ISL6217A_14 Datasheet, PDF (13/20 Pages) Intersil Corporation – Precision Multi-Phase Buck PWM Controller for Intel, Mobile Voltage Positioning IMVP-IV and IMVP-IV+

ISL6217A
Overcurrent Setting - OCSET
The ISL6217A overcurrent protection essentially compares a
user-selectable overcurrent Threshold to the scaled and
sampled output current. An overcurrent condition is defined
when the sampled current is equal to or greater than the
threshold current. A step by step process to design for the
user-desired overcurrent set point is detailed next.
STEP 1: SETTING THE OVERCURRENT THRESHOLD
The overcurrent threshold is represented by the DC current
flowing out of the OCSET pin (See Figure 8). Since the
OCSET pin is held at a constant 1.75V, the user need only
populate a resistor from this pin to ground to set the desired
overcurrent threshold, I_OCSET. The user should pick a value
of I_OCSET between 10µA and 25µA. Once this is done, use
Ohm’s Law to determine the necessary resistor to place from
OCSET to ground:
R_OCSET
=
1.75V
I_OCSET
= R₁ + R₂ + R₃
(EQ. 2)
For example, if the desired overcurrent threshold is 15µA,
the total resistance from OCSET must equal 117kΩ.
STEP 2: SELECTING ISEN RESISTANCE FOR DESIRED
OVERCURRENT LEVEL
After choosing the I_OCSET level, the user must then decide
what level of total output current is desired for overcurrent.
Typically, this number is between 150% and 200% of the
maximum operating current of the application. For example,
if the max operating current is 46A, and the user chooses
150% overcurrent, the ISL6217A will shut down if the output
current exceeds 46A*1.5 or 69A. According to the Block
Diagram, the equation below should be used to determine
R_ISEN once the overcurrent level, I_OC, is chosen.
R_ISEN
=
I_OC
⋅
r_(DSON)
M
⋅ 0.2175
(I_OCSET + 2µA) ⋅ N − 4µA
− 130
(EQ. 3)
In Equation 3, M represents the number of Low-Side
MOSFETs in one channel, and N represents the number of
channels. Using the examples above (I_OC = 69A,
I_OCSET = 15µA) and substituting the values M = 2, N = 2,
r_DS(ON) = 6mΩ, R_ISEN is calculated to be 1.5kΩ.
STEP 3: THERMAL COMPENSATION FOR r_DS(ON) (IF
DESIRED)
If PTCs are used for thermal compensation, then RISEN is
found using the room temperature value of r_DS(ON). If
standard resistors are used for RISEN, then the “HOT” value
of r_DS(ON) should be used for this calculation.
MOSFET r_DS(ON) sensing provides advantages in cost,
efficiency, and board area. However, if more precise current
feedback is desired, a discrete Precision Current Sense
Resistor, RPOWER, may be inserted between the SOURCE
of each channels lower MOSFET and ground. The small
RISEN resistor, as described above, is then replaced with a
standard 1% resistor and connected from the ISEN pin of the
ISL6217A controller to the SOURCE of the lower MOSFET.
13
FN9107.3
June 30, 2005