MIC7401 Datasheet, PDF (27/68 Pages) Micrel Semiconductor – Configurable PMIC, Five-Channel Buck Regulator plus One-Boost with HyperLight Load®, I2C Control, and Enable

Micrel, Inc.
The ramp time is determined by Equation 2:
∆t
=

V_OUT
− V_OUT
50mV
_ INIT

×
t_RAMP
Eq. 2
MIC7401
After the T1 period, the DAC output ramp starts, T2. The
total soft-start time, t_SS, is the sum of both periods. Figure
6 displays the actual boost soft-start waveform.
Where:
V_{OUT_INIT} = Initial output voltage
V_OUT = Final output voltage
t_RAMP = Output dwell time
When the regulator is set in stand-by mode or
programmed to a lower voltage, then the output voltage
ramps down at a rate determined by the output ramp rate
(t_RAMP), the output capacitance and the external load.
Small loads result in slow output voltage decay and
heavy loads cause the decay to be controlled by the DAC
ramp rate.
In Figure 4, V_OUT1 is switched to stand-by mode with an
I²C command and then switched back to normal mode
either by an I²C command or a low-to-high transition of
the STBY pin. In this case, the rise and fall times are the
same due to a 1A load on V_OUT1.
Figure 5. Boost Soft-Start Ramp
Figure 4. Buck DVC Control Ramp
Programmable Boost Soft-Start Control
The boost soft-start time is divided into two parts as
shown in Figure 5. T1 is a fixed 367µs delay starting from
when the internal enable goes high. This delay gives
enough time for the disconnect switch to turn on and
bring the inductor voltage to V_IN before the boost is turned
on. There is a 50µs delay which is controlled by the
parasitic capacitance (Cgd) of the disconnect switch
before the output starts to rise.
March 16, 2015
27
Figure 6. Boost Soft-Start
Revision 1.0