EVAL6730 STMicroelectronics, EVAL6730 Datasheet - Page 5

• Manufacturer Part Number: EVAL6730
• Description: EVAL BOARD FOR L6730XX
• Manufacturer: STMicroelectronics
• Type: Motor / Motion Controllers & Driversr

Specifications of EVAL6730

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Power - Output: 66W
• Voltage - Output: 3.3V
• Current - Output: 20A
• Voltage - Input: 4.5 ~ 14V
• Regulator Topology: Buck
• Frequency - Switching: 400kHz
• Board Type: Fully Populated
• Utilized Ic / Part: L6730
• Input Voltage: 8 V to 52 V
• Product: Power Management Modules
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: L6208
• Other names: 497-5501

Figure 4. Voltage at the two outputs at the beginning of the off-time.
2.3 Choosing the Bulk Capacitor
Since the bulk capacitor, placed between V
AC current capability must be greater than the r.m.s. value of the charge/discharge current. This current flows
from the capacitor to the IC during the on-time (t
slow decay) to the capacitor during the off-time (t
pacitor depends on peak output current, output current ripple, switching frequency, duty-cycle and chopping
style. It also depends on power supply characteristics. A power supply with poor high frequency performances
(or long, inductive connections to the IC) will cause the bulk capacitor to be recharged slowly: the higher the
current control switching frequency, the higher the current ripple in the capacitor; r.m.s. current in the capacitor,
however, does not exceed the r.m.s. output current. Bulk capacitor value ( C ) and the ESR determine the amount
of voltage ripple on the capacitor itself and on the IC. In slow decay, neglecting the dead-time and output current
ripple, and assuming that during the on-time the capacitor is not recharged by the power supply, the voltage at
the end of the on-time is:
so the supply voltage ripple is:
where I
the supply voltage to exceed the nominal voltage. This can be very dangerous if the nominal supply voltage is
close to the maximum recommended supply voltage (52V). In fast decay the supply voltage ripple is about:
always assuming that the power supply does not recharge the capacitor, and neglecting the output current ripple
and the dead-time. Usually (if C > 100 µF) the capacitance role is much less than the ESR, then supply voltage
ripple can be estimated as:
OUT
is the output current. With fast decay, instead, recirculating current recharges the capacitor, causing
I
OUT
V
Out 2
S
S
I
Out 1
and GND pins, is charged and discharged during IC operation, its
OUT
–


I
2 ESR
OUT
ON
OFF


) and from the IC (in fast decay; from the power supply in
ESR
). The r.m.s. value of the current flowing into the bulk ca-


ESR
+
t
--------------------------- -
+
ON
t
-------- -
+
O N
C
+
C
t
-------- -
O N
C


t
O FF
,


,


,
AN1451 APPLICATION NOTE
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