TPS62095_16 Datasheet, PDF (8/27 Pages) Texas Instruments – TPS62095 4A, High Efficiency Step Down Converter with DCS-Control and Low Profile Solution

TPS62095
SLVSBD8A – APRIL 2014 – REVISED MAY 2014
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7.3 Feature Description
7.3.1 PWM Operation
In PWM mode, the device operates with a fixed ON-time switching pulse at medium to heavy load currents. A
quasi fixed switching frequency of typical 1.4MHz over the input and output voltage range is achieved by using
an input feed forward. The ON-time is calculated as shown in Equation 2. As the load current decreases, the
converter enters Power Save Mode operation reducing its switching frequency. The device enters Power Save
Mode at the boundary to discontinuous conduction mode (DCM).
7.3.2 Low Dropout Operation (100% Duty Cycle)
The device offers low input to output voltage difference by entering 100% duty cycle mode. In this mode the high
side MOSFET switch is constantly turned on. This is particularly useful in battery powered applications to achieve
longest operation time by taking full advantage of the whole battery voltage range. The minimum input voltage
where the output voltage falls below set point is given by:
V_IN(min) = V_OUT(min) + I_OUT x ( R_DS(on) + R_L )
(1)
Where
R_DS(on) = High side FET on-resistance
R_L = DC resistance of the inductor
V_OUT(min) = Minimum output voltage the load can accept
7.3.3 Power Save Mode Operation
As the load current decreases, the converter enters Power Save Mode operation. During Power Save Mode, the
converter operates with reduced switching frequency and with a minimum quiescent current to maintain high
efficiency. The Power Save Mode is based on a fixed on-time architecture following Equation 2.
ton = ^VOUT × 360ns × 2
VIN
2×I
f=
OUT
æ V -V
ö V -V
ton² ç1 + ^{IN OUT} ÷ x ^{IN OUT}
çè
VOUT ÷ø
L
(2)
In Power Save Mode, the output voltage rises slightly above the nominal output voltage in PWM mode. This
effect is reduced by increasing the output capacitance or the inductor value. This effect is also reduced by
programming the output voltage of the TPS62095 lower than the target value. As an example, if the target output
voltage is 3.3V, then the TPS62095 can be programmed to 3.3V - 0.3%. As a result, the output voltage accuracy
is now -1.7% to +1.7% instead of -1.4% to 2%. The output voltage accuracy in PFM operation is reflected in the
electrical specification table and given for a 2 x 22µF output capacitance.
7.4 Device Functional Modes
7.4.1 Soft Startup
To minimize inrush current during startup, the device has an adjustable startup time depending on the capacitor
value connected to the SS pin. The device charges the SS capacitor with a constant current of typically 7.5µA.
The feedback voltage follows this voltage divided by 1.56, until the internal reference voltage of 0.8V is reached.
The soft startup operation is completed once the voltage at the SS capacitor has reached typically 1.25V. The
soft startup time is calculated using Equation 3. The larger the SS capacitor, the longer the soft startup time. The
relation between the SS pin voltage and the FB pin voltage is estimated using Equation 4. Leaving the SS pin
floating sets the minimum startup time.
t_SS
= C_SS
x
1.25V
7.5μA
(3)
V_FB
=
V_SS
1.56
(4)
8
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