SC185LEVB datasheet(15/18 Pages) SEMTECH

15 / 18 page

SC185

in circuit whenever the EN pin is below its threshold, or

when the device is in under voltage lockout and AVIN

exceeds 0.8V. When the device is enabled, it takes about

50µs for the internal circuitry wake up and begin the soft-

start up sequence.

Operation Mode Selection

The MODE input is used to select between forced PWM

and automatic PSAVE modes. When the MODE pin is held

high, the device operates in forced continuous PWM mode

regardless of the output load condition. When the MODE

pin is held low (grounded), the device is permitted to

operate in Power Save mode (PSAVE). The MODE pin can

be changed on-the-fly. When the MODE pin is switched

from low to high, the device will transition to forced

continuous PWM mode immediately. When the MODE

pin is switched from high to low, and the load current is

below the PSAVE entry level, the device will transition to

PSAVE mode after 64 clock cycles. The MODE pin has a

500kΩ internal pull-down resistor. This resistor is switched

in circuit whenever the MODE pin is below its threshold,

or when the device is in under voltage lockout but AVIN

exceeds 0.8V.

100% Duty-Cycle Operation

SC185 is capable of operating at 100% duty-cycle. When

the difference between input voltage to output voltage

is less than the minimum dropout voltage, the PMOS

switch is completely on, operating in 100% duty-cycle.

The minimum dropout voltage is the output current

multiplied by the on-resistance of the internal PMOS

switch and the DC-resistance of the inductor when PMOS

switch is on continuously.

Output L-C filter Selection

SC185 has fixed internal loop-gain compensation. It is

optimized for X5R or X7R ceramic output capacitors and

an output L-C filter corner frequency of less than 34KHz.

The output L-C corner frequency can be determined by

Equation 2.

OUT

⋅

(2)

In general, the inductor is chosen to set the inductor ripple

current to approximately 30% of the maximum output

current. It is recommended to use a typical inductor

Applications Information (continued)

value of 1μH to 2.2μH with output ceramic capacitors of

44μF or higher capacitance. Lower inductance should

be considered in applications where faster transient

response is required. More output capacitance will reduce

the output deviation for a particular load transient. When

using low inductance, the maximum peak inductor

current at any condition (normal operation and start up)

can not exceed 5A which is the guaranteed minimum

current limit. The saturation current rating of the inductor

needs to be at least larger than the peak inductor current

which is the maximum output current plus half of inductor

ripple current.

SC185LEVB Datasheet(PDF) 15 Page - Semtech Corporation

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