CS5111 datasheet(5/10 Pages) CHERRY

5 / 10 page

The 5V linear regulator consists of an error amplifier,

bandgap voltage reference, and a composite pass transistor.

The 5V linear regulator circuitry is shown in Figure 2.

When an unregulated voltage greater than 6.6V is applied

to the VREG input, a 5V regulated DC voltage will be pre-

sent at VLIN. For proper operation of the 5V linear regula-

tor, the IBIAS lead must have a 64.9kΩ pull down resistor to

ground. A 100µF or larger capacitor with an ESR <8Ω

must be connected between VLIN and ground. To operate

the 5V linear regulator as an independent regulator (i.e.

separate from the switching supply), the input voltage

must be tied to the VREG lead.

As the voltage at the VREG input is increased, Q1 is turned

on. Q1 provides base drive for Q2 which in turn provides

base current for Q3. As Q3 is turned on, the output voltage,

VLIN, begins to rise as Q3’s output current charges the out-

put capacitor, COUT. Once VLIN rises to a certain level, the

error amplifier becomes biased and provides the appropri-

ate amount of base current to Q1. The error amplifier mon-

itors the scaled output voltage via an internal voltage

divider, R2 through R5, and compares it to the bandgap

voltage reference. The error amplifier output or error sig-

nal is an output current equal to the error amplifier’s input

differential voltage times the transconductance of the

amplifier. Therefore, the error amplifier varies the base

current to Q1, which provides bias to Q2 and Q3, based on

the difference between the reference voltage and the

scaled VLIN output voltage.

The watchdog timer circuitry monitors an input signal

(WDI) from the microprocessor. It responds to the falling

edge of this watchdog signal which it expects to see within

an externally programmable time (see Figure 3).

The watchdog time is given by:

tWDI = 1.353 × CDelay RBIAS

Using CDelay = 0.1µF and RBIAS = 64.9kΩ gives a time rang-

ing from 6.25ms to 11ms assuming ideal components. Based

on this, the software must be written so that the watchdog

arrives at least every 6.25ms. In practice, the tolerance of

CDelay and RBIAS must be taken into account when calculat-

ing the minimum watchdog time (tWDI).

Figure 3. Timing diagram for normal regulator operation.

Figure 4. Timing diagram when WDI fails to appear within the preset

time interval, tWDI.

VLIN

WDI

RESET

VREG

tPOR

A: Watchdog waiting for

low-going transition on

WDI

50% Duty

Cycle

B: RESET stays low for

tWDI time.

VLIN

WDI

RESET

VREG

tPOR

Normal Operation

Control Functions

5V Linear Regulator

Circuit Description

Over

Temperature

Linear

Error

Amplifier

1.25V

VREG

VLIN

Cdelay

Over Voltage

RESET &

Watchdog Timer

Current

Limit

WDI

RESET

Bandgap

Reference

IBIAS

RBIAS

64.9kΩ

COUT = 100µF

ESR < 8Ω

Figure 2. Block diagram of 5V linear regulator portion of the CS5111.

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