MAX1519 Datasheet, PDF(35/43 Page) Maxim Integrated Products – Dual-Phase, Quick-PWM Controllers for Programmable CPU Core Power Supplies

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

MAX1519 Datasheet, PDF (35/43 Pages) Maxim Integrated Products – Dual-Phase, Quick-PWM Controllers for Programmable CPU Core Power Supplies

◁

Dual-Phase, Quick-PWM Controllers for

Programmable CPU Core Power Supplies

MAX1519

MAX1545

ERROR

COMPARATOR

CMN

CMP

MAIN

PHASE

OAIN+

OAIN-

SECOND

PHASE

CSP

CSN

RSENSE

RA RB

PC BOARD TRACE

RESISTANCE

RFBS

CPU SENSE

POINT

RA RB

RSENSE

PC BOARD TRACE

RESISTANCE

Figure 10. Voltage-Positioning Gain

For the low-side MOSFET (NL), the worst-case power

dissipation always occurs at maximum input voltage:

(NL

RESISTIVE)

ï£®

ï£¯1 â

ï£°

ï£«

ï£ï£¬

VOUT

VIN(MAX)

ï£¶

ï£¸ï£·

ï£¹

ï£º

ï£»

ï£«

ï£¬

ï£

ILOAD

Î· TOTAL

ï£¶2

ï£·

ï£¸

RDS(ON)

The worst-case for MOSFET power dissipation occurs

under heavy overloads that are greater than

ILOAD(MAX) but are not quite high enough to exceed

the current limit and cause the fault latch to trip. To pro-

tect against this possibility, you can âoverdesignâ the

circuit to tolerate:

ILOAD

Î·TOTA L ï£«ï£ï£¬IVALLEY(MAX)

âIINDUCTOR

ï£¶

ï£¸ï£·

Î·TOTALIVALLEY(MAX)

ï£«

ï£ï£¬

ILOAD(MAX)LIR

ï£¶

ï£¸ï£·

where IVALLEY(MAX) is the maximum valley current

allowed by the current-limit circuit, including threshold

tolerance and on-resistance variation. The MOSFETs

must have a good size heatsink to handle the overload

power dissipation.

Choose a Schottky diode (DL) with a forward voltage

low enough to prevent the low-side MOSFET body

diode from turning on during the dead time. As a gen-

eral rule, select a diode with a DC current rating equal

to 1/3 of the load current-per-phase. This diode is

optional and can be removed if efficiency is not critical.

Boost Capacitors

The boost capacitors (CBST) selected must be large

enough to handle the gate-charging requirements of

the high-side MOSFETs. Typically, 0.1ÂµF ceramic

capacitors work well for low-power applications driving

medium-sized MOSFETs. However, high-current appli-

cations driving large, high-side MOSFETs require boost

capacitors larger than 0.1ÂµF. For these applications,

select the boost capacitors to avoid discharging the

capacitor more than 200mV while charging the high-

side MOSFETâs gates:

CBST

N x QGATE

200mV

where N is the number of high-side MOSFETs used for

one regulator, and QGATE is the gate charge specified

in the MOSFETâs data sheet. For example, assume (2)

IRF7811W N-channel MOSFETs are used on the high

side. According to the manufacturerâs data sheet, a sin-

gle IRF7811W has a maximum gate charge of 24nC

(VGS = 5V). Using the above equation, the required

boost capacitance would be:

CBST

2 x 24nC

200mV

0.24ÂµF

Selecting the closest standard value, this example

requires a 0.22ÂµF ceramic capacitor.

______________________________________________________________________________________ 35

▷