FAN5038 Datasheet, PDF (9/15 Pages) Fairchild Semiconductor – Dual Voltage Controller for DSP Power

FAN5038
PRODUCT SPECIFICATION
Linear Control Loop
The low-offset op-amp is configured to be the controlling
element in a precision low-drop-out linear regulator. As can
be seen from Figure 2, the op-amp is used to compare the
divided down output of the linear regulator to the precision
reference. The error signal is used to control either an
N-channel MOSFET or a power NPN transistor.
High Current Output Drivers
The FAN5038 switching high current output driver (SDRV)
contains high speed bipolar power transistors configured in a
push-pull configuration. The output driver is capable of sup-
plying 0.5A of current in less than 100ns. The driver’s power
and ground are separated from the overall chip power and
ground for added switching noise immunity.
Internal Reference
The reference in the FAN5038 is a precision band-gap type
reference. Its temperature coefficient is trimmed to provide a
near zero TC. For guaranteed stable operation under all condi-
tions, a 0.1µF capacitor is recommended on the VREF output
pin. No load may be attached to this pin.
Constant-On-Time Oscillator
The FAN5038 switch-mode oscillator is designed as a fixed
on-time, variable off-time oscillator. The constant-on-time
oscillator consists of a comparator, an external capacitor, a
fixed current source, a variable current source, and an analog
switch that selects between two threshold voltages for the
comparator. The external timing capacitor is alternately
charged and discharged through the enabling and disabling
of the fixed current source. The variable current source is
controlled from the error inputs that are received from the
current and voltage feedback signals. The oscillator off-time
is controlled by the amount of current that is available from
the variable current source to charge the external capacitor up
to the high threshold level of the comparator. The on-time is
set be the constant current source that discharges the external
capacitor voltage down to the lower comparator threshold.
Using SWCTRL and LIN_EN
When the SWCTRL pin is HIGH, the switching regulator
will set its output at 3.5V using two internal precision resis-
tors. When this pin is LOW, the switching regulator output
can be set to any voltage between 1.5V and 3.6V using exter-
nal precision resistors. The LIN_EN pin is used to enable or
disable the linear regulator. When the LIN_EN pin is HIGH,
the linear regulator will be disabled. If this pin is LOW, the
linear regulator output can be set from 1.5V to 3.5V using
external precision resistors.
Power Sequencing
The linear regulator output can be sequenced with the circuit
shown in Figure 1. The combination R4 = 100KΩ and
CS = 1µF sets a delay of approximately 25 msec. Diode D2
prevents core voltage from exceeding I/O voltage, so that I/O
tracks core until after the delay.
+5V
+12V
gm
gm
CONSTANT ON-TIME
OSCILLATOR
gm
IO _ANALOG
SWITCH
VH
VL
ION
VREF
REF
FAN5038
SWCTRL
LIN_EN
+12V
SWITCHER
SELECT
LINEAR
ENABLE
Figure 2. FAN5038 Block Diagram
9
VCORE
VI/O
REV. 1.0.2 7/6/00