Applications Information: continued
used as the source for the regulator output current, the fol-
Period × (1 - duty cycle)
lowing gate drive is provided;
GATE(H) = 12V - 5V = 7V, VGATE(L) = 12V (see Figure 17).
COFF
=
,
4848.5
V
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 CS5157 reference circuit does not use this device due to
it’s 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
Trace 4 = V
Trace 2 = Inductor Switching Node (5V/div.)
- 5V
GATE(H)
IN
Power = Vbd × ILOAD × conduction time × switching frequency
(10V/div.)
GATE(L)
Where Vbd = the forward drop of the MOSFET body diode.
For the CS5157 demonstration board as shown in Figure 6;
Figure 17: CS5157 gate drive waveforms depicting rail to rail swing.
Power = 1.6V × 13A × 100ns × 233kHz = 0.48W
The most important aspect of MOSFET performance is
RDSON, which effects regulator efficiency and MOSFET
thermal management requirements.
This is only 1.3% of the 36.4W being delivered to the load.
The power dissipated by the MOSFETs may be estimated
as follows;
Input and Output Capacitors
Switching MOSFET:
These components must be selected and placed carefully to
yield optimal results. Capacitors should be chosen to pro-
vide acceptable ripple on the input supply lines and regula-
tor output voltage. Key specifications for input capacitors
are their ripple rating, while ESR is important for output
capacitors. For best transient response, a combination of
low value/high frequency and bulk capacitors placed close
to the load will be required.
Power = ILOAD2 × RDSON × duty cycle
Synchronous MOSFET:
Power = ILOAD2 × RDSON × (1 - duty cycle)
Duty Cycle =
VOUT + (ILOAD × RDSON OF SYNCH FET
)
VIN + (ILOAD × RDSON OF SYNCH FET) - (ILOAD × RDSON OF SWITCH FET
)
Output Inductor
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the induc-
tor value will decrease output voltage ripple, but degrade
transient response.
Off Time Capacitor (COFF
The COFF timing capacitor sets the regulator off time:
)
TOFF = COFF × 4848.5
Thermal Management
When the VFFB pin is less than 1V, the current charging the
OFF capacitor is reduced. The extended off time can be cal-
culated as follows:
C
Thermal Considerations for Power MOSFETs and Diodes
In order to maintain good reliability, the junction tempera-
ture of the semiconductor components should be kept to a
maximum of 150°C or lower. The thermal impedance (junc-
tion to ambient) required to meet this requirement can be
calculated as follows:
TOFF = COFF × 24,242.5.
Off time will be determined by either the TOFF time, or the
time out timer, whichever is longer.
The preceding equations for duty cycle can also be used to
calculate the regulator switching frequency and select the
COFF timing capacitor:
T
JUNCTION(MAX) - TAMBIENT
Power
Thermal Impedance =
11
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
