S85S1P datasheet(13/38 Pages) SII

13 / 38 page

POWER MONITORING OUTPUT, 5.5 V INPUT, 200 mA SYNCHRONOUS STEP-DOWN SWITCHING REGULATOR WITH 260 nA QUIESCENT CURRENT

Rev.1.3_00

S-85S1P Series

2. Power monitor block

The power monitoring output is a function that divides the input voltage (VIN) of the DC-DC converter into VIN/2 or

VIN/3 and outputs the voltage. For example, the microcontroller can monitor battery voltage by inputting the output

voltage (VPMOUT) to the A/D converter in the microcontroller. Connecting the IC and the microcontroller makes it

possible that it is used as a remained battery capacity monitor for lithium-ion rechargeable batteries, coin batteries,

and other batteries.

VIN is divided into VIN/2 in S-85S1PCxx, and VIN/3 in S-85S1PDxx.

Low output impedance is realized since the buffer amp in the power monitor block constitutes a voltage follower.

Each the power monitor block and DC-DC converter block operate independently. When the PMEN pin is "L" and the

power monitor block is in standby status, the electrical charge in the output capacitor connected to the PMOUT pin is

discharged by an impedance of approximately 2.8 k

Ω.

2. 1 Basic operation

Figure 7 shows the block diagram of the power monitor block to describe basic operation.

Reference voltage (Vrefpm) is generated by dividing the input voltage (VIN) to VIN/2 or VIN/3 using the dividing

resistance (Rpm1 and Rpm2). Since the buffer amplifier constitutes a voltage follower, it can perform the feedback

control so that VPMOUT and Vrefpm are the same. Low output impedance is realized by the buffer amplifier, while

outputting VPMOUT according to VIN.

When "L" is input to the PMEN pin the current which flows to Rpm1 and Rpm2 and the current which flows to the

buffer amplifier can be stopped. The buffer amplifier output is pulled down to VSS by the built-in N-channel

transistor, and VPMOUT is set to the VSS level.

The difference, the output offset voltage (VPOF), is generated between VPMOUT and VPMOUT(S), and it is expressed

with VPMOUT = VPMOUT(S)

+ VPOF.

In addition, VPMOUT will change slightly according to the load current, and the value of change is expressed as the

output impedance (RPS).

PMEN

VSS

VIN

PMOUT

Power monitor

enable circuit

Rpm1

Rpm2

Vrefpm

Buffer amplifier

−

Figure 7

S85S1P 데이터시트(PDF) 13 Page - Seiko Instruments Inc