cs5157h ON Semiconductor, cs5157h Datasheet - Page 12

• Manufacturer Part Number: cs5157h
• Description: Cpu 5-bit Synchronous Buck Controller
• Manufacturer: ON Semiconductor

capacitive load they present to the controller IC. For the
typical application where V
used as the source for the regulator output current, the
following gate drive is provided;
RDS
thermal management requirements.
as follows;
Off Time Capacitor (C
the C
calculated as follows:
time out timer, whichever is longer.
to calculate the regulator switching frequency and select the
C
(see Figure 19.)
V GATE(H) + 12 V * 5.0 V + 7.0 V, V GATE(L) + 12 V
Figure 19. CS5157H Gate Drive Waveforms Depicting
OFF
Trace 3 = V
Math 1 = V
Trace 4 = V
Trace 2− Inductor Switching Nodes (5.0 V/div.)
The most important aspect of MOSFET performance is
The power dissipated by the MOSFETs may be estimated
Switching MOSFET:
Synchronous MOSFET:
Duty Cycle =
The C
When the V
Off time will be determined by either the T
The preceding equations for duty cycle can also be used
ON
Power + I LOAD 2
OFF
timing capacitor:
V OUT ) (I LOAD
, which effects regulator efficiency and MOSFET
V IN )(I LOAD
* (I LOAD
Power + I LOAD 2
OFF
capacitor is reduced. The extended off time can be
GATE(H)
GATE(H)
GATE(L)
timing capacitor sets the regulator off time:
FFB
− 5.0 V
(10 V/div.)
(10 V/div.)
T OFF + C OFF
T OFF + C OFF
pin is less than 1.0 V, the current charging
IN
Rail to Rail Swing
RDS ON OF SWITCH FET )
OFF
RDS ON OF SYNCH FET )
M 1.00 ms
RDS ON
RDS ON OF SYNCH FET )
)
CC1
RDS ON
= V
4848.5
24, 242.5
CC2
( 1 * duty cycle )
= 12 V and 5.0 V is
duty cycle
OFF
time, or the
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CS5157H
12
Power + V BD
where:
Schottky Diode for Synchronous MOSFET
synchronous MOSFET to conduct the inductor current upon
turn off of the switching MOSFET to improve efficiency.
The CS5157H reference circuit does not use this device due
to it’s excellent design. Instead, the body diode of the
synchronous MOSFET is utilized to reduce cost and
conducts the inductor current. For a design operating at
200 kHz 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 8,
channel 2). The power dissipation in the synchronous
MOSFET due to body diode conduction can be estimated by
the following equation:
diode. For the CS5157H demonstration board as shown in
Figure 8;
load.
Input and Output Capacitors
to yield optimal results. Capacitors should be chosen to
provide acceptable ripple on the input supply lines and
regulator 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
current capability, and DC resistance. Increasing the
inductor value will decrease output voltage ripple, but
degrade transient response.
Thermal Considerations for Power
MOSFETs and Diodes
temperature of the semiconductor components should be
kept to a maximum of 150°C or lower. The thermal
impedance (junction to ambient) required to meet this
requirement can be calculated as follows:
Thermal Impedance +
Power + 1.6 V
A Schottky diode may be placed in parallel with the
Where V
This is only 1.3% of the 36.4 W being delivered to the
These components must be selected and placed carefully
The inductor should be selected based on its inductance,
In order to maintain good reliability, the junction
C OFF +
BD
THERMAL MANAGEMENT
Period +
= the forward drop of the MOSFET body
I LOAD
Perioid
13 A
T JUNCTION(MAX) * T AMBIENT
switching frequency
conduction time
100 ns
4848.5
( 1 * duty cycle )
1
233 kHz + 0.48 W
Power
switching frequency