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LTC1553CG

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

Linear Technology 

LTC1553

APPLICATIONS INFORMATION

Table 4. Recommended Minimum RIMAX Resistor (kΩ) vs Maximum Operating Load Current and External MOSFET Q1

MAXIMUM OPERATING

LOAD CURRENT (A)

SUD50N03-10

SUD50N03-10

(TWO IN PARALLEL)

MTD20N03HDL

MTD20N03HDL

(TWO IN PARALLEL)

12

2.4

1.2

4.3

2.2

14

2.7

1.3

5.1

2.7

16

3.0

1.5

6.2

3.0

18

3.6

1.8

6.8

3.3

20

3.9

2.0

7.5

3.6

OUTEN and Thermistor Input

The LTC1553 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. OT and PWRGD are

pulled low, and FAULT is left floating. In shutdown, the

LTC1553 quiescent current will drop to about 130µA. The

remaining 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 consumed by the circuit, especially at elevated

temperature.

OUTEN is designed with multiple thresholds to allow it to

also be utilized for over-temperature protection. The power

MOSFET operating temperature can be monitored with an

external negative temperature coefficient (NTC) thermistor

mounted next to the external MOSFET which is expected

to run the hottest –– often the high-side device, Q1. Elec-

trically, the thermistor should form a voltage divider with

another resistor, R1, connected to VCC. Their midpoint

should be connected to OUTEN (see Figure 6). As the

temperature increases, the OUTEN pin voltage is reduced.

Under normal operating conditions, the OUTEN pin should

stay above 2V. All circuits will function normally, and the

OT pin will remain in a high state. If the temperature gets

abnormally high, the OUTEN pin voltage will eventually

drop below 2V. OT will switch to a logic low, providing an

over-temperature warning to the system. As OUTEN drops

below 1.7V, the LTC1553 disables both FET drivers. If

VCC

PENTIUM II

SYSTEM

R2

NTC THERMISTOR

MOUNT IN CLOSE

THERMAL PROXIMITY

TO Q1

5.6k

OT

G1

VCC

R1

LTC1553

OUTEN G2

VIN

Q1

LO

VOUT

+

Q2

COUT

1553 F06

Figure 6. OUTEN Pin as a Thermistor Input

OUTEN is less than 1.2V, the LTC1553 will enter shutdown

mode. To activate any of these three modes, the OUTEN

voltage must drop below the respective threshold for

longer than 30µs.

Clock Synchronization

The internal oscillator can be synchronized to an external

clock by applying the external clocking signal to the

OUTEN pin. The synchronizing range extends from the

initial operating frequency up to 500kHz. If the external

frequency is much higher than the natural free-running

frequency, the peak-to-peak sawtooth amplitude within

the LTC1553 will decrease. Since the loop gain is inversely

proportional to the amplitude of the sawtooth, the com-

pensation network may need to be adjusted slightly. Note

that the temperature sensing circuitry does not operate

when external synchronization is used.

12

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