Datasheet

Tss(ms) Iss( A)
Css(nF) =
Vref(V)
´ m
-
Vref
R7 = R6
Vo Vref
( ) ( )
Voutmin Ontimemin Fsmax Vinmax Ioutmin 2 RDS Ioutmin RL RDS= ´ ´ - ´ ´ - ´ +
( ) ( ) ( )
Voutmax 1 Offtimemax Fsmax Vinmin Ioutmax 2 RDS Ioutmax RL RDS= - ´ ´ - ´ ´ - ´ +
TPS57112-Q1
SLVSAL8 –DECEMBER 2010 www.ti.com
nominal programmed value during power up. This is useful if a load requires a controlled voltage slew rate. This
is also used if the output capacitance is large and would require large amounts of current to quickly charge the
capacitor to the output voltage level. The large currents necessary to charge the capacitor may make the
TPS57112-Q1 reach the current limit or excessive current draw from the input power supply may cause the input
voltage rail to sag. Limiting the output voltage slew rate solves both of these problems.
The slow start capacitor value can be calculated using Equation 32. For the example circuit, the slow start time is
not too critical since the output capacitor value is 44 mF which does not require much current to charge to 1.8 V.
The example circuit has the slow start time set to an arbitrary value of 4ms which requires a 10 nF capacitor. In
TPS57112-Q1, Iss is 2.2 mA and Vref is 0.800 V.
(32)
BOOTSTRAP CAPACITOR SELECTION
A 0.1 mF ceramic capacitor must be connected between the BOOT to PH pin for proper operation. It is
recommended to use a ceramic capacitor with X5R or better grade dielectric. The capacitor should have 10 V or
higher voltage rating.
OUTPUT VOLTAGE AND FEEDBACK RESISTORS SELECTION
For the example design, 100 kΩ was selected for R6. Using Equation 33, R7 is calculated as 80 kΩ. The nearest
standard 1% resistor is 80.5 kΩ.
(33)
Due to the internal design of the TPS57112-Q1, there is a minimum output voltage limit for any given input
voltage. The output voltage can never be lower than the internal voltage reference of 0.827 V. Above 0.827 V,
the output voltage may be limited by the minimum controllable on time. The minimum output voltage in this case
is given by Equation 34
Where:
Voutmin = minimum achievable output voltage
Ontimemin = minimum controllable on-time (65 ns typical. 120 nsec no load)
Fsmax = maximum switching frequency including tolerance
Vinmax = maximum input voltage
Ioutmin = minimum load current
RDS = minimum high side MOSFET on resistance (15 - 19 mΩ)
RL = series resistance of output inductor (34)
There is also a maximum achievable output voltage which is limited by the minimum off time. The maximum
output voltage is given by Equation 35
Where:
Voutmax = maximum achievable output voltage
Offtimeman = maximum off time (60 nsec typical)
Fsmax = maximum switching frequency including tolerance
Vinmin = minimum input voltage
Ioutmax = maximum load current
RDS = maximum high side MOSFET on resistance (19 - 30 mΩ)
RL = series resistance of output inductor (35)
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