RT8208BGQW Datasheet, PDF (13/18 Pages) Richtek Technology Corporation – Resistor Programmable Output Voltage from 0.75V to 3.3V with Integrated Transition Support

RT8208A/B
For an up transition (from lower to higher VOUT) as shown
in Figure5, the Gx change affects Dx and causes FB to
drop below the 0.75V trip point. This quickly trips the FB
comparator regardless of whether DEM is active or not,
generating an UGATE on-time and a subsequent LGATE
will be turned on. At the end of the minimum off-time
(400ns), if FB is still below 0.75V then another UGATE
on-time is started. This sequence continues until the FB
pin exceeds 0.75V.
Gx
GND
FB
UGATE
FB
Threshold
LGATE
VOUT
Minimum
off-time
Final V_OUT
Initial V_OUT
Figure 6. Output Voltage Up Transition
with Overshooting
If the VOUT change is significant, there can be several
consecutive cycle of UGATE on-time followed by minimum
LGATE time. This can cause a rapid increase in inductor
current: typically it takes only a few switching cycles for
inductor current to rise up to the current limit. At some
point the FB voltage will rise up to the 0.75V reference
and the UGATE pulses will cease, but the inductor’ s LI²
energy must then flow into the output capacitor. This can
create a significant overshoot as shown in Figure6.
The overshooting can be approximated by the following
equation, where I_CL is the current limit, V_FINAL is the
desired set point for the final voltage, L is in μH and C_OUT
is in μF.
V_MAX =
⎛
⎜⎜⎝
(I^{CL2 × L}
C_OUT
)
+
V_FIANL2
⎞
⎟⎟⎠
The Overshoot eliminator (Patent Pending) prevents output
voltage overshooting after rapid changes of Gx. This results
in a gradual change from V_OUT(INITIAL) to V_OUT(FINAL) and
prevents the buildup of high inductor current and reducing
overshoot.
Current Limit Setting (OCP)
RT8208A/B has cycle-by-cycle current limiting control.
The current limit circuit employs a unique “valley” current
sensing algorithm. If the magnitude of the current sense
signal at CS is above the current limit threshold, the PWM
is not allowed to initiate a new cycle (Figure 7).in order to
provide both good accuracy and a cost effective solution,
the RT8208A/B supports temperature compensated
MOSFET R_DS(ON) sensing. The CS pin should be
connected to GND through the trip voltage setting resistor,
R_CS. The CS terminal source 10μA I_CS current, and the
trip level is set to the CS trip voltage, V_CS as in the following
equation.
V_CS(mV) = R_CS(kΩ) x 10( μA)
Inductor current is monitored by the voltage between the
PGND pin and the PHASE pin. So the PHASE pin should
be connected to the drain terminal of the low-side
MOSFET. I_CS has temperature coefficient to compensate
the temperature dependency of the R_DS(ON). PGND is used
as the positive current sensing node. So PGND should
be connected to the source terminal of the bottom
MOSFET.
As the comparison is done during the OFF state, V_CS
sets the valley level of the inductor current. Thus, the
load current at over current threshold, I_{LOAD_OC}, can be
calculated as follows ;
I_{LOAD_OC}
=
VCS
RDS(ON)
+
IRipple
2
= ^VCS + ¹ × ^{( VIN − VOUT ) × VOUT}
R_DS(ON) 2 × L × f
V_IN
In an over current condition, the current to the load exceeds
the current to the output capacitor thus the output voltage
tends to fall. Eventually, it crosses the under-voltage
protection threshold and shutdown.
DS8208A/B-04 May 2011
www.richtek.com
13