SC418 datasheet(14/30 Pages) SEMTECH

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SC418

comparator and a capacitor. One comparator input is con-

nected to V

OUT, the other input is connected to the

capacitor. When the on-time begins, the internal capaci-

tor charges from zero volts through a current which is

proportional to V

IN. When the capacitor voltage reaches

OUT, the on-time is completed and the high-side MOSFET

turns off.

Gate

Drives

FB Comparator

One-Shot

Timer

On-time = K x RTON x (VOUT/VIN)

VOUT

VIN

500mV

COUT

VIN

ESR

VOUT

VLX

RTON

Figure 2 — On-Time Generation

This method automatically produces an on-time that is

proportional to V

OUT and inversely proportional to VIN.

Under steady-state conditions, the switching frequency

can be determined from the on-time by the following

equation.

OUT

The SC418 uses an external resistor to set the on-time which

indirectly sets the frequency. The on-time can be pro-

grammed to provide an operating frequency from 200kHz

to 1MHz using a resistor between the TON pin and ground.

The resistor value is

selected by the following equation.

OUT

TON

)

(

The maximum R

TON value allowed is shown by the follow-

ing equation.

MIN

MAX

TON

Immediately after the on-time, the DL output drives high

to energize the low-side MOSFET. DL has a minimum high

time of ~250ns, after which DL continues to stay high until

one of the following occurs:

VFB falls below the 500mV reference

•

The Zero Cross Detector trips if power-save is

active

TON Limitations and VDDA Supply Voltage

For VDDA below 4.5V, the TON accuracy may be limited

by V

IN. The previous RTON equation is accurate if VIN satis-

fies the below relation over the entire V

IN range:

IN < (VDDA - 1.6V) x 10

If V

IN exceeds ((VDDA - 1.6V) x 10) for all or part of the VIN

range, the previous RTON equation is not accurate. In all

cases where VIN > ((VDDA - 1.6V) x 10), the RTON equation

must be modifi ed as follows.

OUT

TON

25pF

1.6V)

(V5V

10ns)

Note that when V

IN is greater than ((VDDA - 1.6V) x 10), the

actual on-time is fixed and does not vary with V

IN. When

operating in this condition, the switching frequency will

vary inversely with V

IN rather than approximating fixed

frequency.

When a large capacitor is placed in parallel with R1 (CTOP)

VOUT is shown by the following equation.

TOP

RIPPLE

OUT

)

(

The switcher output voltage can be programmed higher

than 5V with careful design. In this case the VOUT pin

cannot connect directly to the switcher output due to its

the maximum voltage rating. An additional resistor

divider network is required to connect from the switcher

output to the VOUT pin. The voltage at the VOUT pin

should be at least 500mV lower than the VDDA supply, to

prevent the VLDO switch-over function. For example, the

voltage at the VOUT pin can be 4V if VDDA is set for 5V.

When the SC418 operates from an external power source

and the LDO is disabled by grounding the ENL pin, the

voltage at the VOUT pin can be as high as shown in

Recommended Operating Conditions. Note that RTON

must be adjusted higher by the same divider ratio to

•

Applications Information (continued)

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