CS5111

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10

fp + 1 (pRLOADCOUT)

(14)

and a zero due to the output capacitor’s ESR at:

fz + 1 (2pESR(COUT))

(15)

Since the error amplifier reference voltage is 1.25 V, the

output voltage must be divided down or attenuated before

being applied to the input of the error amplifier. The

feedback resistor divider attenuation is:

1.25 V

VOUT

The error amplifier in the CS5111 is an operational

transconductance amplifier (OTA), with a gain given by:

GOTA + gmZOUT

(16)

where:

gm

+ D

IOUT

DVIN

(17)

For the CS5111, gm = 2700

µA/V typical.

One possible error amplifier compensation scheme is

shown in Figure 14. This gives the error amplifier a gain plot

as shown in Figure 15.

For the error amplifier gain shown in Figure 15, a low

frequency pole is generated by the error amplifier output

–20 dB/decade slope in Figure 15. The slope changes to zero

at point B due to the zero at:

fz + 1 (2pR4C1)

(18)

Figure 14. RC Network Used to Compensate

the Error Amplifier (OTA)

+

–

1.25 V

M

U

X

SELECT

Error

Amplifier

A pole at point C:

fp + 1 (pR4C2)

(19)

offsets the zero set by the ESR of the output capacitors.

An alternative scheme uses a single capacitor as shown in

Figure 16, to roll the gain off at a relatively low frequency.

Step 9

Finally the watchdog timer period and Power on Reset

time is determined by:

tDelay + 1.353

CDelayRBIAS

(20)

Figure 15. Bode Plot of Error Amplifier (OTA) Gain and Modulator Gain Added to the Feedback

Resistor Divider Attenuation

G

–G

0

A

B

Pole due to error amplifier

–20 dB/dec

error amplifier gain

+G

C

modulator gain +

feedback resistor divider attenuation