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ADP3031

2 MHz PWM Boost Switching Regulator

Analog Devices 

ADP3031

PRELIMINARY TECHNICAL DATA

THEORY OF OPERATION

The ADP3031 is a boost converter driver which stores

energy from an input voltage in an inductor, and delivers

that energy, augmented by the input, to a load at a higher

output voltage. It includes a voltage reference and error

amplifier to compare some fraction of the load voltage to the

reference, and amplify any difference between them. The

amplified error signal is compared to a dynamic signal

produced by an internal ramp generator incorporating

switch current feedback. The comparator output timing sets

the duty ratio of a switch driving the inductor to maintain

the desired output voltage.

Referring to Figure 1, a typical application will power both

the IC and the inductor from the same input voltage. The

on chip MOSFET will be driven on, pulling pin SW close to

PGND. The resulting voltage across the inductor will cause

its current to increase aproximately linearly, with respect to

time.

When the MOSFET switch is turned off the inductor

current cannot drop to zero, and so this current drives the

SW node capacitance rapidly positive until the diode

becomes forward biased. The inductor current will now

begin to charge the load capacitor, causing a slight increase

in output voltage. Generally, the load capacitor is made

large enough that this increase is very small during the time

the switch is off. During this time inductor current is also

delivered to the load. In steady state operation, the inductor

current will exceed the load current, and the excess will be

what charges the load capacitor. The inductor current will

fall during this time, though not necessarily to zero.

During the next cycle, initiated by the on-chip oscillator, the

switch will again be turned on so that the inductor current

will be ramped back up. The charge on the load capacitor

will provide load current, during that interval. The remain-

der of the chip is arranged to control the duty ratio of the

switch, to maintain a chosen output voltage despite changes

in input voltage or load current.

The output voltage is scaled down by a resistor voltage

divider and presented to the gm amplifier. This amplifier

operates on the difference between an on-chip reference and

the voltage at the FB pin so as to bring them to balance.

This will be when the output voltage equals the reference

voltage, multiplied by the resistor voltage divider ratio.

The gm amplifier drives an internal comparator, which has at

its other input a positive going ramp produced by the

Oscillator and modified by the current sense amplifier. The

MOSFET switch is turned on as the modified ramp voltage

rises. When this voltage exceeds the output of the gm

amplifier the comparator will turn off the switch, by reset-

ting the flip-flop, previously set by the oscillator. The output

of the flip-flop is buffered by a high current driver which

turned on the MOSFET switch at the beginning of the

Oscillator cycle.

In the steady state with constant load and input voltage, the

current in the inductor will cycle around some average

current level. The increasing ramp of current will depend on

input voltage and t1, the switch on-time, while the decreas-

ing ramp will depend on the difference between input and

output voltage and t2, the remainder of the cycle. In order

for the peaks of these two ramps to be equal and opposite to

maintain steady state we can say that t1*VIN will equal

t2*(VOUT-VIN), if we neglect the effect of resistance in the

inductor and switch, and the forward voltage drop of the

diode. From this equality we can derive t1/T=1-VIN/VOUT,

where T is the period of a cycle, t1+t2. This result gives us

the switch duty ratio, t1/T in terms of the input and output

voltages.

In practice the duty ratio will need to be slightly higher than

this calculation. Because of series resistance in the inductor

and the switch, the voltage across the actual inductance is

somewhat less than applied VIN, and the actual output

voltage is less than our aproximation by the amount of the

diode forward voltage drop. However, the feedback control

within the ADP3031 will adjust the duty ratio to maintain

the output voltage. Changes in load current and input

voltage are also accomodated by the feedback control.

Changes in load current alone require a change in duty

ratio, in order to change the average inductor current. But

once the inductor current adapts to the new load current,

the duty ratio should return to nearly its original value, as

we see from the duty cycle calculation which depends on

input and output voltages, but not on current. Increasing

the switch duty ratio initially reduces the output voltage,

until the average inductor current increases enough to offset

the reduction of the t2 interval. By limiting the duty ratio we

prevent this effect from regeneratively increasing the duty

ratio to 100%, which would cause the output to fall and the

switch current to rise without limit. The duty ratio is limited

to about 80% by the design of the Oscillator and an addi-

tional flip-flop reset.

A comparator compares the current sense amplifier output

to a factory set limit which resets the flip-flop, turning off

the switch. This prevents runaway or overload conditions

from damaging the switch and reflecting fault overloads

back to the input. Of course, the load is directly connected

to the input by way of the diode and inductor, so protection

against short circuited loads must be done at the power

input.

The gm amplifier has high voltage gain, to insure the output

voltage accuracy and invariance with load and input voltage.

However, because it is a gm amplifier with a specified

current response to input signal voltages, its high frequency

response can be controlled by the compensation impedance.

This permits the high frequency gain of the gm amplifier to

be optimized for the best compromise between speed of

response and frequency stability.

The stable closed loop bandwidth of the system can be

extended by the current feedback shown. A signal represent-

ing the magnitude of the switch current is added to the

ramp. This dynamically reduces the duty ratio, as the

current in the inductor increases, until the gm amplifier

restores it, improving the closed loop frequency stability.

The ADP3031 is intended to operate over a range of

frequencies, set by the RT pin. If the pin is open, the

oscillator runs at its lowest frequency: if the pin is

–4–

REV. PrB

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