CS5165
Application Information: continued
Synchronous MOSFET:
Power = I
LOAD2
×
RDS
ON
×
(1 - duty cycle)
Duty Cycle =
V
OUT
+ (I
LOAD
×
RDS
ON OF SYNCH FET
)
V
IN
+ (I
LOAD
×
RDS
ON OF SYNCH FET
) - (I
LOAD
×
RDS
ON OF SWITCH FET
)
Off Time Capacitor (C
OFF
)
The C
OFF
timing capacitor sets the regulator off time:
T
OFF
= C
OFF
×
4848.5
Trace 3 =
GATE(H)
(10V/div.)
Trace 1=
GATE(H)
- 5V
IN
Trace 4 =
GATE(L)
(10V/div.)
Trace 2 = Inductor Switching Node (5V/div.)
Figure 18: Gate drive waveforms depicting rail to rail swing.
The preceding equation for Duty Cycle can also be used to
calculate the regulator switching frequency and select the
C
OFF
timing capacitor:
C
OFF
=
Period
×
(1-Duty Cycle)
4848.5
where
period =
1
switching frequency
Trace 1 - GATE(H) (5V/div)
Trace 2 - GATE(L) (5V/div)
Figure 19: Normal Operation showing the guaranteed Non-Overlap
time between the High and Low - Side MOSFET Gate Drives, I
LOAD
=
14A.
The CS5165 provides adaptive control of the external NFET
conduction times by guaranteeing a typical 65ns non-over-
lap between the upper and lower MOSFET gate drive puls-
es. This feature eliminates the potentially catastrophic
effect of “shoot-through current”, a condition during
which both FETs conduct causing them to overheat, self-
destruct, and possibly inflict irreversible damage to the
processor.
The most important aspect of FET performance is RDS
ON
,
which effects regulator efficiency and FET thermal man-
agement requirements.
The power dissipated by the MOSFETs may be estimated
as follows:
Switching MOSFET:
Power = I
LOAD2
×
RDS
ON
×
duty cycle
Schottky Diode for Synchronous FET
For synchronous operation, A Schottky diode may be
placed in parallel with the synchronous FET to conduct the
inductor current upon turn off of the switching FET to
improve efficiency. The CS5165 reference circuit does not
use this device due to its excellent design. Instead, the
body diode of the synchronous FET is utilized to reduce
cost and conducts the inductor current. For a design oper-
ating at 200kHz or so, the low non-overlap time combined
with Schottky forward recovery time may make the bene-
fits of this device not worth the additional expense. The
power dissipation in the synchronous MOSFET due to
body diode conduction can be estimated by the following
equation:
Power = V
bd
×
I
LOAD
×
conduction time
×
switching fre-
quency
Where V
bd
= the forward drop of the MOSFET body diode.
For the CS5165 demonstration board:
Power = 1.6V
×
14.2A
×
100ns
×
200kHz = 0.45W
This is only 1.1% of the 40W being delivered to the load.
13
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
