E-L6919ETR STMicroelectronics, E-L6919ETR Datasheet - Page 17

• Manufacturer Part Number: E-L6919ETR
• Description: IC CTRLR 5BIT PROG 2PHASE 28SOIC
• Manufacturer: STMicroelectronics
• Type: Step-Down (Buck)r

Specifications of E-L6919ETR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 1.55 V
• Current - Output: 2A
• Frequency - Switching: 150kHz
• Voltage - Input: 5 ~ 12 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 28-SOIC (7.5mm Width)
• Power - Output: 2W
• Mounting Style: SMD/SMT
• Operating Supply Voltage: 6.5 V
• Maximum Operating Temperature: 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-4592-2

to reduce switching noise above all during load transient. Ceramic capacitor can also introduce benefits in high
frequency noise decoupling, noise generated by parasitic components along power path.
OUTPUT CAPACITOR
Since the microprocessors require a current variation beyond 50A doing load transients, with a slope in the
range of tenth A/ s, the output capacitor is a basic component for the fast response of the power supply.
Dual phase topology reduces the amount of output capacitance needed because of faster load transient response
(switching frequency is doubled at the load connections). Current ripple cancellation due to the 180° phase shift
between the two phases also reduces requirements on the output ESR to sustain a specified voltage ripple.
When a load transient is applied to the converter's output, for first few microseconds the current to the load is sup-
plied by the output capacitors. The controller recognizes immediately the load transient and increases the duty
cycle, 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):
A minimum capacitor value is required to sustain the current during the load transient without discharge it. The
voltage drop due to the output capacitor discharge is given by the following equation:
Where D
transient and the lower is the output voltage static ripple.
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
culated with this relationship:
Where f
The worst condition depends on the input voltage available and the output voltage selected. Anyway the worst
case is the response time after removal of the load with the minimum output voltage programmed and the max-
imum input voltage available.
Increasing the value of the inductance reduces the ripple current but, at the same time, reduces the converter
response time to a load transient. The response time is the time required by the inductor to change its current
from initial to final value. Since the inductor has not finished its charging time, the output current is supplied by
the output capacitors. Minimizing the response time can minimize the output capacitance required.
The response time to a load transient is different for the application or the removal of the load: if during the ap-
plication of the load the inductor is charged by a voltage equal to the difference between the input and the output
voltage, during the removal it is discharged only by the output voltage. The following expressions give approx-
imate response time for I load transient in case of enough fast compensation network response:
SW
MAX
is the switching frequency, V
is the maximum duty cycle value. The lower is the ESR, the lower is the output drop during load
L
between 20% and 30% of the maximum output current. The inductance value can be cal-
t
a pplic atio n
V
OUT
IN
=
=
L
is the input voltage and V
-----------------------------------------------------------------------------------
4 C
----------------------------- -
V
=
V
IN
OUT
V
----------------------------- -
L
–
OUT
f
IN
S W
V
= I
–
OUT
I
V
OUT
OUT
V
I
L
I
IN
2
OUT
· ESR
D
V
-------------- -
t
V
OUT
rem ov al
MAX
L
IN
OUT
–
V
OUT
=
is the output voltage.
-------------- -
V
L
OUT
I
L6919E
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