XRP6142EL2-0-F Datasheet, PDF (8/17 Pages) Exar Corporation – Synchronous Step-Down Controller with DDR Memory Termination

XRP6142
Synchronous Step-Down Controller with DDR
Memory Termination
THEORY OF OPERATION
The XRP6142 synchronous buck controller
utilizes the constant-on-time principle. The on-
time is internally set and is available in three
different set points to allow for different
frequency options. The XRP6142 automatically
adjusts the on-time during operation inversely
with the input voltage V_IN, to maintain a
constant frequency. Therefore, the switching
frequency is independent of the inductor and
capacitor size, unlike hysteretic controllers.
At the beginning of the cycle, the XRP6142
turns on the high-side FET for a fixed duration.
The on time is internally set and adjusted by
V_IN. At the end of the on time, the high-side
FET is turned off, for a predetermined
minimum off time (nominally 300ns). After
T_OFF-MIN has expired, the high-side FET will stay
off until the feedback comparator trip point of
0.5V has been reached. Then the high-side
FET turns on again and the cycle repeats. The
operation of the low-side FET is
complementary to the high-side FET. A short
dead-time prevents shoot-through from
occurring.
TIMING OPTIONS
Three versions of XRP6142 (Timing Options)
are identified by their on times at V_IN=3.3V.
For each version, T_ON is inversely proportional
to V_IN. The constant of proportionality K, is
shown in the table below. Variation of T_ON
versus V_IN is shown graphically in figures 4
and 5.
Part Number
XRP6142EL0.5-F
XRP6142EL1.0-F
XRP6142EL2.0-F
T_ON at V_IN=3.3V
0.5μs
1.0μs
2.0μs
K=T_ONxV_IN
(μs.V)
1.65
3.3
6.6
Note that for a Buck converter the switching
frequency is given by:
f = ^VOUT
V_IN × T_ON
Since for each XRP6142 Timing Option, the
product of V_IN and T_ON is a constant, then
frequency is determined by V_OUT as shown in
the following table.
V_OUT
0.8
1.0
1.2
1.5
1.8
2.5
3.3
f(kHz) for each Timing Option
0.5μs
485
606
727
909
1091
---
---
1.0μs
242
303
364
455
545
758
---
2.0μs
121
152
182
227
273
379
500
INTERNAL SOFT-START
Soft-start time is internally set at 5ms
(nominal). This removes the need for external
components associated with soft-start
function, and helps save cost and reduce PCB
space.
ENABLE
A precision enable function is provided (1.20V
±0.05V). EN should be tied to V_CC in
applications that do not require this function.
INTERNAL REFERENCE VOLTAGE
A high-precision 0.5V internal reference is
provided at the V_REF pin. This is normally tied
to the REFIN pin, thus setting the threshold of
the voltage comparator.
INTERNAL BOOTSTRAP DIODE
XRP6142 includes an internal low-Vf bootstrap
diode. Place a 0.1uF capacitor between BST
and SW pins to provide drive voltage for the
high-side FET.
UNDER-VOLTAGE LOCKOUT
UVLO monitors V_CC and ensures adequate
voltage exists before starting to switch the
FETs.
SHORT CIRCUIT PROTECTION
An internal short-circuit comparator monitors
the feedback voltage. If feedback voltage falls
below 65% of reference voltage (this is
equivalent to output voltage falling below 65%
of nominal value) the IC will latch off. V_CC has
© 2010 Exar Corporation
Page 8 of 17
Rev. 1.0.0