FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
VIN < 3.0V
SD = 0
Power Off
VOUT = OFF
VREF = OFF
PG = 1
Shutdown
VOUT = Discharge
VREF = Discharge
PG = 1
3.0V < = VIN < = 6.0V
SD = 1
Short Circuit
VOUT = OFF
VREF = OFF
Overvoltage
VOUT = ON
VREF = ON
Normal
VOUT= ON
VREF = ON
VOUT
< VOV
tTIMEOUT Expired
VOUT
> VOV
IOUT > = ISHORT
PG = 1
PG = 1
PG = 0
t
TIMEOUT Expired
VOUT > VUV
Over Current
VOUT = OFF
VREF = ON
Undervoltage
VOUT= ON
VREF = ON
TJ < = 145°C &
TIMEOUT Expired
IOUT > = ILIM
For > = 10ms
t
VOUT < VUV
Thermal
Shutdown
VOUT = OFF
VREF = OFF
PG = 1
PG = 1
PG = 1
TJ > = 170°C
Figure 5. Operation Modes Diagram
switching frequency and soft start values are determined by
reading the FREQ and ILIM pins respectively. A soft start
cycle is then initiated to ramp up the output of the buck
converter. The buck converter error amplifier uses the
voltage on the VREFIN pin as its reference voltage until
VREFIN is equal to 0.7 V, then the error amplifier defaults to
the internal 0.7 V reference voltage. This method helps
achieve multiple tracking configurations as will be explained
later in this document.
MODES OF OPERATION
The 34713 has two primary modes of operation:
Normal Mode
In normal mode, all functions and outputs are fully
operational. To be in this mode, the VIN needs to be within its
operating range, Shutdown input is high, and no faults are
present. This mode consumes the most amount of power.
Soft start is used to prevent the output voltage from
overshooting during startup. At initial startup, the output
capacitor is at zero volts; VOUT = 0 V. Therefore, the voltage
across the inductor will be PVIN during the capacitor charge
phase which will create a very sharp di/dt ramp. Allowing the
inductor current to rise too high can result in a large
difference between the charging current and the actual load
current that can result in an undesired voltage spike once the
capacitor is fully charged. The soft start is active each time
the IC goes out of standby or shutdown mode, power is
recycled, or after a fault retry.
Shutdown Mode
In this mode, activated by pulling the SD pin low, the chip
is in a shutdown state and the output is disabled and
discharged. In this mode, the 34713 consumes the least
amount of power since almost all of the internal blocks are
disabled.
START-UP SEQUENCE
When power is first applied, the 34713 checks the status
of the SD pin. If the device is in a shutdown mode, no block
will power up and the output will not attempt to ramp. Once
the SD pin is released to enable the device, the VDDI internal
supply voltage and the bias currents are established and the
internal VDDI POR signal is also released. The rest of the
internal blocks will be enabled and the buck converter
After a successful start-up cycle where the device is
enabled, no faults have occurred, and the output voltage has
reached its regulation point, the 34713 pulls the power good
output signal low after a 10 ms reset delay, to indicate to the
host that the device is in normal operation.
34713
Analog Integrated Circuit Device Data
Freescale Semiconductor
12
FREESCALE [ Freescale ]
