SI9172DB Vishay, SI9172DB Datasheet - Page 5

• Manufacturer Part Number: SI9172DB
• Description: Power Management Modules & Development Tools SI9172 Demo Board
• Manufacturer: Vishay
• Type: DC/DC Switching Converters, Regulators & Controllersr

Specifications of SI9172DB

• Input Voltage: 6.5 V
• Output Voltage: 1.5 V
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 25 C
• Product: Power Management Modules
• Silicon Manufacturer: Vishay
• Application Sub Type: Synchronous Buck Controller
• Kit Application Type: Power Management - Voltage Regulator
• Silicon Core Number: SI9172
• Kit Contents: Board
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: Si9172
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Start-Up
The built-in UVLO circuit in the Si9172 prevents the internal
MOSFET from turning on if the voltage on the V
than 2.4 V.
controller is continuously powered ON until the V
drops below 2.3 V. This hysteresis prevents the converter
from oscillating during the start-up phase and unintentionally
locking up the system. Once the V
UVLO threshold, and with no other shutdown condition
detected, an internal power-on-reset timer is activated. The
circuitry, except the output driver, is turned on. After the POR
time-out of about 3 ms, the soft-start begins. If pin 4 is in BP
mode, the high side PMOS will turn on gradually in about
100 ms to avoid any inrush current. If pin 4 is in PSM mode, the
converter will soft-start in PSM operation, forcing the output
voltage to rise smoothly with minimum overshoot. The rise
time is approximatly 100-ms. In order to start-up properly in
PSM mode, the load current has to be 170 mA or less.
Pulse Skipping Mode
The gate charge losses produced from the Miller capacitance
of the MOSFET are the dominant power dissipation parameter
during light load. Therefore, reduced gate switching frequency
improves overall converter efficiency. This is exactly why the
Si9172 is designed to operate in pulse-skipping mode rather
than PWM mode. If the BP/PSM pin is connected to logic low
level, the converter runs in pulse skipping modulation mode.
In this mode, the internal MOSFET operates with a constant
on-time. The on-time is reduced if necessary by the peak
current detector circuitry. There is an enforced minimum
off-time acting as the feedback comparator blanking time. If
the output voltage drops below the desired level, the main
switch is first turned on and then off. If the applied on-time did
not deliver enough energy to keep the output at the desired
voltage level, the controller will force another on and off
sequence, until the desired voltage is accomplished. If the
applied on-time forces the output to exceed the desired level,
the converter stays off. The excess energy is delivered to the
output slowly, forcing the converter to skip pulses as needed
to maintain regulation. The on-time and off-time are set
internally based on the inductor value (1.5 mH Typical) and
maximum load current. With a 1.5-mH inductor, the Si9172
guarantees to deliver minimum of 170-mA load current. This
current capability decreases as the inductance increases. In
pulse-skipping mode, the switching frequency, f
load current. When the load increases, f
The typical conversion efficiency in PSM mode is 90%.
Efficiency is higher at high output voltage and decreases with
the output voltage.
Adjustable Output
In PSM mode, the output voltage regulation point can be
adjusted by an external analog voltage signal at the ADJ pin.
When this voltage varies from 0.4 V to 2.5 V, the output voltage
also increases linearly from 1.5 V to 3.6 V. When the
programmed output voltage is higher than the input voltage
Document Number: 71379
S-02890—Rev. A, 21–Dec-00
With typical UVLO hysteresis of 0.1 V, the
IN
voltage exceeds the
sw
increases as well.
sw
IN
, varies with
pin is less
IN
voltage
New Product
can support, the converter will be in drop out mode and the
output voltage will be the input voltage minus the resistive drop
of the MOSFET and inductor.
BYPASS Mode
The Si9172 can also operate in Bypass mode to handle heavy
load current. In this mode the IC ignores the feedback signal
at the FB pin, forcing the internal PMOS to turn on at 100% duty
cycle. The input-output voltage differential is merely the
resistive voltage drop on the MOSFET and the inductor. The
Si9172 can bypass at least 800mA at 95% typical efficiency.
Whenever the converter enters BP mode, regardless
switching from PSM or starting up, the PMOS turns on
gradually within 100 ms.
Shutdown
The Si9172 is designed to conserve as much battery life as
possible by decreasing current consumption of the IC during
normal operation as well as the shutdown mode. With logic
low level on the ENABLE pin, the current consumption of the
IC is decreased to less than 1 mA by shutting off most of the
circuits. A logic high enables the controller and starts up as
described in the “Start-Up” section above.
Reference
The reference voltage of the Si9172 is set to 1.215 V. It is
internally connected to the non-inverting inputs of the error
amplifier. A 0.1-mF decoupling capacitor is required at the V
pin.
Power Switches
The main MOSFET switch is integrated in the Si9172 for
optimum performance and minimum overall converter size.
The internal MOSFET is designed to minimize the gate charge
loss as well as the conduction loss. The typical on-resistance
of the PMOS is 150 mW with a minimum V
3.3 V.
An external Schottky diode is mandatory for PSM mode
operation. It freewheels the inductor current after the main
switch is turned off, which is typical in basic non-synchronous
buck converter operation. It must be rated at 800 mA or higher,
with low forward drop to minimize power loss. The diode has
to be connected at PGND with the cathode connected to the
L
Over Temperature Protection
The Si9172 includes an over temperature protection circuit to
prevent thermal runaway in the MOSFET switch. If the
temperature reaches 165_C, an internal soft-start capacitor is
discharged, shutting down the output stage. The converter
remains in the disabled mode until the temperature in the IC
decreases below 140_C.
X
pin.
Vishay Siliconix
IN
pin voltage of
www.vishay.com
Si9172
REF
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