FAN5069EMTCX【PDF 10/22 页】IC REG DL BCK/LINEAR 16TSSOP

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Sheet目录40

6页 7页 8页 9页 [10页]11页 12页 13页 14页

?2005 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5069 Rev. 1.1.5

Block Diagram

Figure 20. Block Diagram

Detailed Operation Description

FAN5069 combines a high-efficiency, fixed-frequency

PWM controller designed for single-phase synchronous

buck Point-Of-Load converters with an integrated LDO

controller to support GTL-type loads. This controller is

ideally suited to deliver low-voltage, high-current power

supplies needed in desktop computers, notebooks,

workstations, and servers. The controller comes with an

integrated boot diode which helps reduce component

cost and increase space savings. With this controller, the

input to the power supply can be varied from 3V to 24V

and the output voltage can be set to regulate at 0.8V to

15V on the switcher output. The LDO output can be con-

figured to regulate between 0.8V to 3V and the input to

the LDO can be from 1.5V to 5V, respectively. An internal

shunt regulator at the VCC pin facilitates the controller

operation from either a 5V or 12V power source.

Bias Supply

FAN5069 can be configured to operate from 5V or 12V

for V

. When 5V supply is used for V

, no resistor is

required to be connected between the supply and the

. When the 12V supply is used, a resistor R

VCC

connected between the 12V supply and the V

CC,

shown in Figure 1. The internal shunt regulator at the

VCC pin is capable of sinking 150mA of current to

ensure that the controllers internal V

is maintained at

5.6V maximum.

Choose a resistor such that:

?SPAN class="pst FAN5069EMTCX_2464147_4"> It is rated to handle the power dissipation.

?SPAN class="pst FAN5069EMTCX_2464147_4"> Current sunk within the controller is minimized to

prevent IC temperature rise.

VCC

Selection (IC)

The selection of R

VCC

is dependent on:

?SPAN class="pst FAN5069EMTCX_2464147_4"> Variation of the 12V supply

?SPAN class="pst FAN5069EMTCX_2464147_4"> Gate charge of the top and bottom FETs (Q

FET

)

?SPAN class="pst FAN5069EMTCX_2464147_4"> Switching frequency (F

)

?SPAN class="pst FAN5069EMTCX_2464147_4"> Shunt regulator minimum current (1mA)

?SPAN class="pst FAN5069EMTCX_2464147_4"> Quiescent current of the IC (I

)

Calculate R

VCC

based on the minimum input voltage for

the V

(EQ. 1)

For a typical example, where: Vin

MIN

= 11.5V, I

= 3mA,

FET

= 30nC, F

= 300KHz, R

VCC

is calculated to be

398.65?

PWM Section

The FAN5069s PWM controller combines the conven-

tional voltage mode control and current sensing through

lower MOSFET R

DS_ON

to generate the PWM signals.

This method of current sensing is loss-less and cost

effective. For more accurate current sense requirements,

an optional external resistor can be connected with the

bottom MOSFET in series.

Current

Sense

Amplifier

COMP

Vref

Vcc

ILIM

10糀

Current Limit

Comparator

Internal

Boot Diode

Internal Vcc 5.6V Max.

OSC

PWM

R(RAMP)

Ramp

Generator

Enable

Vcc

BOOT

Vout

RAMP

Error

Amplifier

PWM

Comparator

BOOT

Shunt Reg

HDRV

LDRV

PGND

Summing

?/DIV>

Amplifier

Adaptive

Gate Drive

Circuit

VCC

Vin

MIN

5.6

1 10

FET

1.2

(

)

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