Micrel, Inc.

MICRF008

August 2008

8

M9999-080108

VDDBB

VSSBB

Compa-

rator

10µA

10µA

DO

Figure 4. DO Pin

REFOSC Pin

The REFOSC input circuit is shown in Figure 5. Input

impedance is quite high (200kΩ). This is a Colpitts

oscillator, with internal 30pF capacitors. This input is

intended to work with standard ceramic resonators,

connected from this pin to VSSBB. The resonators

should not contain integral capacitors, since these

capacitors are contained inside the IC. Externally applied

signals should be AC-coupled, amplitude limited to

this pin is 1.4V.

250

200k

Active

Bias

REFOSC

30pF

30pF

15µA

VDDBB

VSSBB

VSSBB

Figure 5. REFOSC Pin

Control Input (SEL0)

Control input circuitry is shown in Figure 6. The standard

input is a logic inverter constructed with minimum

geometry MOSFETs (Q2, Q3). P-Channel MOSFET Q1

is a large channel length device which functions

essentially as a “weak” pullup to VDDBB. Typical pullup

current is 5μA, leading to an impedance to the VDDBB

supply of typically 1MΩ.

to Interna

Circuits

VDDBB

VSSBB

Logic

Input

Q2

Q3

Q1

VSSBB

Figure 6. SEL0