L6731D STMicroelectronics, L6731D Datasheet - Page 17

• Manufacturer Part Number: L6731D
• Description: IC CTRLR ADJ STPDN SYNC 16-TSSOP
• Manufacturer: STMicroelectronics

Specifications of L6731D

• Applications: Controller, DDR
• Voltage - Input: 1.8 ~ 14 V
• Number Of Outputs: 1
• Voltage - Output: Adjustable down to 0.6V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Output Voltage: 0.6 V to 18 V
• Output Current: 30 A
• Input Voltage: 1.8 V to 14 V
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Package: HTSSOP16
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

L6731D
6
6.1
6.2
Application details
Inductor design
The inductance value is defined by a compromise between the transient response time, the
efficiency, the cost and the size. The inductor has to be calculated to sustain the output and
the input voltage variation to maintain the ripple current (∆I
maximum output current. The inductance value can be calculated with the following
relationship:
Equation 2
Where F
voltage. Increasing the value of the inductance reduces the ripple current but, at the same
time, increases the converter response time to a load transient. If the compensation network
is well designed, during a load transient the device is able to set the duty cycle to 100 % or
to 0 %. When one of these conditions is reached, the response time is limited by the time
required to change the inductor current. During this time the output current is supplied by
the output capacitors. Minimizing the response time can minimize the output capacitor size.
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 in 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):
Equation 3
Moreover, there is an additional drop due to the effective capacitor discharge or charge that
is given by the following formulas:
Equation 4
Equation 5
Formula (4) is valid in case of positive load transient while the formula (5) is valid in case of
negative load transient. D
SW
is the switching frequency, Vin is the input voltage and Vout is the output
∆
Vout
MAX
∆
COUT
Vout
is the maximum duty cycle value that in the L6731D is 100%.
∆
=
COUT
L
Vout
2
≅
⋅
Cout
Vin
Fsw
=
ESR
2
−
=
⋅
⋅
∆
∆ ⋅
Cout
Vout
(
Vin
Iout
∆
I
Iout
∆
L
,
Iout
min
2
⋅
⋅
Vout
Vout
⋅
⋅
Vin
L
ESR
2
⋅
⋅
D
L
max
L
) between 20 % and 30 % of the
−
Vout
)
Application details
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