Applications Information: continued
External Output Enable Circuit
On/off control of the regulator can be implemented
through two additional discrete components (see Figure 12).
This circuit operates by pulling the soft start pin high, and
the VFFB pin low, emulating a short circuit condition.
5V
MMUN2111T1 (SOT-23)
5
SS
CS5156
8
V
Trace 3 = 12V Input (V
Trace 4 = 5V Input (2V/div.)
) and V
) (10V/div.)
CC2
FFB
CC1
IN4148
Trace 1 = Regulator Output Voltage (1V/div.)
Trace 2 = Power Good Signal (2V/div.)
Shutdown
Input
Figure 14: CS5156 demonstration board during power up. Power Good
signal is activated when output voltage reaches 1.70V.
Figure 12: Implementing shutdown with the CS5156.
Selecting External Components
The CS5156 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.
External Power Good Circuit
An optional Power Good signal can be generated through
the use of four additional external components (see Figure
15). The threshold voltage of the Power Good signal can be
adjusted per the following equation:
NFET Power Transistors
(R1 + R2) × 0.65V
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.
VPower Good
=
R2
This circuit provides an open collector output that drives
the Power Good output to ground for regulator voltages
less than VPower Good
.
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 VCC1 = VCC2
= 12V and 5V is used as the source for the regulator output
current, the following gate drive is provided;
5V
R3
Power Good
PN3904
10k
R1
10k
PN3904
VOUT
CS5156
R2
6.2k
VGATE = 12V - 5V = 7V (see Figure 15).
Figure 13: Implementing Power Good with the CS5156.
10
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
