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

Impala Linear Corporation

6

(408) 574-3939

www.impalalinear.com

Jan 1999

ILC6360 1.1

At the start of one of the buck mode cycles, current starts to

flow in the inductor. If the input voltage is greater than the

output voltage, a normal PWM cycle will not stop the current

build up in the inductor. Current continues to build up in the

inductor and flows into the capacitor causing the capacitor

exceeded and the regulator will stop the normal PWM cycle

be dissipated in the pass transistor. The current in the

inductor will drop to zero and the “buck” cycle will start all

over again.

This mode of operation has similar efficiency as a linear

regulator. The power dissipation and efficiency of the regu-

lator is similar to a low dropout linear regulator :

Efficiency,

η = V

The advantage of this circuit is that there is no mode

switching required by the user, it is automatic in the opera-

cycles will occur to maintain the output voltage in regula-

tion. For the ILC6360 the buck mode of operation is

limited to an input voltage 800mV higher than the out-

put voltage or less.

The output ripple will increase because of the larger cur-

rent ripple associated with this mode of operation. The

output current. A large output capacitor with low ESR will

decrease the output ripple voltage. A smaller inductor will

reduce the time needed to charge up the inductor to maxi-

mum current. As a result, the output voltage ripple will

decrease. A first order approximate equation for output rip-

ple is as shown below :

PWM Mode Operation

The ILC6360 uses a PWM or Pulse Width Modulation tech-

nique. The switches are constantly driven at typically

300kHz. The control circuitry varies the power being deliv-

ered to the load by varying the on-time, or duty cycle, of the

switch SW1 (see fig. 2). Since more on-time translates to

higher current build-up in the inductor, the maximum duty

cycle of the switch determines the maximum load current

that the device can support.

There are two key advantages of the PWM type controllers.

First, because the controller automatically varies the duty

cycle of the switch’s on-time in response to changing load

conditions, the PWM controller will always have an opti-

mized waveform for a steady-state load. This translates to

very good efficiency at high currents and minimal ripple on

the output. Ripple is caused by the constant charging and

discharging of the output capacitor.

The other key advantage of the PWM type controllers is that

the radiated noise due to the switching transients will

always occur at the (fixed) switching frequency harmonics.

Many applications do not care much about switching noise,

but certain types of applications, especially communication

equipment, need to minimize the high frequency interfer-

ence within their system as much as possible. Using a

boost converter will cause higher frequency noise to be

generated; using a PWM converter makes that noise high-

ly predictable; thus easier to filter out.

There are downsides of PWM approaches, especially at

very low currents. Because the PWM technique relies on

constant switching and varying duty cycle to match the load

conditions, there is some point where the load current gets

too small to be handled efficiently. An actual switch con-

sumes some finite amount of current to switch on and off; at

very low currents this can be of the same magnitude as the

load current itself, driving switching efficiencies down to

50% and below.

PFM Mode Operation

The ILC6360 overcomes this limitation by automatically

switching over to a PFM, or Pulse Frequency Modulation,

technique at low currents. This technique conserves power

loss by only switching the output if the current drain

requires it. As shown in figure 3, the waveform actually

skips pulses depending on the power needed by the out-

put. This technique is also called “pulse skipping” because

of this characteristic.

V

SET

V

OUT

Switch Waveform

Fig 3: PFM Waveform