PT6943A Datasheet, PDF (5/11 Pages) Texas Instruments – 6-A Dual Output 5-V/3.3-V Input Integrated Switching Regulator

Application Notes
PT6940 Series
Not Recommended For New Designs
Operating Features of the PT6940 Series
of Dual-Output Voltage Regulators
Over-Current Protection
The PT6940 series of regulators incorporate independent
current limit protection at both outputs (Vo₁ & Vo₂) with a
periodic shutdown of both outputs. Applying a load current,
in excess of the current limit threshold to either output,
results in the shutdown of both voltages after a short
period; typically 15ms. Following shutdown the module
periodically attempts to recover by executing a soft start
power-up at intervals of approximately 100ms. If the over-
current fault persists, each attempted restart will result in a
corresponding over-current trip and shutdown. During
the 15ms period prior to each successive shutdown, the
output with the load fault may not reach full regulation.
Power-Up Voltage Sequencing
The output voltages from the PT6940 series regulators are
independently regulated, and internally sequenced to meet
the power-up requirements of popular microprocessors
and DSP chipsets. Figure 1 shows the output voltage wave-
forms of a PT6942 (3.3V/1.8V) after either input power is
applied, or the regulator is enabled. In this example turning
Q₁ off in Figure 2, removes the low-voltage signal at pin 1
and enables the regulator. Following a delay of about 3–5ms,
Vo₁ and Vo₂ rise together until the lower voltage, Vo₂,
reaches its regulation voltage. Vo₁ then continues to rise
until both outputs reach full regulation. The total power-
up time is less than 15ms, and is relatively independent of
load, temperature, and output capacitance. The turn-off of
Q₁ corresponds to the rise in V_STBY. The waveforms were
measured with a 5V input voltage, and with resistive loads
of 4A at both the Vo₁ and Vo₂ outputs.
Standby Control
The output voltages from the PT6940 may be disabled
using the regulator’s Standby control. The standby function
is provided by the “STBY*” control (pin 1). If pin 1 is left
open-circuit the regulator operates normally, and provides
a regulated output at both Vo₁ (pins 4–6) and Vo₂ (pins 23–
25) whenever a valid input source voltage is applied to V_in
(pins 13–16) with respect to GND (pins 7-12 & 17–22).
Applying a low-impedance sink to ground¹ at pin 1, simul-
taneously disables both regulated outputs. This places the
regulator in standby mode, and reduces the input current
drawn by the ISR to typically 10mA. The Standby control
may also be used to maintain both regulator outputs at zero
volts during the period that input power is applied.
The standby pin is ideally controlled using an open-collector
(or open-drain) discrete transistor (See Figure 2). The
open-circuit voltage is the input voltage, V_in.
Figure 2
V_in
13–16
Vin
STBY*
1
+
C_IN
Q1
Inhibit
BSS138
3
26
V₁ Sns
V₂ Sns
4–6
Vo₁
PT6940
23–25
Vo₂
7–12
V₂ Adj V₁ Adj
17–22 27 2
+
+
Co₁
Co₂
Vo ₁
Vo ₂
COM
COM
Figure 1
V1 (1V/Div)
V2 (1V/Div)
Vstby (5V/Div)
Notes:
1. The standby control input is Not compatible with TTL or
other devices that incorporate a totem-pole output drive. Use
only a true open-collector device, preferably a discrete bipolar
transistor (or MOSFET). To ensure the regulator output is
disabled, the control pin must be pulled to less than 0.4Vdc
with a low-level 0.5mA sink to ground.
2 Do not use an an external pull-up resistor. The control pin
has its own internal pull-up. Adding an external pull-up
could disable the over-current protection. The open-circuit
voltage of the “STBY*” pin is the input voltage, V_in.
HORIZ SCALE (2ms/Div)
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