CS5157H
http://onsemi.com
12
capacitive load they present to the controller IC. For the
typical application where V
CC1
= V
CC2
= 12 V and 5.0 V is
used as the source for the regulator output current, the
following gate drive is provided;
V
GATE(H)
+ 12 V * 5.0 V + 7.0 V, V
GATE(L)
+ 12 V
(see Figure 19.)
Figure 19. CS5157H Gate Drive Waveforms Depicting
Rail to Rail Swing
M 1.00 ms
Math 1 = V
GATE(H)
− 5.0 V
IN
Trace 3 = V
GATE(H)
(10 V/div.)
Trace 4 = V
GATE(L)
(10 V/div.)
Trace 2− Inductor Switching Nodes (5.0 V/div.)
The most important aspect of MOSFET performance is
RDS
ON
, which effects regulator efficiency and MOSFET
thermal management requirements.
The power dissipated by the MOSFETs may be estimated
as follows;
Switching MOSFET:
Power + I
LOAD
2
RDS
ON
duty cycle
Synchronous MOSFET:
Power + I
LOAD
2
RDS
ON
(
1 * duty cycle
)
Duty Cycle =
V
OUT
) (I
LOAD
RDS
ON OF SYNCH FET
)
ƪ
V
IN
)(I
LOAD
RDS
ON OF SYNCH FET
)
* (I
LOAD
RDS
ON OF SWITCH FET
)
ƫ
Off Time Capacitor (C
OFF
)
The C
OFF
timing capacitor sets the regulator off time:
T
OFF
+ C
OFF
4848.5
When the V
FFB
pin is less than 1.0 V, the current charging
the C
OFF
capacitor is reduced. The extended off time can be
calculated as follows:
T
OFF
+ C
OFF
24, 242.5
Off time will be determined by either the T
OFF
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
C
OFF
timing capacitor:
C
OFF
+
Perioid
(
1 * duty cycle
)
4848.5
where:
Period +
1
switching frequency
Schottky Diode for Synchronous MOSFET
A Schottky diode may be placed in parallel with the
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:
+
Where V
BD
= the forward drop of the MOSFET body
diode. For the CS5157H demonstration board as shown in
Figure 8;
Power + 1.6 V 13 A 100 ns 233 kHz + 0.48 W
This is only 1.3% of the 36.4 W being delivered to the
load.
Input and Output Capacitors
These components must be selected and placed carefully
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
The inductor should be selected based on its inductance,
current capability, and DC resistance. Increasing the
inductor value will decrease output voltage ripple, but
degrade transient response.
THERMAL MANAGEMENT
Thermal Considerations for Power
MOSFETs and Diodes
In order to maintain good reliability, the junction
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 +
T
JUNCTION(MAX)
* T
AMBIENT
Power
