Datasheet

EN
START EN
V
Ren2 =
V - V
+ 1 A
Ren1
m
START STOP
V - V
Ren1 =
3 Am
EN
1.25V
VIN
+
-
Ren1
Ren2
1 Am 3 Am
TPS54231
SLUS851C –OCTOBER 2008–REVISED JULY 2012 www.ti.com
ECO-MODE
TM
The TPS54231 is designed to operate in pulse skipping Eco-mode
TM
at light load currents to boost light load
efficiency. When the peak inductor current is lower than 100 mA typically, the COMP pin voltage falls to 0.5 V
typically and the device enters Eco-mode
TM
. When the device is in Eco-mode
TM
, the COMP pin voltage is
clamped at 0.5 V internally which prevents the high side integrated MOSFET from switching. The peak inductor
current must rise above 100 mA for the COMP pin voltage to rise above 0.5 V and exit Eco-mode
TM
. Since the
integrated current comparator catches the peak inductor current only, the average load current entering Eco-
mode
TM
varies with the applications and external output filters.
VOLTAGE REFERENCE (V
ref
)
The voltage reference system produces a ±2% initial accuracy voltage reference (±3.5% over temperature) by
scaling the output of a temperature stable bandgap circuit. The typical voltage reference is designed at 0.8 V.
BOOTSTRAP VOLTAGE (BOOT)
The TPS54231 has an integrated boot regulator and requires a 0.1 μF ceramic capacitor between the BOOT and
PH pin to provide the gate drive voltage for the high-side MOSFET. A ceramic capacitor with an X7R or X5R
grade dielectric is recommended because of the stable characteristics over temperature and voltage. To improve
drop out, the TPS54231 is designed to operate at 100% duty cycle as long as the BOOT to PH pin voltage is
greater than 2.1 V typically.
ENABLE AND ADJUSTABLE INPUT UNDER-VOLTAGE LOCKOUT (VIN UVLO)
The EN pin has an internal pull-up current source that provides the default condition of the TPS54231 operating
when the EN pin floats.
The TPS54231 is disabled when the VIN pin voltage falls below internal VIN UVLO threshold. It is recommended
to use an external VIN UVLO to add Hysteresis unless VIN is greater than (V
OUT
+ 2 V). To adjust the VIN UVLO
with Hysteresis, use the external circuitry connected to the EN pin as shown in Figure 11. Once the EN pin
voltage exceeds 1.25 V, an additional 3 μA of hysteresis is added. Use Equation 1 and Equation 2 to calculate
the resistor values needed for the desired VIN UVLO threshold voltages. The V
START
is the input start threshold
voltage, the V
STOP
is the input stop threshold voltage and the V
EN
is the enable threshold voltage of 1.25 V. The
V
STOP
should always be greater than 3.5 V.
Figure 11. Adjustable Input Undervoltage Lockout
(1)
(2)
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