Datasheet

Q
Q
FB
VIN
VCC
SW
RTN
ON TIMER
DRIVER
Vin
BST
LEVEL
SHIFT
THERMAL
SHUTDOWN
FINISH
START
RCL
FINISH
START
RT
L1
R
VCC
UVLO
FB
D1
UVLO
REGULATION
COMPARATOR
SD
FINISH
START
REGULATOR
CURRENT LIMIT
OFF TIMER
SHUTDOWN
GD
BUCK
SWITCH
CURRENT
SENSE
GND
7V BIAS
BYPASS
SWITCH
R
T
2.875
V
V
IN
SENSE
Q2
C1
C5
6V to 95V
Input
RT/SD
0.7
V
300 ns MIN
OFF TIMER
CLR
R
SET
S
C2
C3
V
OUT
R3
R
FB1
FB2
R
CL
R
CL
2.5A
OVER-VOLTAGE
COMPARATOR
CHARGE
PRE
-
C4
SM72485
0.3A
SM72485
SNVS697D –JANUARY 2011–REVISED APRIL 2013 www.ti.com
BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
The SM72485 Step Down Switching Regulator features all the functions needed to implement a low cost,
efficient, Buck bias power converter. This high voltage regulator contains a 100 V N-Channel Buck Switch, is
easy to implement and is provided in the VSSOP and the thermally enhanced WSON package. The regulator is
based on a control scheme using an on-time inversely proportional to V
IN
. The control scheme requires no loop
compensation. Current limit is implemented with forced off-time, which is inversely proportional to V
OUT
. This
scheme ensures short circuit control while providing minimum foldback.
The SM72485 can be applied in numerous applications to efficiently regulate down higher voltages. This
regulator is well suited for high voltage PV panel junction boxes, 48 Volt Telecom and the new 42V Automotive
power bus ranges. Features include: Thermal Shutdown, V
CC
under-voltage lockout, Gate drive under-voltage
lockout, Max Duty Cycle limit timer, intelligent current limit off timer, and a pre-charge switch.
Control Circuit Overview
The SM72485 is a Buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with
line voltage (V
IN
). Control is based on a comparator and the on-time one-shot, with the output voltage feedback
(FB) compared to an internal reference (2.5V). If the FB level is below the reference the buck switch is turned on
for a fixed time determined by the line voltage and a programming resistor (R
T
). Following the ON period the
switch will remain off for at least the minimum off-timer period of 300ns. If FB is still below the reference at that
time the switch will turn on again for another on-time period. This will continue until regulation is achieved.
The SM72485 operates in discontinuous conduction mode at light load currents, and continuous conduction
mode at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero
and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-
time period starts when the voltage at FB falls below the internal reference - until then the inductor current
remains zero. In this mode the operating frequency is lower than in continuous conduction mode, and varies with
load current. Therefore at light loads the conversion efficiency is maintained, since the switching losses reduce
with the reduction in load and frequency. The discontinuous operating frequency can be calculated as follows:
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