PTB48520W Datasheet, PDF (9/18 Pages) Texas Instruments – 25-A, 48-V INPUT ISOLATED DC/DC CONVERTER WITH AUTO-TRACK™ SEQUENCING

www.ti.com
PTB48520W
SLTS233A – NOVEMBER 2004 – REVISED APRIL 2005
Using the Output Enable Control on the PTB48520 Auto-Track Compatible DC/DC Converter
The V_O
on and
Enable (pin 2) control is an active low input
off while it is connected to the input source.
that
The
allows the
V_O Enable
output voltage from
input is referenced
the converter
to the –V_I (pin
to be
3) ⁽¹⁾,
turned
on the
primary side of the converter isolation, and has its own internal pullup. The open-circuit voltage is approximately
20% of the applied input source voltage.
For the converter to
produces a regulated
function
voltage
normally,
whenever
pin 2 must be pulled
a valid source voltage
low to –V_I potential⁽²⁾.
is applied between +V_I
The
(pin
converter
1) and –V_I
output then
(pin 3)⁽³⁾. If
the voltage at pin 2 is allowed to rise above VIH(min), (see specification table), the output from the converter is
turned off.
Figure 6 is an application schematic that shows the typical use of the Output Enable function. Note the discrete
transistor (Q1). Either a discrete MOSFET or bipolar transistor is recommended to control this input. Table 2
gives the threshold requirements.
When placed in Off state, the output neither sources or sinks output current. The load voltage then decays as the
output capacitance is discharged by the load circuit. With the output turned off, the current drawn from the input
source is typically reduced to 2 mA.
(1) The V_O Enable control uses –V_I (pin 3) as ground reference. All voltages are with respect to –V_I.
(2) Use an open-collector (or open-drain) discrete transistor to control the V_O Enable input. A pullup resistor is not necessary. To disable the
converter, the control pin should be pulled low to less than +0.8 V. If the Output Enable feature is not used, pin 2 should be permanently
connected to –V_I (pin 3).
(3) The converter incorporates a UVLO. The UVLO does not allow the converter to power up until the input voltage is close to its minimum
specified operating voltage. This is regardless of the state of the Output Enable control. Consult the specifications for the UVLO
thresholds.
Table 2. Output Enable Control Requirements⁽¹⁾
V_IH
V_IL
V_O/C [Open-Circuit]
I_I [pin 1 at –V_in]
PARAMETER
MIN TYP
2
MAX
0.8
15
–0.8
UNIT
V
V
V
mA
(1) The V_O Enable control uses –V_I (pin 3) as its ground reference. All voltages are with respect to –V_I.
4
Track
PTB48520W
8
+Sense
10
+V_I
1
+V_I
V_OSeq
9
V_OBus
1 = Enable
2
7
V_OEnable _VOAdjust
L
R1
10 kW
Q1
BSS138
R2
O
887 W
A
5
D
V_OCOM
−V_I
3
−V_I
6
−Sense
Figure 6. Output Enable Operation
Turn-On Time: In the circuit of Figure 6, turning Q1 off allows the voltage at pin 2 to rise to its internal pullup
voltage. This disables the converter output. When Q1 is then turned on, it applies a low-level voltage to pin 2,
9