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AIC1573

5-bit DAC, Synchronous PWM Power Regulator with Simple PWM Power Regulator, LDO And Linear Controller

Analog Intergrations 

AIC1573

Output Inductor Selection

Inductor value and type should be chosen based on

output slew rate requirement, output ripple require-

ment and expected peak current. Inductor value is

primarily controlled by the required current respon-

se time. The AIC1573 will provide either 0% or

100% duty cycle in response to a load transient.

The response time to a transient is different for the

application of load and remove of load.

tRISE = L × ∆IOUT

VIN − VOUT ,

tFALL = L × ∆IOUT

VOUT .

Where ∆IOUT

load current step.

is transient

In a typical 5V input, 2V output application, a 3µH

inductor has a 1A/µS rise time, resulting in a 5µS

delay in responding to a 5A load current step. To

optimize performance, different combinations of in-

put and output voltage and expected loads may re-

quire different inductor value. A smaller value of in-

ductor will improve the transient response at the

expense of increase output ripple voltage and ni -

ductor core saturation rating.

Peak current in the inductor will be equal to the

maximum output load current plus half of inductor

ripple current. The ripple current is approximately

equal to:

IRIPPLE = (VIN − VOUT) × VOUT

f × L × VIN ;

f = AIC1573 oscillator frequency.

The inductor must be able to withstand peak cur-

rent without saturation, and the copper resistance

in the winding should be kept as low as possible to

minimize resistive power loss

Input Capacitor Selection

Most of the input supply current is supplied by the

input bypass capacitor, the resulting RMS current

flow in the input capacitor will heat it up. Use a mix

of input bulk capacitors to control the voltage over-

shoot across the upper MOSFET. The ceramic ca-

pacitance for the high frequency decoupling should

be placed very close to the upper MOSFET to sup-

press the voltage induced in the parasitic circuit

impedance. The buck capacitors to supply the

RMS current is approximate equal to:

IRMS = (1− D) ×

D×

I2 OUT

+

1

12

×



VfIN××LD

2

D = VOUT

, where

VIN

The capacitor voltage rating should be at least 1.25

times greater than the maximum input voltage.

PWM MOSFET Selection

In high current PWM application, the MOSFET

power dissipation, package type and heatsink are

the dominant design factors. The conduction loss is

the only component of power dissipation for the

lower MOSFET, since it turns on into near zero

voltage. The upper MOSFET has conduction loss

and switching loss. The gate charge losses are

proportional to the switching frequency and are dis-

sipated by the AIC1573. However, the gate charge

increases the switching interval, tSW, which increase

the upper MOSFET switching losses. Ensure that

both MOSFETs are within their maximum junction

temperature at high ambient temperature by calcu-

lating the temperature rise according to package

thermal resistance specifications.

PUPPER = IOUT2 × RDS(ON) × D + IOUT × VIN × tSW × f

2

PLOWER = IOUT2 × RDS(ON) × (1− D)

The equations above do not model power loss due

to the reverse recovery of the lower MOSFET’s bo-

dy diode.

The RDS(ON) is different for the two previous equa-

tions even if the type devices is used for both. This

17

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