LT4276
10
4276fa
For more information www.linear.com/LT4276
applicaTions inForMaTion
Inrush and Powered On
Once the PSE detects and optionally classifies the PD, the
PSE then powers on the PD. When the port voltage rises
above the V
HSON
threshold, it begins to source I
GPU
out
of the HSGATE pin. This current flows into an external
capacitor (C
GATE
in Figure 5) that causes a voltage to ramp
up the gate of the external MOSFET. The external MOSFET
acts as a source follower and ramps the voltage up on
the output bulk capacitor (C
PORT
in Figure 5), thereby
determining the inrush current (I
INRUSH
in Figure 5). To
meet IEEE requirements, design I
INRUSH
to be ~100mA.
The LT4276 internal charge pump provides an N-channel
MOSFET solution, eliminating a larger and more costly
P-channel FET. The low R
DS(ON)
MOSFET also maximizes
power delivery and efficiency, reduces power and heat
dissipation, and eases thermal design.
Figure 5. Programming I
INRUSH
Figure 6. V
CC
Buck Regulator
EXTERNAL V
CC
SUPPLY
The external V
CC
supply must be configured as a buck
regulator shown in Figure 6. To optimize the buck regulator,
use the external component values in Table 2 correspond
-
ing to the V
IN
operating range. This buck regulator runs
in discontinuous mode with the inductor peak current
considerably higher than average load current on V
CC
.
Thus, the saturation current rating of the inductor must
exceed the values shown in Table 2. Place the capacitor, C,
as close as possible to V
CC
pin 21 and GND pin 19. For
optimal performance, place the external components as
close as possible to the LT4276.
LT4276
HSGATE
GND
4276 F05
VPORT HSSRC
C
GATE
I
GPU
3.3k
C
PORT
VPORT
I
INRUSH
I
INRUSH
=I
GPU
•
C
PORT
C
GATE
DELAY START
After the HSGATE charges up to approximately 7V above
HSSRC, fully enhancing the external Hot Swap MOSFET,
the switching regulator controller operates after a delay
of t
START
. During this delay, the LT4276 draws I
MPS
from
VPORT to ensure that the PSE does not DC disconnect
the PD due to Maintain Power Signature requirements.
V
IN
R
e
V
CC
V
IN
V
CC
GND
SWVCC
LT4276
FMMT723
PBSS9110T
L(µH)
C(µF)
4276 F06
AUXILIARY SUPPLY OVERRIDE
If the AUX pin is held above V
AUXT
, the LT4276 enters
auxiliary power supply override mode. In this mode the
signature resistor is disconnected, classification is dis
-
abled, and HSGATE is pulled down. The T2P pin pulls up
to V
CC
on the LT4276B (or the LT4276A when no R
CLS
++
resistor is present). The T2P pin alternates between pulling
up and floating at f
T2P
on the LT4276A when the R
CLS
++
resistor is present.
The AUX pin allows for setting the auxiliary supply turn on
(V
AUXON
) and turn off (V
AUXOFF
) voltage thresholds. The
auxiliary supply hysteresis voltage (V
AUXHYS
) is set by
sinking current (I
AUXH
) only when the AUX pin voltage is
Table 2 . Buck Regulator Component Selection
V
IN
C L I
SAT
R
e
9V-57V
PoE
22µF
10µF
22µH
100µH
≥1.2A
≥300mA
1Ω
20Ω
