TPS51275 Datasheet, PDF (14/28 Pages) Texas Instruments – Dual Synchronous, Step-Down Controller with 5-V and 3.3-V LDOs

TPS51275, TPS51275C
SLUSB45 – JUNE 2012
www.ti.com
Soft-Start and Discharge
The TPS51275/C operates an internal, 0.8-ms, voltage servo soft-start for each channel. When the ENx pin
becomes higher than the enable threshold voltage, an internal DAC begins ramping up the reference voltage to
the PWM comparator. Smooth control of the output voltage is maintained during start-up. When ENx becomes
lower than the lower level of threshold voltage, TPS51275/C discharges outputs using internal MOSFETs through
VO1 (CH1) and SW2 (CH2).
VREG5/VREG3 Linear Regulators
There are two sets of 100-mA standby linear regulators which output 5 V and 3.3 V, respectively. The VREG5
pin provides the current for the gate drivers. The VREG3 pin functions as the main power supply for the analog
circuitry of the device. VREG3 is an Always ON LDO and TPS51275C has Always ON VREG5. (See Table 1 and
Table 2)
Add ceramic capacitors with a value of 1 µF or larger (X5R grade or better) placed close to the VREG5 and
VREG3 pins to stabilize LDOs.
The VREG5 pin switchover function is asserted when three conditions are present:
• CH1 internal PGOOD is high
• CH1 is not in OCL condition
• VO1 voltage is higher than VREG5-1V
In this switchover condition, three things occur:
• the internal 5-V, LDO regulator is shut off
• the VREG5 output is connected to VO1 by internal switchover MOSFET
• VREG3 input pass is changed from VIN to VO1
VCLK for Charge Pump
The 260-kHz VCLK signal can be used in the charge pump circuit. The VCLK signal becomes available when
EN1. The VCLK driver is driven by VO1 voltage. In a design that does not require VCLK output, leave the VCLK
pin open.
Overcurrent Protection
TPS51275/C has cycle-by-cycle over current limiting control. The inductor current is monitored during the OFF
state and the controller maintains the OFF state during the inductor current is larger than the overcurrent trip
level. In order to provide both good accuracy and cost effective solution, TPS51275/C supports temperature
compensated MOSFET R_DS(on) sensing. The CSx pin should be connected to GND through the CS voltage
setting resistor, R_CS. The CSx pin sources CS current (I_CS) which is 10 µA typically at room temperature, and the
CSx terminal voltage (V_CS= R_CS × I_CS) should be in the range of 0.2 V to 2 V over all operation temperatures.
　The trip level is set to the OCL trip voltage (V_TRIP) as shown in Equation 3.
V_TRIP
=
R_CS ´ I_CS
8
+ 1 mV
(3)
The inductor current is monitored by the voltage between GND pin and SWx pin so that SWx pin should be
connected to the drain terminal of the low-side MOSFET properly.The CS pin current has a 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 source terminal of the low-side MOSFET.
As the comparison is done during the OFF state, V_TRIP sets the valley level of the inductor current. Thus, the load
current at the overcurrent threshold, I_OCP, can be calculated as shown in Equation 4.
( ) _IOCP
=
V_TRIP
R_DS(on)
I_IND(ripple)
+
2
=
V_TRIP
R_DS(on)
+
1
2 ´ L ´ f_SW
´
V_IN - V_OUT
V_IN
´ V_OUT
(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 undervoltage protection threshold and
shutdown both channels.
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