SC453 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC453' PDF : 23 Pages View PDF
SC453
POWER MANAGEMENT
Applications Information (Cont.)
VHYS is created by a current source, IHYS, through R9 (see
the diagram below). The current source value is con-
trolled via RHYS. For simplicity it is easier to select a value
for R7 (the resistor in series with the CMP pin) first, and
then calculate RHYS, as follows:
N.
R7 := 1K⋅Ω
2⋅VREF
RHYS := ⎛ VHYS ⎞
⎜
.
⎝ R7 ⎠
RHYS = 102.0 KΩ
Step 3: Setting RHYS
The next step is to calculate R . Since the SC453 is a
HYS
hysteretic controller, it regulates the amount of output
ripple according to a hysteresis value set by RHYS. The de-
signer must therefore decide upon the amount of desired
output ripple, and then set RHYS 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 VRIPPLE to VHYS.
In the SC453 application circuit, RHYS 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, RHYS 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.
SC453
To achieve tight accuracy, it is recommended that the out-
put ripple be set to 20mV peak-to-peak.
R14
RESR
ESR :=
NCAP
ESR
=
1.5
×
−
10
3Ω
VRIPPLE = 0.02 V
HYS
R5
BOOTV
R4
SLPV
(RCS + ESR)
R3
M.
VHYS := VRIPPLE⋅
ESR
VHYS = 0.033 V
© 2006 Semtech Corp.
17
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