CS5151
Applications Information: continued
Trace 3 = 12V Input (V
CC1
) and V
CC2
) (10V/div.)
Trace 4 = 5V Input (2V/div.)
Trace 1 = Regulator Output Voltage (1V/div.)
Trace 2 = Power Good Signal (2V/div.)
Channel 3 = V
GATE
M1= V
GATE
- 5V
IN
Channel 2 = Inductor Switching Node
Figure 14: CS5151 demonstration board during power up. Power Good
signal is activated when output voltage reaches 1.70V.
Figure 15: CS5151 gate drive waveforms depicting rail to rail swing.
Selecting External Components
The CS5151 can be used with a wide range of external
power components to optimize the cost and performance of
a particular design. The following information can be used
as general guidelines to assist in their selection.
NFET Power Transistors
Both logic level and standard MOSFETs can be used. The
reference designs derive gate drive from the 12V supply
which is generally available in most computer systems and
use logic level MOSFETs. A charge pump may be easily
implemented to support 5V only systems. Multiple
MOSFETs may be paralleled to reduce losses and improve
efficiency and thermal management.
Voltage applied to the MOSFET gate depends on the appli-
cation circuit used. The gate driver output is specified to
drive to within 1.5V of ground when in the low state and to
within 2V of its bias supply when in the high state. In prac-
tice, the MOSFET gate will be driven rail to rail due to
overshoot caused by the capacitive load it presents to the
controller IC. For the typical application where V
CC1
= V
CC2
= 12V and 5V is used as the source for the regulator output
current, the following gate drive is provided;
V
GATE
= 12V - 5V = 7V (see Figure 15).
The most important aspect of MOSFET performance is
RDS
ON
, which effects regulator efficiency and MOSFET
thermal management requirements.
The power dissipated by the MOSFET and the Schottky
diode may be estimated as follows;
Switching MOSFET:
Power = I
LOAD2
×
RDS
ON
×
duty cycle
Schottky diode:
Power = V
FORWARD
×
I
LOAD
×
(1 - duty cycle)
Duty Cycle =
V + V
IN
FORWARD
- (I
LOAD
×
RDS
ON OF SWITCH FET
)
V
OUT
+ V
FORWARD
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 1V, the current charging the
C
OFF
capacitor is reduced. The extended off time can be cal-
culated 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
10
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
