RT8101 Datasheet, PDF (9/15 Pages) Richtek Technology Corporation – 12V Synchronous Buck PWM DC-DC

Preliminary
RT8101/A
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.
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
=
30μA × 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
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.
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.
UGATE
(10V/Div)
IOUT
(10A/Div)
Time (2.5ms/Div)
Figure 3. Power On then Shorted
OCP
UGATE
(10V/Div)
IOUT
(10A/Div)
Time (2.5ms/Div)
Figure 4. Shorted then Power On
DS8101/A-01 March 2007
www.richtek.com
9