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AIC1653

Micropower Inverting DC/DC Converter in SOT-23-5

Analog Intergrations 

AIC1653

APPLICATION INFORMATIONS

Principle of Operation

AIC1653 uses a constant off-time control scheme,

which is represented in Fig. 8, to provide high

efficiency over a range of output current. Q1 and

Q2 along with R3 and R4 form a bandgap

reference used to regulate the output voltage.

When the voltage at NFB pin is slightly below

-1.23V, comparator A1 disables most of the

internal circuitry. Output current is then provided by

output capacitor, which slowly discharges until the

voltage at the NFB pin goes above the hysteresis

point of A1. A1 then enables the internal circuitry to

turn power switch NMOS on, and the current in

inductor begins ramping up. Once the switch

current reaches 100mA, comparator A2 resets

one-shot, which turns NMOS off for 400ns. In the

meantime, the inductor continues to deliver current

to the output. When NMOS turns back on, the

inductor current ramps up. And A2 resets one-shot

again when switch current gets to 100mA. This

switching action continues until the output voltage

is charged up with NFB pin reaching -1.23V. Then

A1 turns the internal circuitry off and the cycle

repeats. The AIC1653 contains additional circuitry

to provide current-limit protection for start-up as

well as short-circuit protection. When FB pin

voltage is higher than –0.6V, switch off-time is

increased to 800nS. This reduces the average

inductor current and helps minimize the power

dissipation in AIC1653 power switch, and in the

external inductor and diode.

Component Selection

Inductor Selection – Inverting Regulator

The following formula calculates the appropriate

inductor value for an inverting regulator. This value

provides a good tradeoff in inductor size and

system performance. In any applications, the

closest value to the one from the formula needs to

be applied to the inductors (both inductors should

have the same value). A use of an inductor value

up to 22µH can induce a slight increase of output

current, but any value beyond that will result in high

output ripple voltage with no further output current

increase. The size of inductor can be reduced by

using a value under 22µH. The formula is shown

as below:

L

=



2

VOUT +

ILIM

VD



 ×

t OFF

(1)

where VD=0.4V (Schottky diode forward voltage),

ILIM=100mA, and tOFF=400nS.

Be aware that, based on formula (1), high output

voltage can raise inductance, which may cause an

increase of inductor size.

For a converter (typical application circuit)

converting from 3.6V to –6V, a 51.2µH inductor is

calculated from the above equation. However, a

22µH inductor is recommended instead to prevent

the loss of output current.

7

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