ISL62871 Datasheet, PDF (14/25 Pages) Intersil Corporation – PWM DC/DC Controller With VID Inputs For Portable GPU Core-Voltage Regulator

ISL62871, ISL62872
Where:
- t_VS is the voltage-step delay
- V_NEW is the new setpoint voltage
- V_OLD is the setpoint voltage that V_NEW is changing
from
- I_VS is the ±100µA setpoint voltage-step current; positive
when V_NEW > V_OLD, negative when V_NEW < V_OLD
- R_T is the sum of the R_SET programming resistors
Fault Protection
Overcurrent
The overcurrent protection (OCP) setpoint is programmed
with resistor R_OCSET which is connected across the OCSET
and PHASE pins. Resistor R_O is connected between the VO
pin and the actual output voltage of the converter. During
normal operation, the VO pin is a high impedance path,
therefore there is no voltage drop across R_O. The value of
resistor R_O should always match the value of resistor
R_OCSET
PHASE
L
DCR
I_L
+
V_DCR
_
R_OCSET
C_SEN
10µ
OCSET
+ ^{_ V}ROCSET
R_O
VO
V_O
C_O
FIGURE 10. OVERCURRENT PROGRAMMING CIRCUIT
Figure 10 shows the overcurrent set circuit. The inductor
consists of inductance L and the DC resistance DCR. The
inductor DC current I_L creates a voltage drop across DCR,
which is given by Equation 22:
V_DCR = I_L ⋅ DCR
(EQ. 22)
The I_OCSET current source sinks 10µA into the OCSET pin,
creating a DC voltage drop across the resistor R_OCSET,
which is given by Equation 23:
V_ROCSET = 10μA ⋅ R_OCSET
(EQ. 23)
The DC voltage difference between the OCSET pin and the
VO pin, which is given by Equation 24:
V_OCSET– V_VO = V_DCR– V_ROCSET =I_L ⋅DCR – I_OCSET⋅ R_OCSET
(EQ. 24)
The IC monitors the voltage of the OCSET pin and the VO
pin. When the voltage of the OCSET pin is higher than the
voltage of the VO pin for more than 10µs, an OCP fault
latches the converter off.
Component Selection For R_OCSET and C_SEN
The value of R_OCSET is calculated with Equation 25, which
is written as:
R_OCSET
=
I_{--O-----C-----}⋅_---D_-----C_-----R_---
I_OCSET
(EQ. 25)
Where:
- R_OCSET (Ω) is the resistor used to program the
overcurrent setpoint
- I_OC is the output DC load current that will activate the
OCP fault detection circuit
- DCR is the inductor DC resistance
For example, if I_OC is 20A and DCR is 4.5mΩ, the choice of
R_OCSET is = 20A x 4.5mΩ/10µA = 9kΩ.
Resistor R_OCSET and capacitor C_SEN form an R-C network
to sense the inductor current. To sense the inductor current
correctly not only in DC operation, but also during dynamic
operation, the R-C network time constant R_OCSET C_SEN
needs to match the inductor time constant L/DCR. The value
of C_SEN is then written as Equation 26:
C_SEN
=
--------------------^L----------------------
R_OCSET ⋅ DCR
(EQ. 26)
For example, if L is 1.5µH, DCR is 4.5mΩ, and R_OCSET is
9kΩ, the choice of C_SEN = 1.5µH/(9kΩ x 4.5mΩ) = 0.037µF.
When an OCP fault is declared, the PGOOD pin will
pull-down to 35Ω and latch off the converter. The fault will
remain latched until the EN pin has been pulled below the
falling EN threshold voltage V_ENTHF or if VCC has decayed
below the falling POR threshold voltage ^V_{VCC_THF}.
Overvoltage
The OVP fault detection circuit triggers after the FB pin
voltage is above the rising overvoltage threshold V_OVRTH for
more than 2µs. For example, if the converter is programmed
to regulate 1.0V at the FB pin, that voltage would have to
rise above the typical V_OVRTH threshold of 116% for more
than 2µs in order to trip the OVP fault latch. In numerical terms,
that would be 116% x 1.0V = 1.16V. When an OVP fault is
declared, the PGOOD pin will pull-down to 65Ω and latch-off
the converter. The OVP fault will remain latched until VCC
has decayed below the falling POR threshold voltage
V_{VCC_THF. An OVP fault cannot be reset by pulling the EN}
pin below the falling EN threshold voltage V_ENTHF.
Although the converter has latched-off in response to an
OVP fault, the LGATE gate-driver output will retain the ability
to toggle the low-side MOSFET on and off, in response to
the output voltage transversing the V_OVRTH and V_OVFTH
thresholds. The LGATE gate-driver will turn-on the low-side
MOSFET to discharge the output voltage, protecting the
load. The LGATE gate-driver will turn-off the low-side
MOSFET once the FB pin voltage is lower than the falling
overvoltage threshold V_OVRTH for more than 2µs. The
falling overvoltage threshold V_OVFTH is typically 102%. That
14
FN6707.0
August 14, 2008