L6917B
Output Voltage Protection and Power Good
The output voltage is monitored by pin VSEN. If it is not within +12/-10% (typ.) of the programmed value, the
powergood output is forced low. Power good is an open drain output and it is enabled only after the soft start is
finished (2048 clock cycles after start-up).
The device provides over voltage protection; when the voltage sensed by the V
pin reaches 2.1V (typ.), the
SEN
controller permanently switches on both the low-side mosfets and switches off both the high-side mosfets in or-
der to protect the CPU. The OSC/INH/FAULT pin is driven high (5V) and power supply (Vcc) turn off and on is
required to restart operations. The over Voltage percentage is set by the ratio between the OVP threshold (set
at 2.1V) and the reference programmed by VID.
2.1V
= ----------------------------------------------------------------------------
OVP[%]
100
(
)
Reference Voltage VID
Under voltage protection is also provided. If the output voltage drops below the 60% of the reference voltage for
more than one clock period the device turns off and the FAULT pin is driven high.
Both Over Voltage and Under Voltage are active also during soft start (Under Voltage after than Vout reaches
0.8V). During soft-start the reference voltage used to determine the OV and UV thresholds is the increasing volt-
age driven by the 2048 soft start digital counter.
Remote Voltage Sense
A remote sense buffer is integrated into the device to allow output voltage remote sense implementation without
any additional external components. In this way, the output voltage programmed is regulated between the re-
mote buffer inputs compensating motherboard trace losses or connector losses if the device is used for a VRM
module. The very low offset amplifier senses the output voltage remotely through the pins FBR and FBG (FBR
is for the regulated voltage sense while FBG is for the ground sense) and reports this voltage internally at VSEN
pin with unity gain eliminating the errors.
If remote sense is not required, the output voltage is sensed by the VSEN pin connecting it directly to the output
voltage. In this case the FBG and FBR pins must be connected anyway to the regulated voltage.
Input Capacitor
The input capacitor is designed considering mainly the input rms current that depends on the duty cycle as re-
ported in figure 8. Considering the dual-phase topology, the input rms current is highly reduced comparing with
a single phase operation.
Figure 8. Input rms Current vs. Duty Cycle (D) and Driving Relationships
Single Phase
0.50
I
OUT
2D (1 − 2D)
if D < 0.5
2
I
=
rms
Dual Phase
I
OUT
2
0.25
(2D - 1) (2 − 2D) if D > 0.5
0.25
0.50
0.75
Duty Cycle (VOUT/VIN)
It can be observed that the input rms value is one half of the single-phase equivalent input current in the worst
case condition that happens for D = 0.25 and D = 0.75.
14/33
STMICROELECTRONICS [ ST ]
