SC461 datasheet(17/31 Pages) SEMTECH

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SC461

Note that the LDO enable pin (ENL) can also disable the

switching regulator through the V

IN UVLO function. Refer

to the ENL Pin and V

IN UVLO section.

Current Limit Protection

The SC461 features programmable current limiting, which

is accomplished using the RDS

(ON) of the lower MOSFET for

current sensing. The current limit is set by R

LIM resistor

which connects from the ILIM pin to the drain of the low-

side MOSFET. When the low-side MOSFET is on, an internal

10μA current flows from the ILIM pin and through the R

LIM

resistor, creating a voltage drop across the resistor. While

the low-side MOSFET is on, the inductor current flows

through it and creates a voltage across the RDS

(ON). The

voltage across the MOSFET is negative with respect to

PGND. If this MOSFET voltage drop exceeds the voltage

across R

LIM, the voltage at the ILIM pin will be negative and

current limit will activate. The current limit then keeps the

low-side MOSFET on and prevents another high-side on-

time, until the current in the low-side MOSFET reduces

enough to bring the ILIM pin voltage up to zero. This

method regulates the inductor valley current at the level

shown by I

LIM in Figure 8.

Time

IPEAK

ILOAD

ILIM

Figure 8 — Valley Current Limit

The current limit schematic with the R

LIM resistor is shown

in Figure 9.

VOUT

VIN

CIN

RLIM

CBST

PGND

ILIM

BST

COUT

Figure 9 — Valley Current Limit

Setting the valley current limit to 10A results in a peak

inductor current of 10A plus peak ripple current. In this

situation the average current through the inductor is 10A

plus one-half the peak-to-peak ripple current.

The R

LIM value is calculated by the next equation.

The internal 10μA current source is temperature compen-

sated at 2800 ppm in order to provide tracking with the

RDS

ON.

Soft-Start of PWM Regulator

The SC461 has a programmable soft-start time that is con-

trolled by an external capacitor at the SS pin. During the

soft-start time, the controller sources 3μA from the SS pin

to charge the capacitor. During the start-up process

(Figure 10), 40% of the voltage ramp at the SS pin is used

as the reference for the FB comparator. The PWM compara-

tor issues an on-time pulse when the FB voltage is less

than 40% of the SS voltage, which forces the output

voltage to follow the SS ramp. The output voltage reaches

regulation when the SS pin voltage exceeds 1.5V and the

FB reaches the 600mV threshold. The time between the

first LX pulse and VOUT reaching the regulation point is

the soft-start time (t

SS). The calculation for the soft-start

time is shown by the following equation.

Applications Information (continued)

SC461 Datasheet(PDF) 17 Page - Semtech Corporation

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