ADP1621ARMZ-R7 datasheet(17/32 Pages) AD

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Data Sheet

ADP1621

Rev. D | Page 17 of 32

The total power dissipation also determines the MOSFET

junction temperature, which is given by

MOSFET

(23)

where:

TJ,MOSFET is the junction temperature.

TA is the ambient temperature.

θJA is the junction-to-ambient thermal resistance of the

MOSFET package. The MOSFET junction temperature must not

exceed its maximum rating at the given power dissipation level.

If lossless current sensing is not used, there is also power

dissipation in the external current-sense resistor, RCS. The power

dissipation, PCS, in the external resistor due to conduction losses

is given by

LOAD













−

(24)

LOOP COMPENSATION

The ADP1621 uses external components to compensate the

regulator loop, allowing optimization of the loop dynamics for

a given application.

The step-up converter produces an undesirable right-half plane

(RHP) zero in the regulation feedback loop. This RHP zero

requires compensating the regulator such that the crossover

frequency occurs well below the frequency of the RHP zero. The

location of the RHP zero is determined by the following equation:

(

)

LOAD

RHP

−

(25)

where:

fZ,RHP is the RHP zero frequency.

RLOAD is the equivalent load resistance or the output voltage divided

by the load current.

To stabilize the regulator, ensure that the regulator crossover

frequency is less than or equal to one-fifth of the RHP zero

frequency and less than or equal to one-fifteenth of the switching

frequency. For an initial practical design, choose the crossover

frequency fC to be the lower of

f =

(26)

and

,RHP

f =

(27)

where:

fC is the crossover frequency.

fSW is the switching frequency.

The regulator loop gain is

(

)

OUT

COMP

OUT

−

(28)

where:

AVL is the loop gain.

VFB is the feedback regulation voltage (typically 1.215 V).

VOUT is the regulated output voltage.

D is the duty cycle.

gm is the error amplifier transconductance gain (typically 300 µs).

ZCOMP is the impedance of the RC network from COMP to GND.

n is the current-sense amplifier gain (typically 9.5).

RCS is the current-sense resistance.

ZOUT is the impedance of the load and output capacitor. In the case

of lossless current sensing, as shown in Figure 28, RCS is equal to the

on resistance, RDSON, of the external power MOSFET. Otherwise,

RCS represents the external current-sense resistor, as shown in

Figure 29.

To determine the crossover frequency, it is important to note

that at that frequency the compensation impedance, ZCOMP, is

dominated by Resistor RCOMP, and the output impedance, ZOUT,

is dominated by the impedance of the output capacitor, COUT.

When solving for the crossover frequency, the equation is

simplified to

| VL

(

)

−

OUT

COMP

OUT

(29)

where:

fC is the crossover frequency.

RCOMP is the compensation resistor.

COUT is the output capacitance.

Solving for RCOMP gives

(

)

OUT

COMP

−

(30)

After the compensation resistor, RCOMP, is known, set the zero

formed by the resistor and compensation capacitor, CCOMP, to

one-fourth of the crossover frequency, or

COMP

(31)

Capacitor C2 is chosen to cancel the zero introduced by the output

capacitance ESR. Thus, set C2 to (see Figure 31)

COMP

OUT

ESR

(32)

where ESR represents the ESR of COUT.

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