EL7520ILZ-T13 查看數據表（PDF） - Intersil

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EL7520ILZ-T13

4-Channel DC/DC Controller

Intersil 

EL7520, EL7520A

Input Capacitor

The input capacitor is used to supply the current to the

converter. It is recommended that CIN be larger than 10µF.

The reflected ripple voltage will be smaller with larger CIN.

The voltage rating of input capacitor should be larger than

maximum input voltage.

Boost Inductor

A 3.3µH inductor is recommended due to the fixed internal

slope compensation. The inductor must be able to handle

the following average and peak current:

ILAVG = 1----I-–-O----D---

ILPK

=

IL

A

V

G

+

-∆----I--L-

2

Switching MOSFET

Due to the parasitic inductance of the trace, the MOSFET

will experience spikes higher that the output voltage when

the MOSFET turns off. Thus, a MOSFET with enough

voltage margin is needed.

The RDS(ON) of the MOSFET is critical for power dissipation

and current limit. A MOSFET with low RDS(ON) is desired to

get high efficiency and output current, but very low RDS(ON)

will reduce the loop stability. A MOSFET with 20mΩ to 50mΩ

RDS(ON) is recommended. Some recommended MOSFETs

are shown in following table.

TABLE 2. RECOMMENDED MOSFETs

PART

NUMBER MANUFACTURER

FEATURE

FDC655AN

Fairchild

6.3A, 30V, RDS(ON) = 23mΩ

Semiconductor

FDS4488

Fairchild

7.9A, 30V, RDS(ON) = 22mΩ

Semiconductor

Si7844DP

SI6928DQ

Vishay

Vishay

10A, 30V, RDS(ON) = 22mΩ

20A, 30V, RDS(ON) = 30mΩ

Rectifier Diode

A high-speed diode is desired due to the high switching

frequency. Schottky diodes are recommended because of

their fast recovery time and low forward voltage. The rectifier

diode must meet the output current and peak inductor

current requirements.

Output Capacitor

The output capacitor supplies the load directly and reduces the

ripple voltage at the output. Output ripple voltage consists of

two components: the voltage drop due to the inductor ripple

current flowing through the ESR of output capacitor, and the

charging and discharging of the output capacitor.

VRIPPLE

=

ILPK

×

ESR

+

-V----O-----–-----V----I--N--

VO

×

------I--O-------

COUT

×

--1--

fS

For low ESR ceramic capacitors, the output ripple is

dominated by the charging and discharging of the output

capacitor. The voltage rating of the output capacitor should

be greater than the maximum output voltage.

Compensation

The EL7520 and EL7520A can operate in either P mode or

PI mode. Connecting CINT pin directly to VIN will enable

P mode. For better load regulation, use PI mode with a

2.2nF capacitor between CINT and ground.

Linear-Regulator Controllers (VON, VLOGIC, and

VOFF)

The EL7520, EL7520A include three independent linear-

regulator controllers, in which two are positive output voltage

(VON and VLOGIC), and one is negative. The VON, VOFF,

and VLOGIC linear-regulator controller functional diagrams,

applications circuits are shown in Figures 27, 28, and 29

respectively.

Calculation of the Linear Regulator Base-Emitter

Resistors (RBL, RBP and RBN)

For the pass transistor of the linear regulator, low frequency

gain (Hfe) and unity gain freq. (fT) are usually specified in the

datasheet. The pass transistor adds a pole to the loop transfer

function at fp = fT/Hfe. Therefore, in order to maintain phase

margin at low frequency, the best choice for a pass device is

often a high frequency low gain switching transistor. Further

improvement can be obtained by adding a base-emitter resistor

RBE (RBP, RBL, RBN in the Functional Block Diagram), which

increase the pole frequency to: fp = fT*(1+ Hfe *re/RBE)/Hfe,

where re = KT/qIc. So choose the lowest value RBE in the

design as long as there is still enough base current (IB) to

support the maximum output current (IC).

We will take as an example the VLOGIC linear regulator. If a

Fairchild FMMT549 PNP transistor is used as the external

pass transistor, Q31 in the application diagram, then for a

maximum VLOGIC operating requirement of 500mA the data

sheet indicates Hfe_min = 100.

The base-emitter saturation voltage is: Vbe_max = 1.25V (note

this is normally a Vbe ~ 0.7V, however, for the Q5 transistor an

internal Darlington arrangement is used to increase it's current

gain, giving a 'base-emitter' voltage of 2 x VBE).

(Note that using a high current Darlington PNP transistor for

Q5 requires that VIN > VLOGIC + 2V. Should a lower input

voltage be required, then an ordinary high gain PNP

transistor should be selected for Q5 so as to allow a lower

collector-emitter saturation voltage).

For the EL7520, EL7520A, the minimum drive current is:

I_DRVL_min = 8mA

12

FN7318.0

July 12, 2005

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