and compensation networks must tie directly to the
AGND pin. Connecting these networks to the PGND can
inject noise into the system and effect performance.
The input bypass capacitor CIN, as shown in Figure 1, must
be placed close to the IC. This will reduce copper trace
resistance which effects input voltage ripple of the IC. For
additional input voltage filtering, a 0.1µF to 4.7µF bypass
capacitors can be placed in parallel with CIN, close to the VIN
pins to shunt any high frequency noise to ground. The output
capacitors, COUT1 and COUT2, should also be placed close to
the IC. Any copper trace connections for the COUTX capacitors can increase the series resistance, which directly effects
output voltage ripple. The feedback network, resistors RFB1
and RFB2, should be kept close to the FB pin, and away from
the inductor to minimize copper trace connections that can
inject noise into the system. Trace connections made to the
inductors and schottky diodes should be minimized to reduce power dissipation and increase overall efficiency. For
more detail on switching power supply layout considerations
see Application Note AN-1149: Layout Guidelines for Switching Power Supplies.
BOOTSTRAP CAPACITOR
A 4.7nF ceramic capacitor or larger is recommended for the
bootstrap capacitor. For applications where the input voltage
is less than twice the output voltage a larger capacitor is
recommended, generally 0.1µF to 1µF to ensure plenty of
gate drive for the internal switches and a consistently low
RDS(ON).
SOFT-START CAPACITOR (BOTH REGULATORS)
The LM2717 does not contain internal soft-start which allows
for fast startup time but also causes high inrush current.
Therefore for applications that need reduced inrush current
the LM2717 has circuitry that is used to limit the inrush
current on start-up of the DC/DC switching regulators. This
inrush current limiting circuitry serves as a soft-start. The
external SS pins are used to tailor the soft-start for a specific
application. A current (ISS) charges the external soft-start
capacitor, CSS. The soft-start time can be estimated as:
TSS = CSS*0.6V/ISS
When programming the softstart time simply use the equation given in the Soft-Start Capacitor section above.
SCHOTTKY DIODE
The breakdown voltage rating of D1 and D2 is preferred to be
25% higher than the maximum input voltage. The current
rating for the diode should be equal to the maximum output
current for best reliability in most applications. In cases
where the input voltage is much greater than the output
voltage the average diode current is lower. In this case it is
possible to use a diode with a lower average current rating,
approximately (1-D)*IOUT however the peak current rating
should be higher than the maximum load current.
Application Information
Some recommended Inductors (others may be used)
Manufacturer
Inductor
Coilcraft
DO3316 and DO5022 series
Contact Information
www.coilcraft.com
Coiltronics
DRQ73 and CD1 series
www.cooperet.com
Pulse
P0751 and P0762 series
www.pulseeng.com
Sumida
CDRH8D28 and CDRH8D43 series
www.sumida.com
Some recommended Input and Output Capacitors (others may be used)
Manufacturer
Capacitor
Vishay Sprague
293D, 592D, and 595D series tantalum
www.vishay.com
Taiyo Yuden
High capacitance MLCC ceramic
www.t-yuden.com
Cornell Dubilier
ESRD seriec Polymer Aluminum Electrolytic
SPV and AFK series V-chip series
www.cde.com
Panasonic
High capacitance MLCC ceramic
EEJ-L series tantalum
www.panasonic.com
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10
Contact Information
LM2717
Application Information
(Continued)
20078558
FIGURE 1. 15V, 3.3V Output Application
20078559
FIGURE 2. 5V, 3.3V Output Application
11
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LM2717 Dual Step-down DC/DC Converter
Physical Dimensions
inches (millimeters)
unless otherwise noted
TSSOP-24 Pin Package (MTC)
For Ordering, Refer to Ordering Information Table
NS Package Number MTC24
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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