RT9245C Datasheet, PDF (18/25 Pages) Richtek Technology Corporation – Multi-Phase PWM Controller for CPU Core Power Supply

RT9245C
PGOOD Function
During start-up, RT9245C will detect 5V_CC and 12V_IN
(through DVD pin). In Figure 21, 5V_CC or 12V_IN is not ready
during T1. V_(SS) (in Figure 20) is pulled to GND by FAULT.
V_(EAP) is also equal to GND. V_(FB) and V_OUT will try to
follow V_(EAP) thus both V_(FB) and V_OUT are equal to GND
during T1. During T2, both 5V_CC and 12V_IN are ready,
FAULT = low, OPSS starts charging up C_SS. In the design
of RT9245C, I_SS (the maximal current sink and source
capability of OPSS) is limited and time-variant. During T2
(V1 = 0.4V > V_(SS) > 0), I_SS(T2) is equal to about 10uA.
T2 = CSS x ^V1 ≅ 4x10⁴ x CSS
ISS(T2)
After V_(SS) > V1, I_SS changes to about 20uA. The rising
speed of V_(SS) becomes about 2 times faster than in T1.
In Figure 20, MOSFET N1 will turn on only if V_(SS) > V_{TH_N1}
(threshold voltage of N1) ≅ 0.7V = V2. Before N1 turns
on, V_(EAP) is still 0V.
T3 = CSS x ^{(V2 - V1)} ≅ 1.5x10⁴ x CSS
ISS(T3)
After V_(SS) > V2, MOSFET N1 turns on, V_(EAP) starts rising.
I_SS(T4) is still equal to about 20uA. V_(SS,EAP) is equal to
V_{TH_N1}. Due to the body effect of MOSFET N1, V_{TH_N1}
increases with higher V_(EAP). For example, if V_OUT target
is 1.4V, V_(SS,EAP) will be equal to about 0.7V at the
beginning of T4 and equal to about 1.1V at the end of T4.
T4 = CSS x ^{(V4 - V2)} ≅ 9x10⁴ x CSS
ISS(T4)
At the end of T4, V_OUT is very close to the target (within
the range of ±40mV). An internal 1ms timer starts. After
about 1ms(T5), The open-drain output PGOOD releases.
After PGOOD releases, I_SS(T6) becomes about 320uA to
accelerate OPSS. RT9245C enters normal operation mode
and is capable to follow VID on the fly.
When any of the fault conditions happens, V_(SS) and
PGOOD will be pulled low immediately. If the fault condition
is one of 5V_CC low, DVD low, OC or VID_OFF, RT9245C
will try to turn off both high side MOSFET and low side
MOSFET. V_OUT will fall slowly to avoid negative V_OUT. The
typical waveform is shown in Figure 22.
www.richtek.com
18
If the fault condition is OV, V_(SS) and PGOOD will be pulled
low immediately also. RT9245C will try to turn on low
side MOSFET and turn off high side MOSFET. V_OUT will
fall quickly to protect CPU from high voltage. The typical
waveform is shown in Figure 23.
V_OUT
5V_CC
V_DAC
Z1
OPSS
+
-
Z2
FB -
+EA
EAP
N1
COMP
FAULT
SS
C_SS
Figure 20. Soft Start Circuit
5V_CC_ready
and DVD_ready
PGOOD
V4
V_(SS)
V3
V2
V1
T1
V_OUT
T2
T3
T4
T5
T6
Figure 21. Soft Start Waveform
PGOOD
V_(SS)
V_OUT
5V_CC_Low + DVD_Low + OC + VID_OFF
Figure 22. Waveform for 5V_CC_Low, DVD_Low, OC or
VID_OFF
DS9245C-02 March 2007