lm5085sdx National Semiconductor Corporation, lm5085sdx Datasheet

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lm5085sdx

Manufacturer Part Number
lm5085sdx
Description
75v Constant On-time Pfet Buck Switching Controller
Manufacturer
National Semiconductor Corporation
Datasheet

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© 2009 National Semiconductor Corporation
75V Constant On-Time PFET Buck Switching Controller
General Description
The LM5085 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 75V 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
300577
LM5085
Features
Package
Wide 4.5V to 75V 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
30057701

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lm5085sdx 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 LM5085 Features ■ Wide 4.5V to 75V input voltage range ■ ...

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... Package Type LM5085MYE MSOP-8EP LM5085MY MSOP-8EP LM5085MYX MSOP-8EP LM5085MME MSOP-8 LM5085MM MSOP-8 LM5085MMX MSOP-8 LM5085SDE LLP-8 LM5085SD LLP-8 LM5085SDX LLP-8 www.national.com 30057702 30057703 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 ...

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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(CL1) t OFF(CL2) Off-time (current limit) (Note 7) 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° 48V 30057705 IN 30057707 CC 30057709 6 Input Operating ...

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Off-Time vs. V and 30057712 ADJ Pin Current vs 30057717 Shutdown Current vs. Temperature 30057753 Voltage at the RT Pin Input Operating Current vs. Temperature VCC vs. Temperature 7 30057713 30057718 30057754 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 30057755 Current Limit Comparator Offset vs. Temperature 30057757 PGATE Driver Output Resistance vs. Temperature 30057714 8 30057756 30057758 30057759 ...

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Feedback Reference Voltage vs. Temperature RT Pin Shutdown Threshold vs. Temperature Soft-Start Time vs. Temperature 30057715 30057760 9 30057716 www.national.com ...

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Block Diagram Sense resistor method shown for current limit detection. Minimum output ripple configuration shown. www.national.com 10 30057719 ...

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Functional Description OVERVIEW The LM5085 is a PFET buck (step-down) DC-DC controller using the constant on-time (COT) control principle. The input operating voltage range of the LM5085 is 4.5V to 75V. The use of a PFET in a buck regulator ...

Page 12

For V = 48V and t = 100 ns, the simplified equation is SHUTDOWN The LM5085 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

PFET with a smaller Total Gate Charge must be used. SOFT-START The internal soft-start feature of the LM5085 allows the reg- ulator to gradually reach a steady state operating point at power up, thereby reducing ...

Page 15

ADJ pin current (32 µA), the required R resistor calculates to: ADJ A standard value 2.1 kΩ resistor is selected. The nominal current limit threshold calculates to: Using the tolerances for the ADJ pin ...

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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 (Circuit of Figure 4) FIGURE 6. Frequency vs (Circuit of Figure 4) 30057705 FIGURE 7. Current Limit vs 30057744 FIGURE 8. LM5085 Power Dissipation 17 30057745 IN (Circuit ...

Page 18

Alternate Output Ripple Configurations The minimum ripple configuration, employing C1, C2, and R3 in figure 4, results in a low ripple amplitude at V mined mainly by the characteristics of the output capacitor and the ripple current in L1. This ...

Page 19

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 to extend the PC board pad from ...

Page 20

Physical Dimensions www.national.com inches (millimeters) unless otherwise noted 8-Lead MSOP-EP Package NS Package Number MUY08A 8-Lead MSOP Package NS Package Number MUA08A 20 ...

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LLP Package NS Package Number SDA08A 21 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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