LTC1753CSW 查看數據表（PDF） - Linear Technology

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LTC1753CSW

5-Bit Programmable Synchronous Switching Regulator Controller for Pentium® III Processor

Linear Technology 

LTC1753

APPLICATIO S I FOR ATIO

Table 3. Recommended RIMAX Resistor (kΩ) vs Maximum Operating Load Current and External MOSFET Q1

MAXIMUM OPERATING

LOAD CURRENT (A)

Si4410

Si4410

(TWO IN PARALLEL)

SUD50N03

8

820

430

680

10

1.2k

560

820

12

—

680

1k

14

—

820

1.2k

16

—

910

1.5k

18

—

1.2k

—

MTD20N03

(TWO IN PARALLEL)

1k

1.2k

1.5k

1.8k

2.0k

2.2k

overloads may not affect the output voltage at all. More

significant overload conditions will allow the SS pin to

reach a steady state, and the output will remain at a

reduced voltage until the overload is removed. Serious

overloads will generate a large overdrive at CC, allowing it

to pull SS down quickly and preventing damage to the

output components.

By using the RDS(ON) of Q1 to measure the output current,

the current limiting circuit eliminates an expensive dis-

crete sense resistor that would otherwise be required. This

helps minimize the number of components in the high

current path. Due to switching noise and variation of

RDS(ON), the actual current limit trip point is not highly

accurate. The current limiting circuitry is primarily meant

to prevent damage to the power supply circuitry during

fault conditions. The exact current level where the limiting

circuit begins to take effect will vary from unit to unit as the

RDS(ON) of Q1 varies.

For a given current limit level, the external resistor from

IMAX to VIN can be determined by:

( )( ) RIMAX =

ILMAX RDS(ON)Q1

IIMAX

where,

IL MAX

=

ILOAD

+

IRIPPLE

2

ILOAD = Maximum load current

IRIPPLE = Inductor ripple current

( )( ) = VIN − VOUT VOUT

( )( )( ) fOSC LO VIN

LTC1753

CC

VIN

190µA

IMAX

7

IFB

8

RIMAX

G1

20Ω

G2

Q1

LO

Q2

+

CIN

VOUT

+

COUT

1753 F05

Figure 5. Current Limit Setting

fOSC = LTC1753 oscillator frequency = 300kHz

LO = Inductor value

RDS(ON)Q1 = Hot on-resistance of Q1 at ILMAX

IIMAX = Internal 190µA sink current at IMAX

OUTEN and Thermistor Input

The LTC1753 includes a low power shutdown mode,

controlled by the logic at the OUTEN pin. A high at OUTEN

allows the part to operate normally. A low level at OUTEN

stops all internal switching, pulls COMP and SS to ground

internally and turns Q1 and Q2 off. PWRGD is pulled low,

and FAULT is left floating. In shutdown, the LTC1753

quiescent current drops to about 130µA. The residual

current is used to keep the thermistor sensing circuit at

OUTEN alive. Note that the leakage current of the external

MOSFETs may add to the total shutdown current con-

sumed by the circuit, especially at elevated temperatures.

OUTEN is designed with two thresholds to allow it to also

be utilized for overtemperature protection. The power

MOSFET operating temperature can be monitored with an

external negative temperature coefficient (NTC) thermistor

12

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