Step-Up/Step-Down DC-DC Converter for 1-Cell Lithium-Ion Batteries

Impala Linear Corporation

5

(408) 574-3939

www.impalalinear.com

Jan 1999

ILC6360 1.1

The ILC6360 performs both buck and boost DC-DC con-

version by controlling the switch element as shown in the

simplified circuit in figure 1 below.

When the switch is closed, current is built up through the

inductor. When the switch opens, this current is forced

through the diode to the output capacitor and load. As this

on and off switching continues, the output capacitor voltage

builds up due to the charge it is storing from the inductor

current. The output voltage is therefore boosted relative to

the input.

In general, the switching characteristic is determined by the

output voltage desired and the current required by the load.

Specifically the energy transfer is determined by the power

stored in the coil during each switching cycle.

Synchronous Rectification

The ILC6360 also uses a technique called “synchronous

rectification” which removes the need for the external diode

used in other circuits. The diode is replaced with a second

switch (SW2) or in the case of the ILC6360, a FET as

shown in figure 2 below.

The two switches now open and close in opposition to each

other, directing the flow of current to either charge the

inductor or to feed the load. The ILC6360 monitors the volt-

age on the output capacitor to determine how much and

how often to drive the switches.

Modes of Operation

There are four modes of operation for the ILC6360

buck/boost DC-DC converter. These four modes are inter-

nally selected by the regulator depending on external con-

ditions such as line voltage, output voltage, load current,

inductor size, output capacitor size and resistive losses.

The first mode is the discontinuous mode. If the load is light

and the inductor value is small enough, the inductor will

transfer all of its energy to the output capacitor before a

cycle is completed. The input current waveform instead of

being continuous with a triangle ripple, will be a series of

discrete triangle shaped pulses as the inductor charges

from the input and discharges into the capacitor. The ripple

on the output capacitor becomes larger than expected com-

pared to continuous mode calculation because of the cur-

rent spikes from the input.

Boost (Step-up) Operation

The second mode is the conventional boost (step-up) mode

of operation. The input current is a smooth waveform with a

triangular ripple current. The output waveform exhibits rip-

ple caused by the charging and discharging of the output

capacitor and the current flowing through the capacitor’s

equivalent series resistance (ESR).

The third mode is the PFM mode. If the output voltage

exceeds an upper limit, for whatever reason, the regulator

enters the PFM mode. The regulator shuts down for one or

more cycles until the output voltage drops below a pre-set

threshold and one cycle is initiated. The inductor current falls

to zero during the off time. The basic cycle is the 3.3mS

PWM cycle but one or more cycles are dropped from the

pulse train (also called pulse skipping). This may be in

response to a light load condition or from a fast transient

load condition where the output capacitor charges too high

during load turn-off. In light load conditions, PFM mode

offers high efficiency due to significantly lower quiescent cur-

rent for the regulator. The output voltage will be a few tens

of millivolts higher in the PFM mode than in the PWM mode.

The fourth mode of operation is the buck (step-down) mode

and is described below.

Buck (Step-down) Operation

The “buck” mode is not a true switching regulator mode but

allows the regulator to operate when the input voltage

exceeds the output voltage. Once the input voltage exceeds

the output voltage, the regulator is not capable of limiting

the current in a non-dissipative fashion.

Fig. 1: Basic Circuit

V

OUT

POK

LBO

LB/SD

SYNC

GND

L

X

V

IN

ILC6360

PWM/PFM

CONTROLLER

SHUTDOWN

CONTROL

V

REF

DELAY

+

+

-

-

SW2

SW1

Fig. 2: Simplified ILC6360 block diagram

APPLICATIONS INFORMATION