ISL6211 Datasheet, PDF (12/16 Pages) Intersil Corporation – Crusoe™ Processor Core-Voltage Regulator

ISL6211
Over-Current Protection
The converter uses the lower MOSFET’s on-resistance,
R_DS(ON), to monitor the current for protection against
shorted outputs. The sensed current from the ISEN pin is
compared with a current set by a resistor connected from
the OCSET pin to the ground,.
R_OCSET = ¹----¹---^.--²-_I--_O-^V----_C-^•----_•(---^R-_R---^C--_D---^S-_S----+₍--_O---¹-_N--⁰---₎⁰----^Ω-----⁾
Where, I_OC is a desired over-current protection threshold
and R_CS is the value of the current sense resistor connected
to the I_SEN pin.
If the lower MOSFET current exceeds the over-current
threshold, a pulse skipping circuit is activated. The upper
MOSFET will not be turned on as long as the sensed
current is higher then the threshold value. This limits the
current supplied by the DC voltage source. This condition
keeps on for eight clock cycles after the over-current
comparator was tripped for the first time. If after these first
eight clock cycles the current exceeds the over-current
threshold again in a time interval of another eight clock
cycles, the overcurrent protection latches and disables the
chip. If the over-current condition goes away during the first
eight clock cycles, normal operation is restored and the
over-current circuit resets itself sixteen clock cycles after
the over-current threshold was exceeded the first time,
Figure 12.
1
IL
2
PGOOD
8 CLK
VOUT
SHUTDOWN
3
CH1 5.0V
CH3 2.0AΩ
CH2 100mV
M 10.0µs
FIGURE 12. OVER-CURRENT PROTECTION WAVEFORMS
Over-Voltage Protection
Should the output voltage increase over 1.9V due to an
upper MOSFET failure, or for other reasons, the over-
voltage protection comparator will force the synchronous
rectifier gate driver high. This action actively pulls down the
output voltage and eventually attempts to blow the battery
fuse. As soon as the output voltage drops below the
threshold, the OVP comparator is disengaged.
This OVP scheme provides a ‘soft’ crowbar function which
helps to tackle severe load transients and does not invert the
output voltage when activated -- a common problem for OVP
schemes with a latch.
Over-Temperature Protection
The chip incorporates an over temperature protection circuit
that shuts the chip down when a die temperature of 150^oC is
reached. Normal operation is restored at die temperature
below 125^oC with a full soft-start cycle.
Design Procedure and Component
Selection Guidelines
As an initial step, define operating voltage range and
minimum and maximum load currents for the controller.
Output Inductor Selection
The minimum practical output inductor value is the one that
keeps inductor current just on the boundary of continuous
conduction at some minimum load. The industry standard
practice is to choose the minimum current somewhere from
10% to 25% of the nominal current. At light load, the
controller can automatically switch to hysteretic mode of
operation to sustain high efficiency. The following equations
help to choose the proper value of the output filter inductor.
∆I = 2 ⋅ I_min
∆I = -^∆----^V----^o----^u----^t
ESR
L=
V_-----i_--n_{-----–----}V_-----o_---u_----t
Fs × ∆I
×
V_-----o_----u_----t
Vin
If the load step is strong enough to pull output voltage
lower than the under-voltage threshold, the chip shuts
down immediately.
Because of the nature of the current sensing technique,
and to accommodate wide range of the R_DS(ON) variation,
the value of the over-current threshold should represent
overload current about 150%...180% of the nominal value.
If accurate current protection is desired, current sense
resistor placed in series with the lower MOSFET source can
optionally be used.
12