Rainbow-electronics MAX1800 Manual de usuario descargar pdf (Pagina 17)

MAX1800

Digital Camera Step-Up

Power Supply

______________________________________________________________________________________ 17

Auxiliary Controllers

The auxiliary controllers use voltage mode to regulate

their output voltages, so the control-loop compensation

is slightly more complex than that for the main convert-

er. Use one of the two following procedures:

Discontinuous Inductor Current

For discontinuous inductor current, the PWM converter

has a single pole. The pole frequency and DC gain of

the PWM controller are dependent on the operating

duty cycle, which is:

where R

is the equivalent load resistance, or:

The frequency of the single pole due to the PWM con-

verter is:

The DC gain of the PWM controller is:

Note that the pole frequency decreases and the DC

gain increases proportionally as the load resistance

LOAD

) is increased. Since the crossover frequency is

the product of the pole frequency and the DC gain, it

remains independent of the load.

The gain through the voltage-divider is:

The DC gain of the error amplifier is A

VEA

= 2000V/V.

Thus, the DC loop gain is:

VDC

= A

VDV

VEA

The compensation resistor-capacitor pair at COMP

cause a pole and zero at frequencies (in Hz):

The equivalent series resistance (ESR) of the output fil-

ter capacitor causes a zero in the loop response at the

frequency (in Hz):

The DC gain, and the poles and zeros are shown in the

Bode plot of Figure 6.

To achieve a stable circuit with the Bode plot of Figure

6, perform the following procedure:

1) Choose the compensation resistor R

that is equiv-

alent to the inverse of the transconductance of the

error amplifier, 1/ R

= G

= 100µS, or R

= 10kΩ.

This sets the high-frequency voltage gain of the

error amplifier to 0dB.

2) Determine the maximum output pole frequency:

where:

LOAD(MIN)

OUT

OUT(MAX)

2(V - V

V-V C

O(MAX)

OUT IN)

OUT IN LOAD(MIN) OUT

2π()R

C ESR

OUT

2 π

4000

410

VDV

REF

OUT

VV V

VVRC

OUT OUT IN

OUT IN LOAD OUT

−

()

VVRC

OUT IN

OUT IN LOAD OUT

−

()

−

()

2π

VV V

IN LOAD

OUT OUT IN

−

()

OSC













FREQUENCY

VDC

GAIN

(dB)

PHASE

180°

90°

0°

-20

PHASE

GAIN

= P

Figure 6. MAX1800 Discontinuous-Current, Voltage-Mode,

Step-Up Converter Bode Plot

1 2 ... 12 13 14 15 16 17 18 19 20 21 22 23 24

Digital Camera Step-Up 1
Power Supply 1

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