Datasheet

TPS62052, TPS62054
TPS62056, TPS62050, TPS62051
SLVS432E –SEPTEMBER 2002–REVISED JUNE 2011 www.ti.com
APPLICATION INFORMATION
Operation
The TPS6205x is a synchronous step-down converter that operates with a 850-kHz fixed frequency pulse width
modulation (PWM) at moderate to heavy load currents and enters the power-save mode at light load current.
During PWM operation the converter uses a unique fast response voltage mode control scheme with input
voltage feed forward to achieve good line and load regulation with the use of small ceramic input and output
capacitors. At the beginning of each clock cycle initiated by the clock signal (S), the P-channel MOSFET switch
is turned on and the inductor current ramps up until the voltage-comparator trips and the control logic turns the
switch off. Also the switch is turned off by the current limit comparator in case the current limit of the P-channel
switch is exceeded. After the dead time preventing current shoot through, the N-channel MOSFET rectifier is
turned on and the inductor current ramps down. The next cycle is initiated by the clock signal again, turning off
the N-channel rectifier and turning on the P-channel switch.
The error amplifier as well as the input voltage determines the rise time of the saw tooth generator; therefore,
any change in input voltage or output voltage directly controls the duty cycle of the converter giving a very good
line and load transient regulation.
Constant Frequency Mode Operation (SYNC = HIGH)
In the constant frequency mode, the output voltage is regulated by varying the duty cycle of the PWM signal in
the range of 100% to 10%. Connecting the SYNC pin to a voltage greater than 1.5 V forces the converter to
operate permanently in the PWM mode even at light or no load currents. The advantage is the converter
operates with a fixed switching frequency that allows simple filtering of the switching frequency for noise sensitive
applications. In this mode, the efficiency is lower compared to the power-save mode during light loads (see
Figure 17). The N-MOSFET of the devices stays on even when the current into the output drops to zero. This
prevents the device from going into discontinuous mode. The device transfers unused energy back to the input.
Therefore, there is no ringing at the output that usually occurs in the discontinuous mode. The duty cycle range
in constant frequency mode is 100% to 10%.
It is possible to switch from forced PWM mode to the power-save mode during operation by pulling the SYNC pin
low. The flexible configuration of the SYNC pin during operation of the device allows efficient power management
by adjusting the operation of the TPS6205x to the specific system requirements.
Power-Save Mode Operation (SYNC = LOW)
As the load current decreases, the converter enters the power-save mode operation. During power-save mode
the converter operates with reduced switching frequency in PFM and with a minimum quiescent current to
maintain high efficiency. Whenever the average output current goes below the skip threshold, the converter
enters the power-save mode. The average current depends on the input voltage. It is 100 mA at low input
voltages and up to 200 mA with maximum input voltage. The average output current must be below the threshold
for at least 32 clock cycles (t
cy
) to enter the power-save mode. During the power-save mode the output voltage is
monitored with a comparator. When the output voltage falls below the comp low threshold set to 0.8% above V
O
nominal, the P-channel switch turns on. The P-channel switch turns off as the peak switch current of typically 200
mA is reached. The N-channel rectifier turns on and the inductor current ramps down. As the inductor current
approaches zero, the N-channel rectifier is turned off and the switch is turned on starting the next pulse. When
the output voltage can not be reached with a single pulse, the device continues to switch with its normal
operating frequency, until the comparator detects the output voltage to be 1.6% above the nominal output
voltage. The converter wakes up again when the output voltage falls below the comp low threshold. This control
method reduces the quiescent current to typically to 12 µA and the switching frequency to a minimum achieving
the highest converter efficiency. Having these skip current thresholds 0.8% and 1.6% above the nominal output
voltage gives a lower absolute voltage drop during a load transient as anticipated with a standard converter
operating in this mode.
Feedforward Capacitor
The feedforward capacitor, C
(ff)
shown in Figure 21, improves the performance in SKIP mode. The comparator is
faster, therefore, there is less voltage ripple at the output in SKIP mode. Use the values listed in Table 2. Larger
values decrease stability in fixed frequency PWM mode. If the TPS6205x is only operated in fixed frequency
PWM mode, the feedforward capacitor is not needed.
10 Submit Documentation Feedback Copyright © 2002–2011, Texas Instruments Incorporated
Product Folder Links: TPS62052 TPS62054 TPS62056 TPS62050 TPS62051