TPS62110-EP_16 Datasheet, PDF (13/25 Pages) Texas Instruments – 17 V, 1.5 A, SYNCHRONOUS STEP-DOWN CONVERTER

TPS62110-EP
TPS62111-EP
TPS62112-EP
www.ti.com .............................................................................................................................................................. SLVS630C – APRIL 2007 – REVISED MAY 2008
DETAILED DESCRIPTION
OPERATION
The TPS6211x is a synchronous step-down converter that operates with a 1 MHz 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 feedforward. Good line and load regulation is achieved with the use of small input and output ceramic
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 comparator trips and the control logic turns the switch
off. The switch is turned off by the current limit comparator if the current limit of the P-channel switch is
exceeded. After the dead time prevents 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 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 sawtooth 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 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 that 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. The
N-MOSFET of the devices stay on even when the current into the output drops to zero. This prevents the device
from going into discontinuous mode, and the device transfers unused energy back to the input. Therefore, there
is no ringing at the output, which usually occurs in 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 TPS6211x 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 pulse frequency modulation (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
about 200 mA at low input voltages and up to 400 mA with maximum input voltage. The average output current
must be below the threshold for at least 32 clock cycles to enter the power save mode. During the power save
mode, the output voltage is monitored with a comparator and the output voltage is regulated in to a typical value
between the nominal output voltage and 0.8% above the nominal output voltage. When the output voltage falls
below the nominal output voltage, the P-channel switch turns on. The P-channel switch is turned off as the peak
switch current is reached. The N-channel rectifier is turned 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 0.8% above the nominal
output voltage. This control method reduces the quiescent current to 20 µA (typical), and reduces the switching
frequency to a minimum that achieves the highest converter efficiency.
Copyright © 2007–2008, Texas Instruments Incorporated
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Product Folder Link(s): TPS62110-EP TPS62111-EP TPS62112-EP