34701_07 Datasheet, PDF (22/38 Pages) Freescale Semiconductor, Inc – 1.5 A Switch-Mode Power Supply with Linear Regulator

FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
FUNCTIONAL DEVICE OPERATION
OPERATIONAL MODES
THERMAL SHUTDOWN
To increase the overall safety of the system designed with
the 34701, an internal Thermal Shutdown function has been
incorporated into the switching regulator circuit. The 34701
senses the temperature of the buck regulator main switching
MOSFET (high-side MOSFET M1; see Figure 2 on page 2),
the low-side (synchronous MOSFET M2), and control circuit.
If the temperature of any of the monitored components
exceeds the limit of safe operation (Thermal Shutdown), the
switching regulator and the LDO shut down. After the
temperature falls below the value given by the Thermal
Shutdown hysteresis window, the switcher tries again to
operate.
The VOUT pull-down MOSFET M3 has an independent
Thermal Shutdown control. If the M3 temperature exceeds
the Thermal Shutdown, the M3 is turned off without affecting
the switcher operation.
The LDO pull-down MOSFET M4 has an independent
Thermal Shutdown control. If the M4 temperature exceeds
the Thermal Shutdown, the M4 will be turned off without
affecting the LDO operation.
SOFT START
A switching regulator soft start feature is incorporated in
the 34701. The soft start is active each time the IC is enabled,
VIN is reapplied, or after a fault retry. Other transient events
do not activate the soft start.
VOLTAGE MARGINING
The 34701 includes a voltage margining feature accessed
through the I²C bus. Voltage margining allows for
independent adjustment of the Switcher VOUT voltage and
the linear output VLDO. Each can be adjusted up and down
in 1.0% steps to a range of ±7.0%. This feature allows for
worst case system validation; i.e., determining the design
margin. Margining details are described in the section entitled
I²C Bus Operation, beginning on page 26 of this datasheet.
POWER SEQUENCING MODES
The power sequencing of the two outputs of this power
supply IC is in compliance with the Freescale Power QUICC
and other 32-bit microprocessor requirements. When the
input voltage is applied, the switcher and linear regulator
outputs follow the supply rail voltage during power-up and
power-down in the limits given by the microcontroller power
sequencing specification, illustrated in Figures 17 through
19. There are two possible power sequencing modes,
Standard and Inverted, as explained in more detail below.
The third mode of operation is Power Sequencing Disabled.
Optional
3.3 V Input
34701
VIN2
VIN1
VBD
VBST
RT
LDRV
CS
LDO
ADDR
SDA
SCL
GND
EN1
EN2
CLKSYN
CLKSEL
FREQ
LFB
RST
BOOT
SW
VOUT
PGND
INV
VDDI
2.5 V
Other
Circuits
3.3 V
VDDL (Core)
VBST
MCU
1.5 V
VDDH (I/Os)
3.3 V Input Supply (I/O Voltage)
∆V = 2.1 V
Max. Lead
1.8V Start-Up
Slope
1.0 V/ms
1.5 V Core Voltage
∆V = 2.1 V
Max. Lead
∆V = 0.4 V
Max. Lag
Figure 17. Standard Power Up / Down Sequence
in +3.3 V Supply System
34701
22
Analog Integrated Circuit Device Data
Freescale Semiconductor