L6725_07 STMICROELECTRONICS [STMicroelectronics], L6725_07 Datasheet - Page 19

• Manufacturer Part Number: L6725_07
• Description: Voltage mode PWM controller with bootstrap anti-discharging system
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

L6725 - L6725A
6.2
6.3
Output capacitors
The output capacitors are basic components for the fast transient response of the power supply.
For example, during a positive load transient, they supply the current to the load until the
converter reacts. The controller recognizes immediately the load transient and sets the duty
cycle at 100%, but 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 that is given by:
Where D
drop due to the ESR is the biggest one while the drop due to the capacitor discharge is almost
negligible. Moreover the ESR value also affects the voltage static ripple, that is:
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:
MAX
∆
is the maximum duty cycle value that in the L6725 is 100%. Usually the voltage
Vout
COUT
=
Irms
2
∆
P
⋅
Vout
Cout
=
=
ESR
∆
ESR
Iout
Vout
⋅
(
Vin
=
⋅
0 (
∆
⋅
∆
,
=
Iout
5 .
Iout
min
D
ESR
⋅
Iout
⋅
2
1 ( D
⋅
⋅
⋅
D
ESR
−
∆ ⋅
L
)
max
2
I
L
)
−
(3)
(7)
Vout
(6)
(5)
)
(4)
OUT
/2 with D = 0.5. The
Application details
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