-

+

Q

R

+

S

Negative Bias

Regulator

Thermal

Shutdown

ON Time

One-Shot

Soft-Start

Gate Driver

Control Logic

VCC

UVLO

Gate

Driver

REGULATION

COMPARATOR

OVER-VOLTAGE

COMPARATOR

CURRENT

LIMIT

COMPARATOR

OFF Time

One-Shot

FB

LM5085

PGATE

ISEN

GND

VCC

ADJ

4.5V to 75V

Input

GND

SHUTDOWN

Q1

D1

L1

1.25V

VCC

R3 C1

C2

1.6V

+

-

+

-

7.7V

40 PA

+

-

0.73V

+

-

VIN

RT

Duty Cycle =

=

LM5085, LM5085-Q1

SNVS565I – NOVEMBER 2008 – REVISED AUGUST 2015

www.ti.com

7 Detailed Description

7.1 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 greatly

simplifies the gate drive requirements and allows for 100% duty cycle operation to extend the regulation range

when operating at low input voltage. However, PFET transistors typically have higher on-resistance and gate

charge when compared to similarly rated NFET transistors. Consideration of available PFETs, input voltage

range, gate drive capability of the LM5085, and thermal resistances indicate an upper limit of 10A for the load

current for LM5085 applications. Constant on-time control is implemented using an on-time one-shot that is

triggered by the feedback signal. During the off-time, when the PFET (Q1) is off, the load current is supplied by

the inductor and the output capacitor. As the output voltage falls, the voltage at the feedback comparator input

(FB) falls below the regulation threshold. When this occurs Q1 is turned on for the one-shot period which is

increases the voltage at FB above the feedback comparator threshold. For a buck regulator the basic relationship

between the on-time, off-time, input voltage and output voltage is:

(1)

where Fs is the switching frequency. Equation 1 is valid only in continuous conduction mode (inductor current

The COT control scheme, with the feedback signal applied to a comparator rather than an error amplifier,

requires no loop compensation, resulting in very fast load transient response.

The LM5085 is available in both an 8-pin HVSSOP package and an 8-pin WSON package with an exposed pad

to aid in heat dissipation. An 8-pin VSSOP package without an exposed pad is also available.

7.2 Functional Block Diagram

Sense resistor method shown for current limit detection.

Minimum output ripple configuration shown.

10

Submit Documentation Feedback

Copyright © 2008–2015, Texas Instruments Incorporated

Product Folder Links: LM5085 LM5085-Q1