PT6440 Datasheet, PDF (7/8 Pages) Texas Instruments – 6-A 5-V/3.3-V Input Adjustable Integrated Switching Regulator

Application Notes
PT6440 Series
Using the Inhibit Function on the PT6440
Excalibur 5V/3.3V Bus Step-Down ISRs
For applications requiring output voltage On/Off control,
the 12-pin PT6440 series products incorporate an Inhibit
function. This function may be used wherever there is a
requirement for the module to be switched off. The
function is provided by the Inhibit control (pin 1) input.
The ISR functions normally with pin 1 open-circuit, ¹
providing a regulated output whenever a valid source
voltage is applied to V_in, (pins 2–4), with respect to GND
(pins 5–8). When a low-level ground signal is applied
to pin 1, the regulator output is disabled.
Figure 1 shows an application schematic, which details
the typical use of the Inhibit function. Note the discrete
transistor (Q₁). The Inhibit control has its own internal
pull-up to +V_in potential. An open-collector or open-
drain device is required to control this pin. ²
The Inhibit pin control thresholds are given in Table 1.
Equation 1 may be used to determine the approximate
current drawn from the input source, and by Q₁ when the
regulator is placed in the inhibit state.
Table 1; Inhibit Control Requirements 3
Parameter
Enable (VIH)
Disable (VIL)
Transition Time
Min
Max
Vin – 0.5
Vin
-0.2V
0.5V
10µs 4
Equation 1; Off Input Current
I_stby
= V_in ÷ 10kΩ
± 20%
Figure 1
+5V
C _in
2,3,4
Vin
PT6442
9,10,11
Vo
+2.5V
Inhibit* GND Vo(adj)
+
1
5–8 12
+
C _out
COM
COM
Inhibit
Q1
BSS138
Turn-On Time: In the circuit of Figure 1, turning Q₁ on
applies a low-voltage to the Inhibit control (pin 1) and
disables the regulator output. Correspondingly, turning
Q₁ off allows the Inhibit control pin to be pulled high by
its internal pull-up resistor. The ISR produces a fully
regulated output voltage within 10 milliseconds of the
release of the Inhibit control pin. The actual turn-on
time will vary with input voltage, output load, and the
total amount of load capacitance. Figure 2 shows the
typical rise in both output voltage and input current for a
PT6441 (3.3V) following the turn-off of Q₁ at time t =0.
The waveform was measured with a 5Vdc input voltage,
and 6 Adc resistive load.
Figure 2
VO (2V/Div)
IIN (2A/Div)
VINH (5V/Div)
-1
0
1
2
3
4
5
6
7
8
9
10
t (milliseconds)
Notes:
1. Use an open-collector device (preferably a discrete
transistor) for the Inhibit input. A pull-up resistor is not
necessary. To disable the output voltage, the control pin
should be pulled low to less than +0.5VDC.
2. Do not control the Inhibit input with an external DC
voltage. This will lead to erratic operation of the ISR and
may over-stress the regulator.
3. Avoid capacitance greater than 500pF at the Inhibit
control pin. Excessive capacitance at this pin will cause the
ISR to produce a pulse on the output voltage bus at turn-
on.
4. Keep the On/Off transition to less than 10µs. This
prevents erratic operation of the ISR, which could cause
the output voltage to be momentarily higher than normal.
For technical support and more information, see inside back cover or visit www.ti.com