Datasheet

=
IN
RT
PWM
V
V
K
( )
= ´
-
UVLO
2 1
ON UVLO
V
R R
V V
-
=
ON OFF
1
UVLO
V V
R
I
TPS40170
SLUS970A –MARCH 2011–REVISED NOVEMBER 2013 www.ti.com
Equations for Programming the Input UVLO:
Components R1 and R2 represent external resistors for programming UVLO and hysteresis and can be
calculated in Equation 1 and Equation 2 respectively.
(1)
where
• V
ON
is the desired turn-on voltage of the converter
• V
OFF
is the desired turn-off voltage for the converter
• I
UVLO
is the hysteresis current generated by the device, 5.0 µA (typ)
• V
UVLO
is the UVLO pin threshold voltage, 0.9 V (typ) (2)
NOTE
If the UVLO pin is connected to a voltage greater than 0.9 V, the programmable UVLO is
disabled and the device defaults to an internal UVLO (V
VBP(on)
and V
VBP(off)
). For example,
the UVLO pin can be connected to VDD or the VBP pin to disable the programmable
UVLO function.
A 1-nF ceramic by-pass capacitor must be connected between the UVLO pin and GND.
Oscillator and Voltage Feed-Forward
TPS40170 implements an oscillator with input-voltage feed-forward compensation that enables instant response
to input voltage changes. Figure 18 shows the oscillator timing diagram for the TPS40170. The resistor from the
RT pin to GND sets the free running oscillator frequency. The voltage V
RT
on the RT pin is made proportional to
the input voltage (see Equation 3).
where
• K
PWM
= 15 (3)
The resistor at the RT pin sets the current in the RT pin. The proportional current charges an internal 100-pF
oscillator capacitor. The ramp voltage on this capacitor is compared with the RT pin voltage, V
RT
. Once the ramp
voltage reaches V
RT
, the oscillator capacitor is discharged. The ramp that is generated by the oscillator (which is
proportional to the input voltage) acts as voltage feed-forward ramp to be used in the PWM comparator.
The time between the start of the discharging oscillator capacitor and the start of the next charging cycle is fixed
at 170 ns (typical). During the fixed discharge time, the PWM output is maintained as OFF. This is the minimum
OFF-time of the PWM output.
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