CS5158 Cherry Semiconductor Corporation, CS5158 Datasheet - Page 11

• Manufacturer Part Number: CS5158
• Description: CPU 5-Bit Synchronous Buck Controller
• Manufacturer: Cherry Semiconductor Corporation

used as the source for the regulator output current, the fol-
lowing gate drive is provided;
Figure 17: CS5158 gate drive waveforms depicting rail to rail swing.
The most important aspect of MOSFET performance is
RDS
thermal management requirements.
The power dissipated by the MOSFETs may be estimated
as follows;
Switching MOSFET:
Synchronous MOSFET:
Off Time Capacitor (C
The C
When the V
C
culated as follows:
Off time will be determined by either the T
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
C
Duty Cycle =
V
OFF
OFF
IN
Trace 3 = V
Math 1= V
Trace 4 = V
Trace 2 = Inductor Switching Node (5V/div.)
V
ON
+ (I
capacitor is reduced. The extended off time can be cal-
timing capacitor:
GATE(H)
OFF
, which effects regulator efficiency and MOSFET
LOAD
Power = I
timing capacitor sets the regulator off time:
GATE(H)
Power = I
GATE(H)
GATE(L)
FFB
× RDS
= 12V - 5V = 7V, V
V
OUT
pin is less than 1V, the current charging the
- 5V
(10V/div.)
(10V/div.)
ON OF SYNCH FET
IN
LOAD
T
+ (I
T
OFF
LOAD
OFF
LOAD
OFF
2
= C
= C
× RDS
2
)
× RDS
× RDS
OFF
OFF
) - (I
× 24,242.5.
ON
GATE(L)
ON OF SYNCH FET
× 4848.5
ON
LOAD
× (1 - duty cycle)
× duty cycle
= 12V (see Figure 17).
× RDS
Applications Information: continued
OFF
ON OF SWITCH FET
)
time, or the
)
11
where:
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 CS5158 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:
Power = V
Where V
For the CS5158 demonstration board as shown in Figure 6;
Input and Output Capacitors
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.
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.
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:
This is only 1.3% of the 36.4W being delivered to the load.
Power = 1.6V × 13A × 100ns × 233kHz = 0.48W
Thermal Impedance =
bd
bd
= the forward drop of the MOSFET body diode.
× I
C
Period =
OFF
LOAD
Thermal Management
=
× conduction time × switching frequency
Period × (1 - duty cycle)
switching frequency
T
JUNCTION(MAX)
4848.5
1
Power
- T
,
AMBIENT