RT8205A RICHTEK [Richtek Technology Corporation], RT8205A Datasheet - Page 21

• Manufacturer Part Number: RT8205A
• Description: High Efficiency, Main Power Supply Controllers for Notebook Computers
• Manufacturer: RICHTEK [Richtek Technology Corporation]

driver waveform to become the complement of the high-
side gate-driver waveform. This in turn causes the inductor
current to reverse at light loads as the PWM loop to
maintain a duty ratio of V
CCM mode is to keep the switching frequency fairly
constant, but it comes at a cost : The no-load battery
current can be 10mA to 40mA, depending on the external
MOSFETs.
Reference and linear Regulators (REF, VREGx)
The 2V reference (REF) is accurate within ± 1% over
temperature, making REF useful as a precision system
reference. Bypass REF to GND with a 0.22uF (min)
capacitor. REF can supply up to 50uA for external loads.
Loading REF reduces the VOUTx output voltage slightly
because of the reference load-regulation error.
VREG5 regulator supplies total of 70mA for internal and
external loads, including MOSFET gate driver and PWM
controller. VREG3 regulator supplies up to 70mA for
external loads. Bypass VREG5 and VREG3 with a 4.7uF
(min) capacitor; use an additional 1uF per 5mA of internal
and external load.
When the 5V main output voltage is above the VREG5
switchover threshold, an internal 1.5Ω N-Channel MOSFET
switch connects VOUT1 to VREG5 while simultaneously
shutting down the VREG5 linear regulator. Similarly, when
the 3.3V main output voltage is above the VREG3
switchover threshold, an internal 1.5Ω N-Channel MOSFET
switch connects VOUT2 to VREG3 while simultaneously
shutting down the VREG3 linear regulator. It can decrease
the power dissipation from the same battery, because the
converted efficiency of SMPS is better than the converted
efficiency of linear regulator.
Current-Limit Setting (ENTRIPx)
The RT8205A/B/C 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 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
Copyright
DS8205A/B/C-06 July 2012
©
2012 Richtek Technology Corporation. All rights reserved.
OUT
/V
IN
. The benefit of forced-
characteristic and maximum load capability are a function
of the sense resistance, inductor value, and battery and
output voltage.
The RT8205A/B/C uses the on-resistance of the
synchronous rectifier as the current-sense element. Use
the worse-case maximum value for R
MOSFET datasheet, and add a margin of 0.5%/°C for the
rise in R
The R
the over current threshold. The resistor R
to a 10uA current source from ENTRIPx. When the voltage
drop across the sense resistor or low-side MOSFET
equals 1/10 the voltage across the R
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
Choose a current limit resistor by following equation :
V
R
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 (LG1_CP or SECFB)
The external 14V charge pump is driven by LGATE1 (Figure
3 and Figure 4). When LGATE1 is low, the C1 will be
charged by D1 from VOUT1. C1 voltage is equal to V
minus a diode drop. When LGATE1 transitions to high,
the charges from C1 will transfer to C2 through D2 and
charge it to V
on the next cycle, C2 will charge C3 to its voltage minus
a diode drop through D3. Finally, C3 charges C4 through
ILIM
ILIM
is a registered trademark of Richtek Technology Corporation.
= (R
= (I
ILIM
DS(ON)
ILIM
resistor between the ENTRIPx pin and GND sets
ILIM
I
0
L
Figure 2. “valley” Current-Limit
x R
LGATE1
x 10uA) / 10 = I
with temperature.
DS(ON)
plus VC1. As LGATE1 transitions low
) x 10 / 10uA
RT8205A/B/C
ILIM
x R
DS(ON)
ILIM
I
I
I
resistor, positive
L, peak
Load
LIM
ILIM
www.richtek.com
DS(ON)
ILIM
t
is connected
resistor.
from the
OUT1
21