Preliminary
Application Information
Power On Reset
The RT8101/A automatically initializes upon applying of
input power V
CC
. The power on reset function (POR)
continually monitors the input bias supply voltage at the
VCC pin. The POR trip level is typically 9.6V at VCC
rising.
VIN Detection
After POR the RT8101/A continuously generates a 10kHz
pulse train with 1μs pulse width to turn on the upper
MOSFET for detecting the existence of V
IN
. RT8101/A
keeps monitoring PHASE pin voltage during the detection
period.
As soon as the PHASE voltage crosses 1.5V two times,
V
IN
existence is recognized and the RT8101/A initiates
its soft start cycle as described in next section.
V
IN
POR_H
PHASE_M
+
-
1.5V
PHASE
UGATE
1st 2nd PHASE
waveform
Internal Counter will count (V
PHASE
> 1.5V)
two times (rising & falling) to recognize when
V
IN
is ready.
RT8101/A
A 30μA current source flows through the internal resistor
21.6kΩ to PHASE pin causing 0.65V voltage drop across
the resistor. OCP is triggered if the voltage at PHASE pin
(drop of lower MOSFET V
DS
) is lower than
−0.25V
when
low side MOSFET conducting. Accordingly inductor
current threshold for OCP is a function of conducting
resistance of lower MOSFET R
DS(ON)
as :
I
OCSET
=
30
μ
A
×
21.6k-0.4V
R
DS(ON)
If MOSFET with R
DS(ON)
= 10mΩ is used,
the OCP
threshold current is about 25A. Once OCP is triggered,
the RT8101/A enters hiccup mode and re-soft starts again.
The RT8101/A shuts down after OCP
hiccups twice.
OCP
UGATE
(10V/Div)
Figure 1
Soft Start
A built-in soft-start is used to prevent surge current from
V
IN
to V
OUT
during power on. After the existence of V
IN
is
detected, soft-start (SS) begins automatically. The
feedback voltage (V
FB)
is clamped by internal linear ramping
up SS voltage, causing PWM pulse width increasing
slowly and thus inducing little surge current. Soft-start
completes when SS voltage exceeds internal reference
voltage (0.8V), the time duration is about 3.2ms.
Over Current Protection
The RT8101/A senses the current flowing through lower
MOSFET for over current protection (OCP) by sensing
the PHASE pin voltage as shown in the Functional Block
Diagram.
I
OUT
(10A/Div)
Time (2.5ms/Div)
Figure 3. Power On then Shorted
OCP
UGATE
(10V/Div)
I
OUT
(10A/Div)
Time (2.5ms/Div)
Figure 4. Shorted then Power On
DS8101/A-01 March 2007
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
