RT6908
VI/O Buck Soft-Start
The recommended value for R11 should be up to 10kΩ
without some sacrificing. Place the resistor divider as
close as possible to the chip can reduce noise sensitivity.
The output voltage also can be adjusted from −7% to 7%
by setting the I2C register 04h [3:0].
The step-down converter has an internal soft-start to reduce
the input inrush current. When the buck converter is
enabled, the output voltage rises slowly from zero to the
regulated voltage. The typical soft-start time is around
300μs.
VCORE Buck Soft-Start
VI/O Buck Over Current Protection
The synchronous buck converter has an internal soft-start
to reduce the input inrush current. When the converter is
enabled, the output voltage rises slowly from zero to the
regulated voltage. The typical soft-start time is around
300μs.
The IC senses the inductor current that is flowing out the
LXB1 pin. The internal MOSFET will be turned off if the
peak inductor current reaches 3.8A (typ.).
VI/O Buck Short Circuit Protection
VCORE Buck Over Current Protection
To limit the short circuit current, the device has a cycle-
by-cycle current limit. To avoid the short circuit current
from rising above the internal current limit when the output
is shorted toGND, the switching frequency is reduced as
well. The switching frequency is reduced to one-half of
original frequency when the output voltage is below 80%
and to one-fourth of the original frequency when the output
voltage is below 20%. If the “short” is removed, the buck
converter will resume operation. If the voltage remains
below 80% after 50ms, the IC will shut down.
The IC has a cycle-by-cycle low side source current
sensing algorithm that uses the on-resistance of the low
side MOSFET as a current sensing element, so that costly
sense resistors are not required. TheDHB2 andDLB2 will
be turned off if the low side MOSFET source current
reaches setting value. Moreover, IC will restart after 50ms
if over current remains 7 cycles. Low side source peak
current limit threshold is 1/8 voltage at the ILIMT2 pin.
Meanwhile, the real current limit value need consider the
on-resistance of the low side MOSFET.
VI/O Buck Loop Compensation
V
ILIMIT2
I
=
(A)
LIM_M2
The voltage feedback loop can be compensated with an
external compensation network consisted of the R4 and
C19. Choose R4 to set high frequency integrator gain for
fast transient response. And choose the C19 to set the
integrator zero to maintain stability.
8×R (switch)
ON
when ILIM_M2 is “Low side switch current limit”.
VCORE Buck Short Circuit Protection
To limit the short circuit current, the device has a cycle-
by-cycle current limit. To avoid the short circuit current
from low side MOSFET source current limit when the
output is shorted to GND, the switching operation will be
stop. The switching operation will stop when the output
voltage is below 20%. If the short is removed, the buck
converter will resume operation. If the voltage remains
below 80% after 50ms, the IC will shut down.
VCORE Synchronous Buck Controller
The synchronous buck controller is a high efficiency PWM
architecture with 500kHz operation frequency and fast
transient response. The controller need external high side
and low sideN-MOSFET as synchronous rectifier and does
not required Schottky diode on the LXB2 pin. The high
side MOSFET is connected between VIN and the LXB2
pin, while the low side MOSFET is connected between
the LXB2 pin andGND.
VCORE Buck Loop Compensation
The voltage feedback loop can be compensated with an
external compensation network consisted of R23 and C25
and C26. Choose R23 to set high frequency integrator
gain for fast transient response, C25 and C26 to set the
integrator zero to maintain stability.
VCORE Buck Output Voltage Setting
The regulated default output voltage as shown in the
following equation :
R11
R12
⎛
⎝
⎞
⎟
⎠
VCORE = VFBB2 × 1 +
, where VFBB2 = 0.8V (typ.)
⎜
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
18
DS6908-01 March 2013
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
