SC4530 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4530' PDF : 23 Pages View PDF
SC453
POWER MANAGEMENT
Applications Information (Cont.)
DESIGN PROCEDURE
Step 1: Define Constants
VINMAX := 20V
VINMIN := 8V
Maximum input voltage
Minimum input voltage
IMAX_FL := 20A
Maximum load current,
highest output voltage
ILKGMAX := 5A
Leakage current, highest
output voltage
VMAX_NL := 1.212V
VMIN_NL := 0.956 ⋅V
VREF := 1.7V
The highest V voltage
CORE
The lowest VCORE voltage
SC453 Internal reference voltage
Step 2: Output Inductor and Capacitor Selection
The SC453 has "passive" droop. The voltage at full load is
less than the DAC voltage by the voltage drop across the
current sense resistor and any PCB copper losses from
the sense resistor to the processor socket. The steady-
state voltage at full load is:
( ) VMAX_FL := VMAX_NL − RCS + RCU ⋅IMAX_FL
VMAX_FL = 1.182 V
Output capacitance and ESR values are a function of
transient requirements and output inductor value. Figure
1 illustrates the response of a hysteretic converter to a
positive transient. In a hysteretic converter with passive
droop, like the SC453, two conditions determine if you
meet the positive transient requirements.
COUT := 330⋅μF
RESR := 6m⋅Ω
Output capacitance per cap
ESR per cap
VPOS_TRANS
A.
( ) ESR ≤
IMAX_FL − ILKGMAX
RCS := 1m⋅Ω
RCU := 0.5m⋅Ω
Current sense resistor
Parasitic resistance from the
current sense resistor to the
processor
B.
( ) VNEG_TRANS ≥ deltaV COUT
VPOS_TRANS := 50mV Voltage droop allowed for a low
current to high current transient
VNEG_TRANS := 50mV Voltage rise allowed for a high
current to low current transient
VRIPPLE := 20m V
Desired maximum output ripple
Figure 1 - Hysteretic Converter Response to a
Positive Transient
The first condition is easy to see; if the ESR is too high, the
transient response will fail.
© 2006 Semtech Corp.
14
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