SC453TSTRT datasheet(17/23 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 at the point

of regulation, which is the point between the inductor and

the current sense resistor. The amount of ripple at the

output is deﬁned by the current sense resistor and the

output cap, ESR. This factor is taken into account in the

equation shown below relating V

RIPPLE to VHYS.

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 a current source, IHYS, through R9 (see

the diagram below). The current source value is con-

trolled via R

HYS. For simplicity it is easier to select a value

for R7 (the resistor in series with the CMP pin) ﬁrst, and

then calculate R

HYS, as follows:

R7 1K Ω

⋅

RHYS

2VREF

⋅

VHYS

⎛

⎜

⎝

⎞

⎠

RHYS 102.0 KΩ

In the SC453 application circuit, R

HYS consists of three re-

sistors (R3, R4 and R5). These resistors also form the

dividers for BOOTV and SLPV. Note also that depending

on circuit layout and parasitics, R

HYS may have to be ad-

justed slightly to obtain optimum performance. (We will

come back to calculate the above resistors after we set

the PBOOT and Deeper Sleep voltages). To increase hys-

teresis without having to change the divider resistors, a

fourth resistor (R14), can be added. Additional hysteresis

is needed when inductances in the current sense paths

cause additional signal that add to the resistive signal,

limiting the accuracy of the calculations.

R14

HYS

BOOTV

SLPV

SC453

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