Applications Information: continued
COFF timing capacitor:
Period × (1 - duty cycle)
COFF
=
,
4848.5
where:
1
Period =
switching frequency
Schottky Diode for Synchronous MOSFET
A Schottky diode may be placed in parallel with the syn-
chronous MOSFET to conduct the inductor current upon
turn off of the switching MOSFET to improve efficiency.
The CS5150 reference circuit does not use this device due to
its excellent design. Instead, the body diode of the syn-
chronous MOSFET is utilized to reduce cost and conducts
the inductor current. For a design operating at 200kHz or so,
the low non-overlap time combined with Schottky forward
recovery time may make the benefits of this device not
worth the additional expense (see Figure 6, channel 2). The
power dissipation in the synchronous MOSFET due to body
diode conduction can be estimated by the following equation:
Trace 3 = V
(10V/div.)
GATE(H)
Math 1= V
- 5V
GATE(H)
IN
Trace 4 = V
Trace 2 = Inductor Switching Node (5V/div.)
(10V/div.)
GATE(L)
Figure 17: CS5150 gate drive waveforms depicting rail to rail swing.
The most important aspect of MOSFET performance is
RDSON, which effects regulator efficiency and MOSFET
thermal management requirements.
Power = Vbd × ILOAD × conduction time × switching frequency
The power dissipated by the MOSFETs may be estimated
as follows;
Where Vbd = the forward drop of the MOSFET body diode.
For the CS5150 demonstration board as shown in Figure 6;
Switching MOSFET:
Power = ILOAD2 × RDSON × duty cycle
Power = 1.6V × 13A × 100ns × 233kHz = 0.48W
This is only 1.3% of the 36.4W being delivered to the load.
Synchronous MOSFET:
Power = ILOAD2 × RDSON × (1 - duty cycle)
“Droop” Resistor for Adaptive Voltage Positioning
Duty Cycle =
Adaptive voltage positioning is used to reduce output volt-
age excursions during abrupt changes in load current.
Regulator output voltage is offset +40mV when the regula-
tor is unloaded, and -40mV at full load. This results in
increased margin before encountering minimum and maxi-
mum transient voltage limits, allowing use of less capaci-
tance on the regulator output (see Figure 7).
VOUT + (ILOAD × RDSON OF SYNCH FET
)
VIN + (ILOAD × RDSON OF SYNCH FET) - (ILOAD × RDSON OF SWITCH FET
)
Off Time Capacitor (COFF
)
The COFF timing capacitor sets the regulator off time:
To implement adaptive voltage positioning, a “droop”
resistor must be connected between the output inductor
and output capacitors and load. This is normally imple-
mented by a PC board trace of the following value:
TOFF = COFF × 4848.5
When the VFFB pin is less than 1V, the current charging the
COFF capacitor is reduced. The extended off time can be cal-
culated as follows:
80mV
RDROOP
=
TOFF = COFF × 24,242.5.
IMAX
Off time will be determined by either the TOFF time, or the
time out timer, whichever is longer.
Adaptive voltage positioning can be disabled for improved
DC regulation by connecting the VFB pin directly to the load
using a separate, non-load current carrying circuit trace.
The preceding equations for duty cycle can also be used to
calculate the regulator switching frequency and select the
11
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
