RT7241A
time to discharge the output capacitor to the level that
requires the next “ON” cycle. In reverse, when the output
current increases from light load to heavy load, the
switching frequency increases to the preset value as the
inductor current reaches the continuous conduction. The
transition load point to the light load operation is shown in
Figure 3. and can be calculated as follows :
VTT Linear Regulator
The RT7241Aintegrates a high performance low dropout
linear regulator that is capable of sourcing and sinking
currents up to 1.5A. This VTT linear regulator employs
ultimate fast response feedback loop so that small ceramic
capacitors are enough for keeping track of VDDQ/2 within
40mV at all conditions, including fast load transient. To
achieve tight regulation with minimum effect of wiring
resistance, a remote sensing terminal, VTTSNS, should
be connected to the positive node of the VTT output
capacitor(s) as a separate trace from the VTT pin. For
stable operation, total capacitance of the VTT output
terminal can be equal to or greater than 10μF.
IL
Slpoe = (VIN - VOUT) / L
IPEAK
ILOAD = IPEAK / 2
Current Limit Setting for VDDQ (CS)
The RT7241A current limit is adjustable by CS pin and it
is a cycle-by-cycle “valley” type, measuring the inductor
current through the synchronous rectifier during the off-
time while the inductor current ramps down. The current
is determined by measuring the voltage between source
and drain of the synchronous rectifier, adding temperature
compensation for greater accuracy. If the current exceeds
the current limit, the on-time one-shot is inhibited until
the inductor current ramps down below the current limit.
Thus, only when the inductor current is well below the
current limit, another on-time is permitted. If the output
current exceeds the available inductor current (controlled
by the current limit mechanism), the output voltage will
drop. If it drops below the output under-voltage protection
level (see next section) the IC will stop switching to avoid
excessive heat.
t
tON
Figure 3. Boundary Condition of CCM/DEM
(V V
)
IN
OUT
I
t
ON
LOAD
2L
where tON is the on-time.
The switching waveforms may appear noisy and
asynchronous when light load causes diode emulation
operation. This is normal and results in high efficiency.
Trade offs in DEM noise vs. light load efficiency is made
by varying the inductor value.Generally, low inductor values
produce a broader efficiency vs. load curve, while higher
values result in higher full load efficiency (assuming that
the coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values include
larger physical size and degraded load transient response
(especially at low input voltage levels).
The RT7241A also includes a negative current limit to
protect the IC against sinking excessive current and
possibly damaging the IC. If the voltage across the
synchronous rectifier indicates the negative current is too
high, the synchronous rectifier turns off until after the next
high side on-time.
During discontinuous switching, the on-time is immediately
increased to add “hysteresis” to discourage the IC from
switching back to continuous switching unless the load
increases substantially. The IC returns to continuous
switching as soon as an on-time is generated before the
inductor current reaches zero. The on-time is reduced back
to the length needed for 500kHz switching and encouraging
the circuit to remain in continuous conduction, preventing
repetitive mode transitions between continuous switching
and discontinuous switching.
The RT7241Aprovides adjustable OCP setting via change
the RCS to decide the current limit. The current limit can
be derived by the following equation :
I
LIM = 2.64 x 106/RCS
The default setting of RCS is 165kΩ, which means current
limit is 16A. The maximum current limit should be lower
than 20A.
Copyright 2017 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7241A-02 January 2017
www.richtek.com
13
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