RT9209 Datasheet, PDF (10/14 Pages) Richtek Technology Corporation – Synchronous Buck PWM DC-DC with Enable & PGOOD

RT9209/A
Application Information
The RT9209/A operates at either single 5V power supply
with a bootstrap UGATE driver or a 5V/12V dual-power
supply form the ATX SMPS. The dual- power supply is
recommended for high current applications, the
RT9209/A can deliver higher gate driving current while
operating with ATX SMPS based on a dual-power supply.
5V
VCC
6.5V
Regulator
BOOT
C1
1uF
UGATE
R1
12V
10
5V
+
A
The Bootstrap Operation
In a single power supply system, the UGATE driver of
RT9209/A is powered by an external bootstrap circuit, as
shown in the Figure1. The boot capacitor, C_BOOT,
generates a floating reference. Typically a 0.1μF C_BOOT
is enough for most of MOSFETs used with the
RT9209/A. The voltage drop between BOOT and A node
is refreshed to a voltage of V_CC − diode drop (V_D) while the
lower MOSFET turning on.
VCC
C2
D1
BOOT
1uF
C1 ⁺
UGATE 0.1uF ^-
5V
+
VCC
C2
1uF
LGATE
RT9209/A
Figure 2. Dual Power Supply Operation
Power On Reset
The Power-On Reset (POR) monitors the supply voltage
(normal +5V) at the VCC pin. The VCC POR level is set
to 4.1V with 0.5V hysteresis. The POR function initiates
soft-start operation after all supply voltages exceed their
POR thresholds.
A
VCC
LGATE
RT9209/A
Figure 1. Single 5V power Supply Operation
Dual Power Operation
The RT9209/A is designed to supply a regulated 6.5V at
VCC pin automatically when BOOT pin is powered by a
12V. In a system with ATX 5V/12V power supply, the
RT9209/A is ideal for higher current applications due to
the higher gate driving capability, V_UGATE = 12V and V_LGATE
= 6.5V. A RC (10Ω/1μF) filter is also recommended at
BOOT pin to prevent the ringing induced from fast power-
on, as shown in Figure 2.
Soft Start
A built-in soft-start is used to prevent surge current from
power supply input during power on. The soft-start voltage
is controlled by an internal 22μA to change a capacitor
slowly. It clamps the ramping of reference voltage at the
input of error amplifier and the pulse-width of the output
driver slowly.
Under Voltage and Over Voltage Protection
The voltage at FB pin is monitored and protected against
OC (over current), and OV (over voltage). The UV
threshold is 0.5V and OV-threshold is 1.0V. Both UV/OV
detection have 30ms triggered delay. When OC or UV
trigged, a hiccup re-start sequence will be initialized, as
shown in Figure 3. Only 3 times of trigger are allowed to
latch off. Hiccup is disabled during soft-start interval.
Shutdown
Pulling low the SS pin by a small single transistor can
shutdown the RT9209/A PWM controller as shown in
typical application circuit.
www.richtek.com
10
DS9209/A-08 March 2007