lm25085sdx National Semiconductor Corporation, lm25085sdx Datasheet

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lm25085sdx

Manufacturer Part Number
lm25085sdx
Description
42v Constant On-time Pfet Buck Switching Controller
Manufacturer
National Semiconductor Corporation
Datasheet

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© 2009 National Semiconductor Corporation
42V Constant On-Time PFET Buck Switching Controller
General Description
The LM25085 is a high efficiency PFET switching regulator
controller that can be used to quickly and easily develop a
small, efficient buck regulator for a wide range of applications.
This high voltage controller contains a PFET gate driver and
a high voltage bias regulator which operates over a wide 4.5V
to 42V input range. The constant on-time regulation principle
requires no loop compensation, simplifies circuit implemen-
tation, and results in ultra-fast load transient response. The
operating frequency remains nearly constant with line and
load variations due to the inverse relationship between the
input voltage and the on-time. The PFET architecture allows
100% duty cycle operation for a low dropout voltage. Either
the R
used to sense current for over-current detection.
Typical Application, Basic Step Down Controller
DS(ON)
of the PFET or an external sense resistor can be
300793
LM25085
Features
Package
Wide 4.5V to 42V input voltage range
Adjustable current limit using R
resistor
Programmable switching frequency to 1MHz
No loop compensation required
Ultra-Fast transient response
Nearly constant operating frequency with line and load
variations
Adjustable output voltage from 1.25V
Precision ±2% feedback reference
Capable of 100% duty cycle operation
Internal soft-start timer
Integrated high voltage bias regulator
Thermal shutdown
MSOP-8EP
MSOP-8
LLP-8 (3 mm x 3 mm)
DS(ON)
or a current sense
February 6, 2009
www.national.com
30079301

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lm25085sdx Summary of contents

Page 1

... Either the R of the PFET or an external sense resistor can be DS(ON) used to sense current for over-current detection. Typical Application, Basic Step Down Controller © 2009 National Semiconductor Corporation LM25085 Features ■ Wide 4.5V to 42V input voltage range ■ ...

Page 2

... Package Type LM25085MYE MSOP-8EP LM25085MY MSOP-8EP LM25085MYX MSOP-8EP LM25085MME MSOP-8 LM25085MM MSOP-8 LM25085MMX MSOP-8 LM25085SDE LLP-8 LM25085SD LLP-8 LM25085SDX LLP-8 www.national.com 30079302 30079303 NSC Package Drawing Junction Temperature Range MUY08A -40°C to +125°C MUY08A MUY08A MUA08A MUA08A MUA08A SDA08A SDA08A SDA08A ...

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Pin Descriptions Pin Name Description No. 1 ADJ Current Limit Adjust 2 RT On-time control and shutdown 3 FB Voltage Feedback from the regulated output 4 GND Circuit Ground 5 ISEN Current sense input for current limit detection. 6 PGATE ...

Page 4

Absolute Maximum Ratings If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN to GND ISEN to GND ADJ to GND RT GND Electrical Characteristics junction temperature (T ) ...

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Symbol Parameter Off-Time t Off-time (current limit) (Note 7) OFF(CL1) t OFF(CL2) t OFF(CL3) t OFF(CL4) Regulation and Over-Voltage Comparators (FB Pin regulation threshold REF V FB over-voltage threshold bias current FB Soft-Start Function t ...

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Typical Performance Characteristics Unless otherwise specified the following conditions apply: T Efficiency (Circuit of Figure 4) Shutdown Current vs vs www.national.com = 25° 24V 30079305 IN 30079307 CC 30079309 6 Input Operating ...

Page 7

Off-Time vs. V and 30079312 ADJ Pin Current vs 30079317 Shutdown Current vs. Temperature 30079352 Voltage at the RT Pin Input Operating Current vs. Temperature VCC vs. Temperature 7 30079313 30079318 30079353 www.national.com ...

Page 8

On-Time vs. Temperature Off-Time vs. Temperature ADJ Pin Current vs. Temperature www.national.com Minimum On-Time vs. Temperature 30079354 Current Limit Comparator Offset vs. Temperature 30079356 PGATE Driver Output Resistance vs. Temperature 30079314 8 30079355 30079357 30079310 ...

Page 9

Feedback Reference Voltage vs. Temperature RT Pin Shutdown Threshold vs. Temperature Soft-Start Time vs. Temperature 30079315 30079358 9 30079316 www.national.com ...

Page 10

Block Diagram Sense resistor method shown for current limit detection. Minimum output ripple configuration shown. www.national.com 10 30079319 ...

Page 11

Functional Description OVERVIEW The LM25085 is a PFET buck (step-down) DC-DC controller using the constant on-time (COT) control principle. The input operating voltage range of the LM25085 is 4.5V to 42V. The use of a PFET in a buck regulator ...

Page 12

For V = 42V and t = 100 ns, the simplified equation is SHUTDOWN The LM25085 can be shutdown by grounding the RT pin (see Figure 1). In this mode the PFET is held off, and the VCC ...

Page 13

For t in Equation 11 use the minimum on-time at the SW ON node. To determine this time period add the “Minimum on- time in current limit” specified in the Electrical Characteristics (t -4) to the difference of the turn-off ...

Page 14

SOFT-START The internal soft-start feature of the LM25085 allows the reg- ulator to gradually reach a steady state operating point at power up, thereby reducing startup stresses and current surges. Upon turn-on, when Vcc reaches its under-voltage lockout threshold, the ...

Page 15

Using equation 7 with the minimum value for the ADJ pin current (32 µA), the required R resistor calculates to: ADJ A standard value 2.1 kΩ resistor is selected. The nominal current ...

Page 16

C : The capacitor at the VCC pin (from VIN to VCC) pro- VCC vides not only noise filtering and stability for the VCC regula- tor, but also provides the surge current for the PFET gate drive. The typical recommended ...

Page 17

FIGURE 5. Efficiency vs. Load Current and V FIGURE 6. Frequency vs 30079305 (Circuit of Figure 4) IN 30079344 (Circuit of Figure 4) www.national.com ...

Page 18

FIGURE 8. LM25085 Power Dissipation (Circuit of Figure 4) www.national.com FIGURE 7. Current Limit vs. V (Circuit of Figure 30079345 30079346 ...

Page 19

Alternate Output Ripple Configurations The minimum ripple configuration, using C1, C2 and R3, used in the example circuit figure 4, results in a low ripple amplitude at V determined mainly by the characteristics of the output OUT capacitor and the ...

Page 20

FIGURE 10. Lowest Cost Ripple Generating Configuration PC Board Layout In most applications, the heat sink pad or tab con- nected to the switch node, i.e. the junction of Q1, L1 and D1. While it is common ...

Page 21

Physical Dimensions inches (millimeters) unless otherwise noted 8-Lead MSOP-EP Package NS Package Number MUY08A 8-Lead MSOP Package NS Package Number MUA08A 21 www.national.com ...

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LLP Package NS Package Number SDA08A 22 ...

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Notes 23 www.national.com ...

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... National Semiconductor and the National Semiconductor logo are registered trademarks of National Semiconductor Corporation. All other brand or product names may be trademarks or registered trademarks of their respective holders. ...

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