Rainbow-electronics MAX1800 Manual do Utilizador descarregar pdf (Página 21)

Choose the MOSFET for low dropout. The maximum

acceptable on-resistance of the MOSFET is determined

by the maximum load current to achieve the required

dropout voltage (minimum input voltage minus output

voltage):

Determine the minimum output capacitance as follows.

The output capacitor and load resistance set the domi-

nant pole (f

The second pole (f

) occurs due to the AO output

resistance and the gate capacitance of the external

MOSFET:

where R

is the output resistance of the gain block at

AO, and C

(GATE-Q1)

is the total gate capacitance of the

MOSFET, Q1. The control loop DC gain is:

where A

V-GB

is the voltage gain from AI to AO (typically

100), and G

(FS-Q1)

is the forward transconductance

gain of Q1. Choose the output capacitance so that the

second pole occurs at or above the loop-gain crossover

frequency:

Since V

REF

is 1.25V, A

(V-GB)

is typically 100, and R

is typically 800Ω, then:

Using the Linear Regulator to Make a

Step-Up/Step-Down Circuit

Some applications have a battery voltage that can be

either greater than or less than the desired output volt-

age. In this case, a step-up or step-down converter will

not be able to generate the required output voltage

under all conditions. To avoid this limitation, use a step-

OUT

FS Q GATE Q

OUT

≥













−

()

−

()

,12 500

AGRC

OUT

REF

OUT

V GB FS Q AO GATE Q

≥













−

()

−

()

−

()

AGR

VLOOP

REF

OUT

V GB FS Q LOAD













−

()

−

()

AO GATE Q

()

−

LOAD OUT

DS ON−

≤

DROPOUT

LOAD(MAX)

OUT

REF

12 1

=−













MAX1800

Digital Camera Step-Up

Power Supply

______________________________________________________________________________________ 21

Figure 11. Linear Regulator

OUT

REF

LOAD

RESISTOR

)

INPUT VOLTAGE

(FROM MAIN

OUTPUT)

PART OF MAX 1800

ONA

OFF

OUT+

OUT-

MAIN

COMP

DCON

PART OF

MAX1800

INPUT

1-CELL

LITHIUM-ION

Figure 10. Auxiliary Charge-Pump Configuration

1 2 ... 16 17 18 19 20 21 22 23 24

Digital Camera Step-Up 1
Power Supply 1

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