Datasheet

95%
5%
t
RAMP
t
Startup
EN
V
OUT
Vin
min
+ Vout
max
) Iout
max
ǒ
RDSon
max
) R
L
Ǔ
TPS62420-Q1
SLVSA56 –DECEMBER 2009 www.ti.com
Figure 27. Soft Start
100% Duty Cycle Low Dropout Operation
The converters offer a low input to output voltage difference while still maintaining operation with the use of the
100% duty cycle mode. In this mode the P-channel switch is constantly turned on. This is particularly useful in
battery-powered applications to achieve longest operation time by taking full advantage of the whole battery
voltage range; i.e., the minimum input voltage to maintain regulation depends on the load current and output
voltage, and can be calculated as:
(3)
With:
Iout
max
= maximum output current plus inductor ripple current
RDSon
max
= maximum P-channel switch RDSon
R
L
= DC resistance of the inductor
Vout
max
= nominal output voltage plus maximum output voltage tolerance
With decreasing load current, the device automatically switches into pulse skipping operation in which the power
stage operates intermittently based on load demand. By running cycles periodically the switching losses are
minimized and the device runs with a minimum quiescent current maintaining high efficiency.
Under-Voltage Lockout
The under voltage lockout circuit prevents the device from malfunctioning at low input voltages and from
excessive discharge of the battery and disables the converters. The under-voltage lockout threshold is typically
1.5V, max 2.35V. In case the default register values are overwritten by the Interface, the new values in the
registers REG_DEF_1_Low and REG_DEF_2 remain valid as long the supply voltage does not fall under the
under-voltage lockout threshold, independent of whether the converters are disabled.
MODE SELECTION
The MODE/DATA pin allows mode selection between forced PWM Mode and Power Save Mode for both
converters. Furthermore, this pin is a multipurpose pin and provides (besides Mode selection) a one-pin interface
to receive serial data from a host to set the output voltage. This is described in the section EasyScale Interface.
Connecting this pin to GND enables the automatic PWM and power save mode operation. The converters
operate in fixed-frequency PWM mode at moderate to heavy loads and in the PFM mode during light loads,
maintaining high efficiency over a wide load current range.
Pulling the MODE/DATA pin high forces both converters to operate constantly in the PWM mode even at light
load currents. The advantage is the converters operate with a fixed 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. For additional flexibility it is possible to switch from Power Save Mode to forced
PWM mode during operation. This allows efficient power management by adjusting the operation of the converter
to the specific system requirements.
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