SC4501 View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC4501

2Amp, 2MHz Step-Up Switching Regulator with Soft-Start

Semtech Corporation 

SC4501

POWER MANAGEMENT

Application Information

The turn off voltage is:

IN

6/8

I HYS

4.6µA

R3

SWITCH CLOSED

WHEN Y = “1”

SHDN

3

+

Y

R4

1.1V

-

COMPARATOR

SC4501

VL = VH − VHYS = 2.75 − 0.69 = 2.06V > 1.4V .

Frequency Compensation

Figure 8 shows the equivalent circuit of a boost converter

using the SC4501. The output filter capacitor and the load

form an output pole at frequency:

ωp2

=

−

2IOUT

VOUTC2

=

2

− ROUTC2

(13)

where

C2

is

the

output

capacitor

and

ROUT

=

VOUT

IOUT

is

the

equivalent load resistance.

Figure 7. Programmable Hysteretic UVLO Circuit

The zero formed by C2 and its equivalent series resistance

(ESR) is neglected due to low ESR of the ceramic output

capacitor.

with VL > 1.4V .

Example: Increase the turn on voltage of a VIN = 3.3V boost

converter from 1.4V to 2.75V.

Using VH = 2.75V and R4 = 100KΩ in (12),

R3 = 150KΩ .

The resulting UVLO hysteresis is:

VHYS = IHYSR3 = 4.6µA •150KΩ = 0.69V .

There is also a right half plane (RHP) zero at angular

frequency:

ωZ2

=

ROUT

(1 − D)2

L

(14)

ωz2 decreases with increasing duty cycle D and increasing

IOUT. Using the 5V to 12V boost regulator (1.35MHz) in

Figure 1(a) as an example,

ROUT

≥

5V

0.74A

=

6.8Ω

VIN

POWER

STAGE

COMP

-

FB

Gm

R3

C6

RO

+

1.242V

C4

VOLTAGE

REFERE N CE

C5 R1

I OUT

VOUT

ESR

C2

ROUT

R2

Figure 8. Simplified Block Diagram of a Boost Converter

© 2005 Semtech Corp.

13

www.semtech.com

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