LM25007_11 NSC [National Semiconductor], LM25007_11 Datasheet

LM25007_11

Manufacturer Part Number

LM25007_11

Description

Boot Capacitor Regulation in LM25007 Constant-On-Time (COT) Converter

Manufacturer

NSC [National Semiconductor]

Datasheet

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Boot Capacitor Regulation

in LM25007 Constant-On-

Time (COT) Converter

The Issue

LM25000 series of constant-on-time (COT) integrated regu-

lators provide a simple, cost-effective way of implementing a

step down buck regulator with nearly fixed frequency. Non-

synchronous operation reduces switching frequency at very

light load resulting in higher efficiency than a comparable

fixed frequency converter. The non-synchronous operation,

however, causes two problems related to boot capacitor reg-

ulation under certain operating conditions.

Bootstrap capacitor (C

ing startup if a voltage is present at the output of the converter

A similar situation may occur under light load or no load con-

ditions see

is discontinuous causing the diode (D1) to turn off for a rela-

tively long off time. During this time the bootstrap capacitor

drive threshold (V

these conditions the top side FET fails to turn on and the out-

put voltage is no longer regulated. Since there is no current

in the inductor, this output voltage appears at the switch node

BST

) may discharge to a level below the top side FET gate

Figure

3. At light load/no load, the inductor current

), which is normally around 5V. Under

BST

) may have insufficient voltage dur-

FIGURE 1. LM25007 Application with Prebiased Output

FIGURE 2. Startup with Prebiased Output

301535

National Semiconductor

Application Note 2129

Vijay Choudhary

March 14, 2011

see

Since, at startup, there is no current in the inductor (L), the

prebias voltage at the output appears at the switch node. If

this prebias voltage is high such that the boot capacitor volt-

age (V

and the converter fails to startup see

persists until prebias is removed. A prebias is often caused

by some leakage path in downstream circuits, e.g., a logic

circuit or FPGA with a pin pulled high, or leftover charge from

a previous power down.

strap capacitor voltage rises and is given by V

and the switching resumes bringing the VOUT to the target

level. At that time, if the low load condition persists, the whole

cycle will repeat itself, causing a hiccup mode operation in

which V

This loss of regulation is undesirable in many applications.

OUT

Figure

) of high side gate, the high side switch fails to turn 'on'

. When V

BST

OUT

). As V

1. This voltage is usually referred to as prebias.

fall to V

-V

OUT

30153502

) is lower than the threshold voltage

, and therefore VSW, drops, the boot-

-V

> V

and rises back to the target level.

, the hi-side FET turns on again

30153501

Figure 2

. This condition

www.national.com

BST

= V

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LM25007_11 Summary of contents

Page 1

Boot Capacitor Regulation in LM25007 Constant-On- Time (COT) Converter The Issue LM25000 series of constant-on-time (COT) integrated regu- lators provide a simple, cost-effective way of implementing a step down buck regulator with nearly fixed frequency. Non- synchronous operation reduces switching ...

Page 2

The Solution For prebiased startup problem, the source of prebias should be removed if possible. If the prebias source is some leakage path in the downstream logic circuit, the system designer should consider the option of tying unused logic pin, ...

Page 3

Notes 3 www.national.com ...

Page 4

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