Datasheet

( ) ( )
( )
f
IN OUT OUT
TRIP RIPPLE TRIP
OCP
IN
DS on DS on
V V V
V I V
1
I
R 2 R 2 L V
- ´
= + = + ´
´ ´
( )
( ) ( )
( )
TRIP TRIP
TRIP
R k I A
V mV 24 mV
9
W ´ m
= -
TPS51123A
SLUSAA6C –APRIL 2011–REVISED SEPTEMBER 2012 www.ti.com
Also note that, in the case of Auto-skip or Out-of-Audio™ mode, if the output voltage goes +10% above the
target value and the power-good signal flags low, then the loop attempts to correct the output by turning on the
low-side driver (forced PWM mode). After the feedback voltage returns to be within +5% of the target value and
the power-good signal goes high, the controller returns back to auto-skip mode or Out-of-Audio™ mode.
Output Discharge Control
When ENC is low, the TPS51123A discharges outputs using internal MOSFET which is connected to VOx and
GND. The current capability of these MOSFETs is limited to discharge slowly.
Low-Side Driver
The low-side driver is designed to drive high current low R
DS(on)
N-channel MOSFET(s). The drive capability is
represented by its internal resistance, which are 4 Ω for VREG5 to DRVLx and 1.5 Ω for DRVLx to GND. A dead
time to prevent shoot through is internally generated between top MOSFET off to bottom MOSFET on, and
bottom MOSFET off to top MOSFET on. 5-V bias voltage is delivered from VREG5 supply. The instantaneous
drive current is supplied by an input capacitor connected between VREG5 and GND. The average drive current
is equal to the gate charge at Vgs = 5 V times switching frequency. This gate drive current as well as the high-
side gate drive current times 5 V makes the driving power which need to be dissipated from TPS51123A
package.
High-Side Driver
The high-side driver is designed to drive high current, low R
DS(on)
N-channel MOSFET(s). When configured as a
floating driver, 5-V bias voltage is delivered from VREG5 supply. The average drive current is also calculated by
the gate charge at Vgs = 5 V times switching frequency. The instantaneous drive current is supplied by the flying
capacitor between VBSTx and LLx pins. The drive capability is represented by its internal resistance, which are 4
Ω for VBSTx to DRVHx and 1.5Ω for DRVHx to LLx.
Current Protection
TPS51123A has cycle-by-cycle over current limiting control. The inductor current is monitored during the ‘OFF’
state and the controller keeps the ‘OFF’ state during the inductor current is larger than the over current trip level.
In order to provide both good accuracy and cost effective solution, TPS51123A supports temperature
compensated MOSFET R
DS(on)
sensing. The TRIPx pin should be connected to GND through the trip voltage
setting resistor, R
TRIP
. TRIPx terminal sources I
TRIP
current, which is 10 μA typically at room temperature, and the
trip level is set to the OCL trip voltage V
TRIP
as below. Note that the V
TRIP
is limited up to about 205 mV
internally.
(3)
Note that when TRIPx voltage is under a certain thershould (typically 0.4V), the switcher channel concerned is
shut down. The inductor current is monitored by the voltage between GND pin and LLx pin so that LLx pin should
be connected to the drain terminal of the bottom MOSFET properly. Itrip has 4500 ppm/°C temperature slope to
compensate the temperature dependency of the R
DS(on)
. GND is used as the positive current sensing node so
that GND should be connected to the proper current sensing device, i.e. the source terminal of the bottom
MOSFET.
As the comparison is done during the OFF state, V
TRIP
sets valley level of the inductor current. Thus, the load
current at over current threshold, I
OCP
, can be calculated in Equation 4.
(4)
In an overcurrent condition, the current to the load exceeds the current to the output capacitor thus the output
voltage tends to fall down. Eventually, it ends up with crossing the under voltage protection threshold and
shutdown both channels.
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