l6712a STMicroelectronics, l6712a Datasheet - Page 13

• Manufacturer Part Number: l6712a
• Description: Two-phase Interleaved Dc/dc Controller
• Manufacturer: STMicroelectronics

2.Constant Current Operation
This happens when ON time limitation is reached after the current in each phase reaches I
35µA).
The device enters in Quasi-Constant-Current operation: the low-side mosfets stays ON until the current
read becomes lower than I
ON with a TON imposed by the control loop at the next available clock cycle and the device works in the
usual way until another OCP event is detected.
This means that the average current delivered can slightly increase also in Over Current condition since
the current ripple increases. In fact, the ON time increases due to the OFF time rise because of the current
has to reach the I
When this happens, the device works in Constant Current and the output voltage decrease as the load
increase. Crossing the UVP threshold causes the device to reset.
Figure 6 shows this working condition.
It can be observed that the peak current (I
Where V
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
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
Figure 6. Constant Current operation
Over current is set anyway when I
to work with convenient values for I
onMAX
) while the OFF time depends on the application:
outMIN
TonMAX
I
peak
is the minimum output voltage (VID-40% as follow).
a) Maximum current for each phase
OCPx
=
T
bottom. The worst-case condition is when the ON time reaches its maximum value.
I
OCPx
OFF
I
OCPx
MAX,TOT
=
+
V
------------------------------------- - Ton
L
(I
TonMAX
IN
INFOx
⋅
INFOx
Ipeak I
------------------------------------- -
–
FB
Vout
=
L
< 35µA) skipping clock cycles. The high side mosfets can be turned
. Since the OCP intervention threshold is fixed, to modify the per-
V
2 I
reaches 35µA (I
OUt
–
⋅
peak
min
MAX
OCPx
) is greater than the I
⋅
=
2
MAX
⋅
§
©
I
OCPx
FB
=
MIN
Ipeak
I
I
=70µA). The full load value is only a convention
MAX
OCPx
I
OCPx
).
+
Ipeak I
------------------------------------- -
f
=
+
OCPx
----------------------------------------- -
T
V
-------------------------------------- - 0.40 T
ONmax
Vout
UVP
(I
IN
DROOP
2
–
but it can be determined as follow:
–
OCPx
Vout
=50 µ A)
b) Output Characteristic
L
1
+
T
·
¹
MIN
OFF
2·I
⋅
OCPx
Droop effect
(I
DROOP
L6712A L6712
⋅
I
MAX,TOT
=70 µ A)
OCPx
Iout
(I
INFOx
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