AIC1577-COTR データシートの表示（PDF） - Analog Intergrations

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AIC1577-COTR

External NMOS Step-Down PWM Controller

Analog Intergrations 

AIC1577

in the voltage of VFB pin. Thus the ITH/RUN voltage

remains increasing until the average inductor cur-

rent matches new load current. While the high-side

MOSFET turns off, the low-side MOSFET is turned

on to recharge bootstrap capacitor C3.

Main control loop is shutdown when ITH/RUN goes

below 0.8V. When ITH/RUN pulled up to 0.8V or up

by error amplifier, main control loop is enabled.

Low Current Operation

During heavy load current operation, AIC1577 op-

erates in PWM mode with a frequency of 500KHz.

Decreasing of the current will cause a drop in

ITH/RUN below 1.33V so that AIC1577 enters PFM

mode operation for better efficiency. If the voltage

across RS does not exceed the offset of current

comparator within a cycle, then the high-side and

internal MOSFETs will disable until ITH/RUN goes

over 1.33V.

Component Selection

AIC1577 can be used in many switching regulator

applications, such as step-down, step-up, SEPIC

and positive-to-negative converters. Among all,

step-down converter is the most common applica-

tion. External component selection, beginning with

selecting RS, depends on load requirement of the

application. Once RS is decided, the choice of in-

ductor, which is followed by selecting power

MOSFET and diode, can be easily chosen. Finally,

CIN and COUT can be determined.

RS Selection

The choice of RS has substantial connection with

required output current. The threshold voltage of

current comparator decides peak inductor current,

which yields a maximum average output current

(IMAX). And the peak current is less than half of the

peak-to-peak ripple current, ΔIL.

Allowing a margin for variation of AIC1577, external

component can be yielded as:

100mV

RS = IMAX

Inductor Selection

With the operating frequency high to 500KHz,

smaller inductor value is favored. In general,

operating in high frequency will cause low efficiency

because of large MOSFET switching loss. Thus the

effect of inductor value on ripple current and low

current operation must be considered as well.

The inductor value has a direct influence on ripple

current ( Δ IL), which decreases with high induc-

tance and increases with high VIN or VOUT:

∆IL

=

VIN

− VOUT

f ×L

 VOUT + VD

 VIN + VD





VD is the drop voltage of the output Schottky diode.

Accepting a large value ofΔIL allows the use of low

inductance, but yields high output ripple voltage

and large core loss. The inductor value also has an

effect on low current operation. Low inductor value

causes the PFM operation to begin at high load cur-

rent. The efficiency of the circuit decreases at the

beginning of low current operation. Generally

speaking, low inductance in PFM mode will cause

the efficiency to decrease.

Power MOSFET Selection

For an application of AIC1577, an external N-

channel power MOSFET, used as the high-side

switch, must be properly selected. To prevent

MOSFET from damage during high input voltage

operation, attention should be taken to the BVDSS

specification for MOSFET.

Other important selection criteria for the power

MOSFET include the “ON” resistance RDS(ON), in-

put voltage and maximum output current.

9

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