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LM25007_11

更新时间： 2024-01-14 14:21:04

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

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LM25007_11 数据手册

National Semiconductor

Application Note 2129

Vijay Choudhary

Boot Capacitor Regulation

in LM25007 Constant-On-

Time (COT) Converter

March 14, 2011

see Figure 1. This voltage is usually referred to as prebias.

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_BST=V_CC-V_SW) is lower than the threshold voltage

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

and the converter fails to startup see Figure 2 . This condition

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.

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_BST) may have insufficient voltage dur-

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

30153501

FIGURE 1. LM25007 Application with Prebiased Output

30153502

FIGURE 2. Startup with Prebiased Output

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

ditions see Figure 3. At light load/no load, the inductor current

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

tively long off time. During this time the bootstrap capacitor

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

drive threshold (V_TH), which is normally around 5V. Under

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

(V_SW=V_OUT). As V_OUT, and therefore VSW, drops, the boot-

strap capacitor voltage rises and is given by V_BST= V_CC

V_OUT. When V_CC-V_OUT> V_TH, the hi-side FET turns on again

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_OUTfall to V_CC-V_THand rises back to the target level.

This loss of regulation is undesirable in many applications.

301535

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与LM25007_11相关器件

型号	品牌	获取价格	描述	数据表
LM25007_14	TI	获取价格	42V, 0.5A Step-Down Switching Regulator
LM25007MM	NSC	获取价格	42V, 0.5A Step-Down Switching Regulator
LM25007MM	TI	获取价格	LM25007 42V, 0.5A Step-Down Switching Regulator
LM25007MM/NOPB	TI	获取价格	9-42V Wide Vin, 0.5A, Constant On-Time Non-Synchronous Buck Regulator 8-VSSOP -40 to 125
LM25007MMX	NSC	获取价格	42V, 0.5A Step-Down Switching Regulator
LM25007MMX	TI	获取价格	LM25007 42V, 0.5A Step-Down Switching Regulator
LM25007MMX/NOPB	TI	获取价格	LM25007 42V, 0.5A Step-Down Switching Regulator
LM25007SD	TI	获取价格	LM25007 42V, 0.5A Step-Down Switching Regulator
LM25007SD	NSC	获取价格	42V, 0.5A Step-Down Switching Regulator
LM25007SD/NOPB	TI	获取价格	9-42V Wide Vin, 0.5A, Constant On-Time Non-Synchronous Buck Regulator 8-WSON -40 to 125

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