SC453TSTRT datasheet(18/26 Pages) SEMTECH

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POWER MANAGEMENT

SC453

Applications Information (Cont.)

Step 3 — Setting RHYS

The next step is to calculate R

HYS

. Since the SC453 is a

hysteretic controller, it regulates the amount of output

ripple according to a hysteresis value set by R

HYS

. The de-

signer must therefore decide upon the amount of desired

output ripple, and then set R

HYS

accordingly.

The hysteresis controls the amount of ripple voltage at the

point of regulation, which is the point between the induc-

tor and the current sense resistor. The amount of ripple at

the output is defined by the current sense resistor and the

output capacitor ESR. This factor is taken into account in

the equation shown below relating V

RIPPLE

to V

HYS

To achieve tight accuracy, it is recommended that the out-

put ripple be set to 20mV peak-to-peak.

ESR

RESR

NCAP

ESR

1.5

−

×Ω

VRIPPLE

0.02 V

VHYS

VRIPPLE

RCS

ESR

()

ESR

⋅

VHYS

0.033 V

HYS

is created by switching a current source (I

HYS

) through

a resistor in series with the CMP pin. The I

HYS

value is

controlled via R

HYS

. For simplicity it is easier to select a

value for CMP resistor (R7, see Typical Application Circuit

on page 2) and then calculate R

HYS

, as follows.

⋅

RHYS

2VREF

⋅

VHYS

⎛

⎜

⎝

⎞

⎠

RHYS

102.0 K

In the SC453 application circuit, R

HYS

consists of three

resistors (R3, R4 and R5) in parallel with R14. These re-

sistors also form the dividers for BOOTV and SLPV. Note

also that depending on circuit layout and parasitics, R

HYS

may have to be adjusted slightly to obtain optimum per-

formance. (The above resistors will be calculated after the

PBOOT and deeper sleep voltages are set later in this pro-

cedure). To increase hysteresis without having to change

the divider resistors, a fourth resistor (R14), can be add-

ed. Additional hysteresis is needed when inductances in

the current sense paths cause additional signal that adds

to the resistive signal, limiting the accuracy of the calcula-

tions.

R14

HYS

BOOTV

SLPV

SC453

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