参数资料
型号: L6918D
厂商: STMICROELECTRONICS
元件分类: 稳压器
英文描述: SWITCHING CONTROLLER, 1200 kHz SWITCHING FREQ-MAX, PDSO28
封装: SO-28
文件页数: 8/35页
文件大小: 431K
代理商: L6918D
L6918 L6918A
16/35
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 VoutMIN is the minimum output voltage (UVP threshold).
The device works in Constant-Current, and the output voltage decreases as the load increase, until the output
voltage reaches the under-voltage threshold (VoutMIN). When this threshold is crossed, all mosfets are turned
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 (TonMAX) while the OFF time depends on the application:
Over current is set anyway when IINFOx reaches 35A. The full load value is only a convention to work with con-
venient values for IFB. Since the OCP intervention threshold is fixed, to modify the percentage with respect to
the load value, it can be simply considered that, for example, to have on OCP threshold of 170%, this will cor-
respond to IINFOx =35A(IFB =70A). The full load current will then correspond to IINFOx = 20.5A(IFB =41A).
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 ROUT. Since the device has an average cur-
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 IINFOx) is sourced from the FB pin. Connecting a resistor between this pin
and Vout, the total current information flows only in this resistor because the compensation network between
TonMAX
IOCPx
Ipeak
IMAX
Vout
Iout
(IFB=50
A)
2IOCPx
(IFB=70
A I
MAX,TO
UVP
Droop effect
I peak
I
OCPx
V
IN
Vout
min
L
------------------------------------- Ton
MAX
I
OCPx
V
IN
Vout
MI N
L
-------------------------------------- 0.5 T
+
=
+
=
I
MAX TOT
,
2I
MAX
2I
OCPx
I peak
I
OCP x
2
--------------------------------------
+
=
=
f
1
Ton
MAX
T
OFF
+
------------------------------------------
=
T
OFF
L
I pea k
I
OCPx
V
OUT
--------------------------------------
=
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