PM6680A STMicroelectronics, PM6680A Datasheet - Page 25

• Manufacturer Part Number: PM6680A
• Description: IC CTLR DUAL SYNC STDN 5X5VFQFPN
• Manufacturer: STMicroelectronics

Specifications of PM6680A

• Applications: Controller, Embedded Computer System
• Voltage - Input: 6 ~ 36 V
• Number Of Outputs: 2
• Voltage - Output: 0.9 ~ 5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-VFQFN, 32-VFQFPN
• For Use With: 497-6379 - BOARD EVALUATION FOR PM6680A497-6378 - BOARD EVALUATION FOR PM6680497-6425 - BOARD EVAL BASED ON PM6680A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PM6680A
7.7
Being fixed the valley threshold, the greater the current ripple is, greater the DC output
current is The valley current limit can be set with resistor RCSENSE:
Equation 7
Where I
Consider the temperature effect and the worst case value in RDSon calculation.
The accuracy of the valley current threshold detection depends on the offset of the internal
comparator (∆V
Equation 8
Where RSNS is the sensing element(RDSon)
PM6680A provides also a fixed negative peak current limit to prevent an excessive reverse
inductor current when the switching section sinks current from the load in PWM mode. This
negative current limit threshold is measured between PHASE and SGND pins, comparing
the magnitude drop on the PHASE node during the conduction time of the low side
MOSFET with an internal fixed voltage of 120 mV.
The negative valley-current limit I
Equation 9
Soft start and soft end
Each switching section is enabled separately by asserting high EN1/EN2 pins respectively.
In order to realize the soft start, at the startup the overcurrent threshold is set 25 % of the
nominal value and the undervoltage protection (see related sections) is disabled. The
controller starts charging the output capacitor working in current limit. The overcurrent
threshold is increased from 25 % to 100 % of the nominal value with steps of 25 % every
700 µs (typ.). After 2.8 ms (typ.) the undervoltage protection is enabled. The soft start time is
not programmable. A minimum capacitor C
overshoot on the output:
Equation 10
CSENSE
∆
I
I
Lvalley
Lvalley
OFF
= 100 µA, RDSon is the drain-source on resistance of the low side switch.
=
) and on the accuracy of the current generator (∆I
∆
I
I
CSENSE
CSENSE
+
R
NEG
C
⎡
⎢
⎣
CSENSE
R
INT
CSENSE
(if the device works in PWM mode) is given by:
I
≥
NEG
∆
I
Lvalley
=
V
×
4
OFF
R
---------------------------------------------
=
INT
6
I
DS on
µ
120
CSENSE
A
+
R
is required to ensure a soft start without any
(
Icsense
DSon
∆
mV
2
)
I
L
×
×
I
×
Lvalley
100
C
out
⎤
⎥
⎦
+
∆
R
R
CSENSE
CSENSE
CSENSE
Device description
+
∆
R
)
R
SNS
SNS
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