AAT1153

2A Step-Down Converter

PRODUCT DATASHEET

12

1153.2008.02.1.2

www .analogictech.com

For output voltages above 2.0V, when light-load effi-

ciency is important, the minimum recommended induc-

tor is 2.2μH.

Manufacturer’s specifications list both the inductor DC

current rating, which is a thermal limitation, and the

peak current rating, which is determined by the satura-

tion characteristics. The inductor should not show any

appreciable saturation under normal load conditions.

Some inductors may meet the peak and average current

ratings yet result in excessive losses due to a high DCR.

Always consider the losses associated with the DCR and

its effect on the total converter efficiency when selecting

an inductor. For optimum voltage-positioning load tran-

sients, choose an inductor with DC series resistance in

the 20mΩ to 100mΩ range. For higher efficiency at

heavy loads (above 200mA), or minimal load regulation

(but some transient overshoot), the resistance should be

kept below 100mΩ. The DC current rating of the induc-

tor should be at least equal to the maximum load cur-

rent plus half the ripple current to prevent core satura-

tion (2A + 600mA). Table 2 lists some typical surface

mount inductors that meet target applications for the

AAT1153.

For example, the 2.2μH CDRH5D16-2R2 inductor select-

ed from Sumida has a 28.7mΩ DCR and a 3.0ADC cur-

rent rating. At full load, the inductor DC loss is 57mW

which gives a 1.6% loss in efficiency for a 1200mA, 1.8V

output.

Slope Compensation

The AAT1153 step-down converter uses peak current

mode control with slope compensation for stability when

duty cycles are greater than 50%. The slope compensa-

tion is set to maintain stability with lower value inductors

which provide better overall efficiency. The output induc-

tor value must be selected so the inductor current down

slope meets the internal slope compensation require-

ments. As an example, the value of the slope compensa-

tion is set to 1A/μs which is large enough to guarantee

stability when using a 2.2μH inductor for all output volt-

age levels from 0.6V to 3.3V.

The worst case external current slope (m) using the

L

m =

=

= 1.5A/µs

3.3

2.2

To keep the power supply stable when the duty cycle is

above 50%, the internal slope compensation (mA)

should be:

1

2

· m = 0.75A/µs

Therefore, to guarantee current loop stability, the slope

of the compensation ramp must be greater than one-half

of the down slope of the current waveform. So the inter-

nal slope compensated value of 1A/μs will guarantee

stability using a 2.2μH inductor value for all output volt-

ages from 0.6V to 3.3V.

Input Capacitor Selection

The input capacitor reduces the surge current drawn

from the input and switching noise from the device. The

input capacitor impedance at the switching frequency

should be less than the input source impedance to pre-

vent high frequency switching current passing to the

input. The calculated value varies with input voltage and

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- ESR

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- ESR

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A low ESR input capacitor sized for maximum RMS cur-

rent must be used. Ceramic capacitors with X5R or X7R

dielectrics are highly recommended because of their low

ESR and small temperature coefficients. A 22μF ceram-

ic capacitor for most applications is sufficient. A large

value may be used for improved input voltage filtering.

The maximum input capacitor RMS current is:

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