RT8205A RICHTEK [Richtek Technology Corporation], RT8205A Datasheet - Page 22

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

RT8205A/B/C
D4 when LGATE1 switched to high. So, V
V
Where V
equal to the VREG5; V
across the Schottky.
LG1_CP in the RT8205B (Figure 3) can be used as clock
signal for charge pump circuit to generate approximately
14V DC voltage and the clock driver uses VOUT1 as its
power supply, SECFB in the RT8205C is used to monitor
the charge pump through resistive divider (Figure 4). In an
event when SECFB dropped below 2V, the detection circuit
forces the high-side MOSFET off and the low-side
MOSFET on for 300ns to allow CP to recharge and SECFB
rise above 2V. In the event of an overload on CP where
SECFB can not reach more than 2V, the monitor will be
cancelled. Special care should be taken to ensure enough
normal voltage ripple on each cycle as to prevent CP shut-
down.
The SECFB pin has ~17mV of hysteresis, so the ripple
should be enough to bring the SECFB voltage above the
threshold by ~3x the hysteresis, or (2V + 3 x 17mV) =
2.051V. Reducing the CP decoupling capacitor and placing
a small ceramic capacitor (10 pF to 47pF) (C
in parallel with the upper leg of the SECFB resistor
feedback network (R
robustness of the charge pump.
Copyright
www.richtek.com
22
CP
SECFB
= V
©
OUT1
LG1_CP
LGATE1
2012 Richtek Technology Corporation. All rights reserved.
LGATE1
V
+ 2 x V
Figure 3. Connect to LG1_CP
OUT1
Figure 4. Connect to SECFB
is the peak voltage of LGATE1 driver and is
C1
D1
D2
V
OUT1
LGATE1
CP1
C1
D1
D2
D
of Figure 4) will also increase the
C2
is the forward diode dropped
− 4 x V
D3
C2
C3
D3
D
D4
C3
C
D4
F
CP
F
CP
R
of Figure 4)
voltage is :
C4
CP1
R
C4
CP2
CP
MOSFET Gate Driver (UGATEx, LGATEx)
The high-side driver is designed to drive high-current, low
R
5-V bias voltage is delivered from VREG5 supply. The
average drive current is also calculated by the gate charge
at V
drive current is supplied by the flying capacitor between
BOOTx and PHASEx pins. A dead time to prevent shoot
through is internally generated between high-side
MOSFET off to low-side MOSFET on, and low-side
MOSFET off to high-side MOSFET on.
The low-side driver is designed to drive high current low
R
that drives LGATEx low is robust, with a 0.6Ω typical on-
resistance. A 5V bias voltage is delivered from VREG5
supply. The instantaneous drive current is supplied by an
input capacitor connected between VREG5 and GND.
For high-current applications, some combinations of high-
and low-side MOSFETs might be encountered that will
cause excessive gate-drain coupling, which can lead to
efficiency-killing, EMI-producing shoot-through currents.
This is often remedied by adding a resistor in series with
BOOTx, which increases the turn-on time of the high-side
MOSFET without degrading the turn-off time (Figure 5).
Soft-Start
A build-in soft-start is used to prevent surge current from
power supply input after ENTRIPx is enabled. The typical
soft-start duration is 2ms period. Furthermore, the
maximum allowed current limit is segmented in 5 steps:
20%, 40%, 60%, 80% and 100% during the 2ms period.
DS(ON)
DS(ON)
GS
is a registered trademark of Richtek Technology Corporation.
= 5 V times switching frequency. The instantaneous
Figure 5. Reducing the UGATEx Rise Time
N-MOSFET(s). When configured as a floating driver,
N-MOSFET(s). The internal pull-down transistor
UGATEx
PHASEx
BOOTx
10
DS8205A/B/C-06 July 2012
V
IN