L6918A STMicroelectronics, L6918A Datasheet - Page 16

• Manufacturer Part Number: L6918A
• Description: 5 BIT PROGRAMMABLE MULTIPHASE CONTROLLER
• Manufacturer: STMicroelectronics

L6918 L6918A
Figure 6. Constant Current operation
It can be observed that the peak current (Ipeak) is greater than the 140% but it can be determined as follow:
Where Vout
The device works in Constant-Current, and the output voltage decreases as the load increase, until the output
voltage reaches the under-voltage threshold (Vout
off, the FAULT pin is driven high and the device stops working. Cycle the power supply to restart operation. The
maximum average current during the Constant-Current behavior results:
In this particular situation, the switching frequency results reduced.
The ON time is the maximum allowed (T
Over current is set anyway when I
venient values for I
the load value, it can be simply considered that, for example, to have on OCP threshold of 170%, this will cor-
respond to I
INTEGRATED DROOP FUNCTION
The devices use the droop function to satisfy the requirements of high performance microprocessors, reducing
the size and the cost of the output capacitor.
This method "recovers" part of the drop due to the output capacitor ESR in the load transient, introducing a de-
pendence of the output voltage on the load current
As shown in figure 7, the ESR drop is present in any case, but using the droop function the total deviation of the
output voltage is minimized. In practice the droop function introduces a static error proportional to the output
current that can be represented by an equivalent output resistance R
rent mode regulation, the information about the total current delivered is used to implement the Droop Function.
This current (equal to the sum of both I
and Vout, the total current information flows only in this resistor because the compensation network between
16/35
INFOx
MIN
is the minimum output voltage (UVP threshold).
= 35 A (I
TonMAX
FB
Ipeak
. Since the OCP intervention threshold is fixed, to modify the percentage with respect to
FB
=
I
= 70 A). The full load current will then correspond to I
OCPx
T
OF F
I
MA X T OT
INFOx
+
=
,
TonMAX
V
------------------------------------ - Ton
L
IN
INFOx
reaches 35 A. The full load value is only a convention to work with con-
onMAX
–
Ipea k I
------------------------------------- -
=
Vout
L
2 I
) is sourced from the FB pin. Connecting a resistor between this pin
V
O UT
) while the OFF time depends on the application:
MAX
–
min
OCPx
MIN
=
2
). When this threshold is crossed, all mosfets are turned
MAX
I
O CPx
Ipeak
I
I
MAX
OCPx
=
f
I
O CPx
+
=
Ipeak I
------------------------------------- -
------------------------------------------
Ton
Vout
UVP
+
OUT
MAX
V
------------------------------------- - 0.5 T
2
–
IN
. Since the device has an average cur-
1
OCP x
–
+
(I
Vout
T
FB
L
=50 A)
OF F
Droop effect
MIN
INFOx
(I
2·I
FB
=70 A
OCPx
= 20.5 A (I
I
MAX,TO
Iout
FB
= 41 A).