Datasheet

LTC4267-1
14
42671fa
For more information www.linear.com/4267-1
applicaTions inForMaTion
the LTC4267-1 will attempt to quickly charge capacitor C1
using an internal secondary current limit circuit. In this
scenario, the PSE current limit should provide the overall
limit for the circuit. For slower rising inputs, the 375mA
current limit in the LTC4267-1 will set the charge rate of
the capacitor C1. In either case, the PWRGD signal may
go inactive briefly while the capacitor is charged up to the
new line voltage. In the design of a PD, it is necessary
to determine if a step in the input voltage will cause the
PWRGD signal to go inactive and how to respond to this
event. In some designs, it may be desirable to filter the
PWRGD signal so that intermittent power bad conditions
are ignored. Figure 7 demonstrates a method to insert a
lowpass filter on the power good interface.
For PD designs that use a large load capacitor and also con-
sume a lot of power, it is important to delay activation of the
switching regulator with the PWRGD signal. If the regulator
is not disabled during the current-limited turn-on sequence,
the PD circuitry will rob current intended for charging up
the load capacitor
and create
a slow rising input, possibly
causing the LTC4267-1 to go into thermal shutdown.
The
`
PWRGD pin connects to an internal open drain,
100V transistor capable of sinking 1mA. Low impedance
to V
PORTN
indicates power is good.
`
PWRGD is high
impedance during signature and classification probing
and in the event of a thermal overload. During turn-off,
PWRGD is deactivated when the input voltage drops below
30V. In addition, PWRGD may go active briefly at turn-on
for fast rising input waveforms. PWRGD is referenced to
the V
PORTN
pin and when active, will be near the V
PORTN
potential. Connect the PWRGD pin to the switching regula-
tor circuitry as shown in Figure 7.
PD Interface Thermal Protection
The LTC4267-1 PD Interface includes thermal overload
protection in order to provide full device functionality in
a miniature package while maintaining safe operating
temperatures. Several factors create the possibility of
significant power dissipation within the LTC4267-1. At
turn-on, before the load capacitor has charged up, the
instantaneous power dissipated by the LTC4267-1 can be
as much as 10W. As the load capacitor charges up, the
power dissipation in the LTC4267-1 will decrease until it
reaches a
steady-state value
dependent on the DC load
current. The size of the load capacitor determines how fast
the power dissipation in the LTC4267-1 will subside. At
room temperature, the LTC4267-1 can typically handle load
capacitors as large as 800µF without going into thermal
shutdown. With large load capacitors, the LTC4267-1 die
temperature will increase by as much as 50°C during a
single turn-on sequence. If for some reason power were
removed from the part and then quickly reapplied so that
the LTC4267-1 had to charge up the load capacitor again,
the temperature rise would be excessive if safety precau-
tions were not implemented.
The LTC4267-1 PD interface protects itself from thermal
damage by monitoring the die temperature. If the die
temperature exceeds the overtemperature trip point, the
Figure 6. LTC4267-1 Power Good
Figure 7. Power Good Interface Examples
42671 F07
LTC4267-1
V
PORTN
P
OUT
PGND
V
PORTP
I
TH
/RUN
TO
PSE
–48V
+
C1
5µF
100V
ALTERNATE ACTIVE-HIGH ENABLE FOR P
VCC
PIN
C15 OPTIONAL
SEE APPLICATIONS INFORMATION SECTION
ACTIVE-HIGH ENABLE FOR RUN PIN WITH INTERNAL PULL-UP
10k
100k
0.047µF
PWRGD
PGND
LTC4267-1
V
PORTN
P
OUT
PGND
V
PORTP
P
VCC
TO
PSE
–48V
+
C1
5µF
100V
10k
100k
0.047µF
PWRGD
PGND
R
START
C
PVCC
PWRGD
C1
5µF
MIN
V
PORTN
P
OUT
1.125V
300k
300k
R9
100k
LTC4267-1
THERMAL SHUTDOWN
UVLO
42671 F06
TO
PSE
+
–
+
+
–
I
TH
/RUN
PGND
PGND