RT8230A/B/C/D/E
20
DS8230A/B/C/D/E-04 February 2014www.richtek.com
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Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
voltage and switching frequency simultaneously. The on-
time decreasing has a limitation and the output voltage
will be lifted up under the slight load condition. The
controller will turn on LGATE first to pull down the output
voltage. When the output voltage is pulled down to the
balance point of the output load current, the controller will
proceed the short on-time sequence as the above
description.
Linear Regulators (LDOx)
The RT8230A/B/C/D/E includes 5V (LDO5) and 3.3V
(LDO3) linear regulators. The regulators can supply up to
100mA for external loads. Bypass LDOx with a minimum
4.7μF ceramic capacitor. When V
OUT1
is higher than the
switch over threshold (4.66V), an internal 1.5Ω P-MOSFET
switch connects BYP1 to the LDO5 pin while
simultaneously disconnects the internal linear regulator.
Current Limit Setting (ENTRIPx)
The RT8230A/B/C/D/E has cycle-by-cycle current limit
control. The current limit circuit employs a unique “valley”
current sensing algorithm. If the magnitude of the current
sense signal at PHASEx is above the current limit
threshold, the PWM is not allowed to initiate a new cycle
(Figure 2). The actual peak current is greater than the
current limit threshold by an amount equal to the inductor
ripple current. Therefore, the exact current limit
characteristic and maximum load capability are a function
of the sense resistance, inductor value, battery and output
voltage.
Figure 2. “Valley” Current Limit
The RT8230A/B/C/D/E uses the on resistance of the
synchronous rectifier as the current sense element and
supports temperature compensated MOSFET R
DS(ON)
sensing. The R
ILIM
resistor between the ENTRIPx pin and
GND sets the current limit threshold. The resistor R
ILIM
is
connected to a current source from ENTRIPx
which is
5μA (typ.) at room temperature. The current source has a
4700ppm/°C temperature slope to compensate the
temperature dependency of the R
DS(ON)
. When the voltage
drop across the sense resistor or low-side MOSFET
equals 1/10 the voltage across the R
ILIM
resistor, positive
current limit will be activated. The high-side MOSFET will
not be turned on until the voltage drop across the MOSFET
falls below 1/10 the voltage across the R
ILIM
resistor.
Choose a current limit resistor according to the following
equation :
V
LIMIT
= (R
LIMIT
x 5μA) / 10 = I
LIMIT
x R
DS(ON)
R
LIMIT
= (I
LIMIT
x R
DS(ON)
) x 10 / 5μA
Carefully observe the PC board layout guidelines to ensure
that noise and DC errors do not corrupt the current sense
signal at PHASEx and GND. Mount or place the IC close
to the low-side MOSFET.
Charge Pump (SECFB)
The external 14V charge pump is driven by LGATEx
(LGATE1 for RT8230B/E, LGATE2 for RT8230C/D). As
shown in Figure 3, when LGATEx is low, C1 will be charged
by V
OUT1
through D1. C1 voltage is equal to V
OUT1
minus
the diode drop. When LGATEx becomes high, C1 transfers
the charge to C2 through D2 and charges C2 voltage to
V
LGATEX
plus C1 voltage. As LGATEx transitions low on
the next cycle, C3 is charged to C2 voltage minus a diode
drop through D3. Finally, C3 charges C4 through D4 when
LGATEx switches high. Thus, the total charge pump
voltage, V
CP
, is :
V
CP
= V
OUT1
+ 2 x V
LGATEx
− 4 x V
D
where V
LGATEx
is the peak voltage of the LGATEx driver
which is equal to LDO5 and V
D
is the forward voltage
dropped across the Schottky diode.
The SECFB pin in the RT8230B/C/D/E is used to monitor
the charge pump via a resistive voltage divider to generate
approximately 14V DC voltage and the clock driver uses
V
OUT1
as its power supply. In the Figure 3 when SECFB
drops below its feedback threshold, an ultrasonic pulse
will occur to refresh the charge pump driven by LGATEx.
If there is an overload on the charge pump in which SECFB
can not reach more than its feedback threshold, the
I
L
t
I
PEAK
I
LOAD
I
LIMIT
