l6730cqtr STMicroelectronics, l6730cqtr Datasheet - Page 31

• Manufacturer Part Number: l6730cqtr
• Description: Adjustable Step-down Controller With Synchronous Rectification
• Manufacturer: STMicroelectronics

L6730C - L6730D
6.2
6.3
Output capacitors
The output capacitors are basic components for the fast transient response of the power
supply. They depend on the output voltage ripple requirements, as well as any output voltage
deviation requirement during a load transient. During a load transient, the output capacitors
supply the current to the load or absorb the current stored into the inductor until the converter
reacts. In fact, even if the controller recognizes immediately the load transient and sets the duty
cycle at 100% or 0%, the current slope is limited by the inductor value. The output voltage has
a first drop due to the current variation inside the capacitor (neglecting the effect of the ESL):
Moreover, there is an additional drop due to the effective capacitor discharge or charge that is
given by the following formulas:
Formula (8) is valid in case of positive load transient while the formula (9) is valid in case of
negative load transient. D
100%. For a given inductor value, minimum input voltage, output voltage and maximum load
transient, a maximum ESR and a minimum C
also affect the static output voltage ripple. In the worst case the output voltage ripple can be
calculated with the following formula:
Usually the voltage drop due to the ESR is the biggest one while the drop due to the capacitor
discharge is almost negligible.
Input capacitors
The input capacitors have to sustain the RMS current flowing through them, that is:
Where D is the duty cycle. The equation reaches its maximum value, I
losses in worst case are:
Vout
COUT
Vout
Vout
MAX
Irms
P
2
Vout
COUT
is the maximum duty cycle value that in the L6730C - L6730D is
Cout
I
L
ESR
ESR
(
Iout
ESR
(
2
Vin
0 (
Cout
5 .
Iout
Iout
,
Iout
D
8
min
Iout
OUT
Cout
1 ( D
2
Vout
2
ESR
D
L
)
1
L
value can be set. The ESR and C
2
max
)
Fsw
(12)
(7)
(9)
(11)
)
Vout
(10)
)
(8)
OUT
/2 with D = 0.5. The
Application details
OUT
values
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