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ADP3041

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ADP3041

THEORY OF OPERATION

Switching Regulator

The ADP3041 is a boost converter driver that 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 an error amplifier to com-

pare some fraction of the load voltage to the reference and to

amplify any difference between them. The amplified error

signal is compared to a dynamic signal produced by an inter-

nal ramp generator incorporating switch current feedback. The

comparator output timing sets the duty ratio of a switch driv-

ing the inductor to maintain the desired output voltage.

Referring to Figure 1, a typical application powers both the IC

and the inductor from the same input voltage. The on-chip

MOSFET is driven on, pulling the SW pin close to PGND. The

resulting voltage across the inductor causes its current to increase

approximately 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 now begins 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 exceeds the load current, and the excess is

what charges the load capacitor. The inductor current falls

during this time, though not necessarily to zero.

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

switch is again turned on so that the inductor current is ramped

up again. The charge on the load capacitor provides load current

during that interval. The remainder 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 is 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 compara-

tor turns off the switch by resetting 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 cycles around some average current

level. The increasing ramp of current depends on input voltage

and t1, the switch-on time, while the decreasing ramp depends

on the difference between the input and output voltage and t2,

the remainder of the cycle. For the peaks of these two ramps

to be equal and opposite to maintain steady state, one 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 one 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 needs 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 the applied VIN, and the actual output voltage is less than

our approximation by the amount of the diode forward voltage

drop. However, the feedback control within the ADP3041 adjusts

the duty ratio to maintain the output voltage. Changes in

load current and input voltage are also accommodated by the

feedback control.

Changes in load current alone require a change in duty ratio in

order to change the average inductor current. Once the inductor

current adapts to the new load current, the duty ratio should

return to nearly its original value, as one can 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, one can 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 additional flip-flop reset.

A comparator compares the current sense amplifier output to a

factory set limit that 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 ensure 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 representing the magni-

tude 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.

Soft Start

The soft start pin can load the COMP pin, forcing a low duty

cycle when its voltage is low. A capacitor on SS initially holds

the pin low; however, a small internal current charges the

capacitor, causing SS to rise after SD goes high. As it rises,

–10–

REV. D

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