iC-DC

2-CHANNEL BUCK/BOOST DC/DC CONVERTER

Rev B1, Page 12/19

Intermittent flow / continuous flow

If the inductor is recharged in the next cycle before the

coil current has run free, there is no gap in the current.

This continuous flow occurs when the supply voltage is

low or the load current high.

If the charge and discharge processes are concluded

within one clock cycle and the coil current drops to zero

each time, intermittent flow prevails. This is the case

when the supply voltage is sufficiently high or the load

current sufficiently low.

When no more current flows through the coil, after fly-

back in intermittent operation both ends of the inductor

are switched to ground. This prevents the oscillations

in no-load operation that are typical of many converters

(RLC resonating circuit). This helps to achieve better

EMC behavior.

Startup behavior

During startup and with low supply voltages the coil’s

maximum cut-off current is reduced (soft start) until the

nominal voltage is reached at VH. Figure 3 shows the

startup behavior for VH, VCC1 (5 V), and VCC2 (3.3 V)

with the voltage monitor outputs V1OK and V2OK.

As three output voltages are generated with this switch-

ing converter topology using just one single inductor,

the sequence must allow that all three converter outputs

are respectively supplied with the required current. The

core of the converter consists of three independent con-

verters. Each converter has its own regulated cut-off

and restart current and its own voltage monitor. Dur-

ing the startup phase the VH converter ramps up until

a voltage of approximately 7 V is obtained. Here, the

cut-off current is initially limited to lower values so that

a soft start with a low startup current ripple is achieved.

When a high enough VH voltage is available, the VH1

and VH2 converters are also switched on and the volt-

ages rise to their nominal values. Once in a steady

state, the individual converters are cyclically supplied

by the inductor with precisely the amount of current that

is required at that moment. The maximum load current

can thus be drawn from each of the voltages VH, VH1,

VH2, VCC1, or VCC2.

Standby

The converters can be individually activated by pins

ENV1 and ENV2. If neither of the two inputs are trig-

gered, the device is in standby mode and the current

consumption is reduced to a minimum. As soon as one

of the two inputs becomes active, the VH voltage is also

available.

Dual 3.3V/5V BOOST/BUCK SMPS, 08.05.2008 08:51:13

V(VH)

V(VCC1)

V(VCC2)

500us

550us

600us

Time (linear) [Plot 0]

0

2V

4V

6V

8V

V(V1OK)

V(V2OK)

0

1V

2V

3V

4V

5V

I(LVBH)

0

200mA

400mA

600mA

Figure 3: Startup principle for voltages VH, VCC1, VCC2, V1OK, and V2OK