SS6845G datasheet(8/13 Pages) SSC

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SS6845G

7/21/2005 Rev.3.01

APPLICATION INFORMATION

Introduction

Short Circuit/Thermal Protection

The SS6

845G is a micropower charge pump DC/DC

converter that produces a regulated 5V output

with an input voltage range from 2.7V to 5.0V. It

utilizes the charge pump topology to boost VIN to

a regulated output voltage. Regulation is obtained

by sensing the output voltage through an internal

resistor divider. A switched doubling circuit

enables the charge pump when the feedback

voltage is lower than the trip point of the internal

comparator, and vice versa. When the charge

pump is enabled, a two-phase non-overlapping

clock activates the charge pump switches. To

maximize battery life for a battery-use application,

quiescent current is limited to no more than 13

µA.

The SS6

845G includes built-in short circuit current

limiting as well as over-temperature protection.

During a short circuit condition, the output current

is automatically constrained to approximately

170mA. This short circuit current will cause a rise

in the internal IC junction temperature. When the

die temperature exceeds 150°C, the thermal

protection will shut down the charge pump switching

operation

and the die temperature will then

reduce. Once the die temperature drops below

135

°C, the charge pump switching circuit will

restart.

If the fault has not been eliminated, the

this protection mechanism will repeat again and

again, allowing the SS6845G to work continuously

in a

short circuit condition without damaging the

device.

Operation

This kind of converter uses capacitors to store

and transfer energy. Since the capacitors can’t

change their voltage level abruptly, the voltage

ratio of VOUT over VIN is limited to some range.

Capacitive voltage conversion is obtained by

switching a capacitor periodically. It first charges

the capacitor by connecting it across a voltage

source and then connects it to the output.

Referring to Fig. 19, during the on state of internal

clock, Q1 and Q4 are closed, which charges C1 to

VIN level. During the off state, Q3 and Q2 are

closed. The output voltage is VIN plus VC1, that is,

2VIN.

Shutdown

In shutdown mode, the output is disconnected

from the input. The input current is extremely low

since most of the circuitry is turned off. Due to

high impedance, the shutdown pin cannot float.

Efficiency

The diagrams,

Fig. 20 and Fig. 21 show the operation

of the charge pump in the on and off states.

R DS-ON is the resistance of the switching element

during

conduction. ESR is the equivalent series

resistance of the flying capacitor C1. ION-AVE and

IOFF-AVE are the average current during the on-state

and off-state, respectively. D is the duty-cycle, which

means the ratio of the on-state time to the total cycle

time. Let's look at capacitor C1 - assuming that

capacitor C 1, has reached its steady state, then the

amount of charge flowing into C 1 during the on-state

is equal to that flowing out of C1 during the off-state.

VOUT

VIN

COUT

CIN

Fig. 19 The circuit of charge pump

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