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LTC1753

1753fa

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

MAXIMUM OPERATING

Si4410

MTD20N03

LOAD CURRENT (A)

Si4410

(TWO IN PARALLEL)

SUD50N03

(TWO IN PARALLEL)

820

430

680

1.2k

560

820

1.2k

—

680

1.5k

—

820

1.2k

1.8k

—

910

1.5k

2.0k

—

1.2k

—

2.2k

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 130

µA. 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

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:

IMAX

LMAX

DS ON Q

IMAX

= ()(

)

() 1

where,

LMAX

LOAD

RIPPLE

ILOAD = Maximum load current

IRIPPLE = Inductor ripple current

−

()( )

()( )( )

OUT

OSC

Figure 5. Current Limit Setting

190

µA

CIN

VOUT

1753 F05

COUT

RIMAX

VIN

–

Ω

LTC1753

IMAX

IFB

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