EN
95%
5%
V
OUT
t
Start
t
RAMP
SLVSAP7 – JANUARY 2011
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Ioutmax = maximum output current plus inductor ripple current
RDSonmax = maximum P-channel switch RDSon.
RL = DC resistance of the inductor
Voutmax = nominal output voltage plus maximum output voltage tolerance
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 DCDC converters and LDOs. The under-voltage lockout
threshold is configurable in the range of typically 2.8V to 3.25V with falling voltage at the SYS pin. The default
undervoltage lockout voltage as well as the hysteresis are defined in register CON_CTRL2. The default
undervoltage lockout voltage is 3.0V with 500mV hysteresis.
SHORT-CIRCUIT PROTECTION
The High Side and Low Side MOSFET switches are short-circuit protected with maximum output current = ILIMF.
Once the High Side MOSFET switch reaches its current limit, it is turned off and the Low Side MOSFET switch is
turned. The High Side MOSFET switch can only turn on again, once the current in the Low Side MOSFET switch
decreases below its current limit.
Soft Start
The 3 step-down converters in have an internal soft start circuit that controls the ramp up of the output voltage.
The output voltage ramps up from 5% to 95% of its nominal value within typ. 250ms. This limits the inrush current
in the converter during start up and prevents possible input voltage drops when a battery or high impedance
power source is used. The Soft start circuit is enabled after the start up time tStart has expired.
During soft start, the output voltage ramp up is controlled as shown in
Figure 31.
Figure 31. Soft Start
ENABLE
To start up each converter independently, the device has a separate enable pin for each of the DCDC
converters. In order to enable any converter with its enable pins, the devices need to be in ON-state by pulling
PB_IN=LOW or POWER_ON=HIGH. The sequencing option programmed needs to be DCDC_SQ[2..0] = 101.
If EN_DCDC1, EN_DCDC2, EN_DCDC3 are set to high, the corresponding converter starts up with soft start as
previously described.
Pulling the enable pin low forces the device into shutdown, with a shutdown quiescent current as defined in the
electrical characteristics. In this mode, the high side and low side MOSFETs are turned-off, and the entire
internal control circuitry is switched-off. If disabled, the outputs of the DCDC converters are pulled low by internal
250
resistors, actively discharging the output capacitor. For proper operation the enable pins must be
terminated and must not be left floating.
32
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