Datasheet

T
SW
1.039
71657
R (k ) =
(kHz)
W
f
SW
1114
tss(ms) =
(kHz)f
( )
UVLO ENAFALLING
UVLO
STOP ENAFALLING UVLO 1 HYS
R 1 V
R 2 =
V V + R 1 I + I
´
- ´
ENAFALLING
START STOP
ENARISING
UVLO
ENAFALLING
HYS
ENARISING
V
V V
V
R 1 =
V
I1 × 1 + I
V
æ ö
-
ç ÷
è ø
æ ö
-
ç ÷
è ø
TPS54061
i
VIN
Ruvlo1
Ruvlo2
EN
Optional
V
EN
ihys1
TPS54061
SLVSBB7C –MAY 2012–REVISED JANUARY 2014 www.ti.com
Figure 18. Adjustable Undervoltage Lock Out
(2)
(3)
Internal Slow Start
The TPS54061 has an internal digital slow start that ramps the reference voltage from zero volts to its final value
in 1114 switching cycles. The internal slow start time is calculated by the following expression:
(4)
If the EN pin is pulled below the stop threshold of 1.18 V, switching stops and the internal slow start resets. The
slow start also resets in thermal shutdown.
Constant Switching Frequency and Timing Resistor (RT/CLK Pin)
The switching frequency of the TPS54061 is adjustable over a wide range from 50 kHz to 1100 kHz by varying
the resistor on the RT/CLK pin. The RT/CLK pin voltage is typically 0.53 V and must have a resistor to ground to
set the switching frequency. To determine the timing resistance for a given switching frequency, use Equation 5.
To reduce the solution size, one would typically set the switching frequency as high as possible, but tradeoffs of
the supply efficiency, maximum input voltage and minimum controllable on time should be considered. The
minimum controllable on time is typically 120ns and limits the operating frequency for high input voltages. The
maximum switching frequency is also limited by the frequency shift circuit. More discussion on the details of the
maximum switching frequency is located below.
(5)
Selecting the Switching Frequency
The TPS54061 implements current mode control which uses the COMP pin voltage to turn off the high side
MOSFET on a cycle-by-cycle basis. Each cycle the switch current and COMP pin voltage are compared, when
the peak switch current intersects the COMP voltage, the high side switch is turned off. During overcurrent
conditions that pull the output voltage low, the error amplifier will respond by driving the COMP pin high,
increasing the switch current. The error amplifier output is clamped internally, which functions as a switch current
limit.
12 Submit Documentation Feedback Copyright © 2012–2014, Texas Instruments Incorporated
Product Folder Links: TPS54061