XRP6142 Datasheet, PDF (9/17 Pages) Exar Corporation – Synchronous Step-Down Controller with DDR

XRP6142
Synchronous Step-Down Controller with DDR
Memory Termination
to be recycled in order for IC to resume
operation.
OVERCURRENT PROTECTION (OCP)
OCP function is implemented by monitoring
the voltage across the low-side FET when it is
on. OCP is programmed via a resistor RLIM
connected between ILIM and SW pins. An
internal constant-current source ILIM (50uA
nominal) establishes a voltage across RLIM.
This voltage sets the trip point of the OCP
comparator. If the OCP comparator is
triggered for eight consecutive switching
cycles, then a hiccup timeout, as described in
the next section, is initiated. Calculate RLIM
from:
RLIM
=
⎢⎣⎡⎜⎝⎛ ^{IOCP +}
ΔIL
2
⎟⎞
⎠
×
R_DS
(ON
)
⎤
⎥⎦
+
20mV
42.5μA
110ms, following which, a soft-start is
attempted. If the OCP condition is still present,
then the timeout and soft-start cycle repeat.
This is referred to as hiccup timeout.
PROGRAMMING VOUT
A pair of output resistors is used to set the
output voltage V_OUT. Calculate R1 from:
R1 =
R2_{⎜⎜⎝⎛}
V_OUT
V_REF
−1_{⎟⎟⎠⎞}
Where:
R2 is nominally set at 10k (bottom resistor)
V_REF is reference voltage (0.5V)
Note that V_OUT must contain some voltage
ripple in order for XRP6142 to regulate the
output. Since XRP6142 regulates the bottom
of the output ripple the average value will be
higher (see figure 16).
Where:
IOCP is the output current at which
overcurrent protection is activated (usually set
20% above maximum I_OUT)
∆IL is inductor current ripple nominally set at
30% of I_OUT
R_DS(ON) is the maximum rated on resistance of
the FET
20mV is the OCP comparator offset spec
42.5μA is the minimum spec of the ILIM
source
The actual IOCP is 50% to 100% higher than
expected IOCP as seen in figure 11. This is
because RLIM in the above equation is
calculated based on worst case parameters.
A temperature coefficient of 0.3%/°C has been
designed into ILIM. This useful feature nulls
out the positive temperature coefficient of the
FET R_DS(ON) to a first order. Thus IOCP should
be largely independent of operating
temperature.
HICCUP TIMEOUT
When an over current condition is detected,
the internal FET drivers are turned off for
Fig. 16: V_OUT Voltage Ripple
V_OUT can be programmed more precisely from:
R1 =
R2_{⎜⎜⎝⎛} ^VOUT
−
(0.5 ×V_OUT
V_REF
,
ripple)
−
1_{⎟⎟⎠⎞}
Where:
V_OUT , ripple = ΔIL × ESR
ESR is the output capacitor’s Equivalent Series
Resistance.
OUTPUT CAPACITOR
C_OUT is the most critical component for proper
operation, since the XRP6142 relies on V_OUT
© 2010 Exar Corporation
Page 9 of 17
Rev. 1.0.0