33730_10 Datasheet, PDF (16/26 Pages) Freescale Semiconductor, Inc – Switch Mode Power Supply with Multiple Linear Regulators

FUNCTIONAL DEVICE OPERATION
OPERATION DESCRIPTION
output current is also used by the following linear regulators
V_{DD3_3}, V_DDL, and sensor supplies V_REF1, and V_REF2.
The direct voltage conversion to V_DDH = 5.0 V together
with the Protection FET Driver circuit allows operation of the
IC at very low battery voltages, which would otherwise
require to use a boost regulator (with an additional system
cost) or a different and more expensive switching converter
topology (e.g. flyback).
Short Circuit Protection
The switching regulator is protected against the over-
current and short-circuit conditions. It integrates a current
limit circuit, which has two threshold levels - the pulse by
pulse, and the extreme.
Pulse by Pulse Current Limit
Pulse-by-Pulse Current Limit threshold has a nominal
value set I_{LIM_SW}. When the current flowing through switching
regulator power FET exceeds this value the power FET is
immediately turned off. During the next switching cycle the
power FET is turned on again until it is commanded off by its
natural duty cycle or until the current reaches the threshold
level again. It should be noted that the current limit is blanked
for several tens of nanoseconds during the turn-on and turn-
off transition times in order to prevent erroneous turn off due
to the current spikes caused by switcher parasitic
components.
Extreme Current Limit.
In some cases, during the over-current or short-circuit
condition, the inductor current does not sufficiently decay
during the off time of the switching period. The current rise
during the current limit blanking time is higher than the decay
during the off time. In this case the current in the inductor
builds up every consecutive switching cycle. In order to
prevent the power FET failure during this condition an
extreme current limit has been implemented. When the
current flowing through the power FET reaches the
I_{LIM_SW_Ext} threshold, the switching regulator will shut off for
500 μs, before the switching regulator is allowed to turn on
again (see Figure 5).
ILim
3.5A
Inductor
Cu rrent
Ex t.
4.5A ^ILim
tB _LANK
0
Swi tc her
FET Gate
500us d elay
TSW
TSW
Figure 5. 33730 Current Limit
Soft Start
The switching regulator has an integrated soft-start
feature. During the soft-start sequence the duty cycle of the
internal power switch will be gradually increased from low
value to the regulation level. This technique prevents any
undesirable inrush current into the buck regulator output
capacitor.
LINEAR REGULATORS
The 33730 integrates two linear regulator control circuits
V_DD3 (programmable), V_DDL (programmable) both capable of
driving up to 15 mA (min.) base current into the external pass
NPN transistors. The output voltage of both linear regulators
is monitored at their feedback pins (V_DD3 and V_DDL). If the
voltage at any of the V_DD3, V_DDL feedback pins fall below their
regulation level, the supervisory Reset control circuits will
assert the corresponding reset signal (RSTL, and/or RST3
lines will be pulled low). See Table 6 for the output voltage
selection details.
The linear regulators will stay in regulation down to 4.5 V
at the KA_VBAT pin.
The 33730 linear regulators offer high flexibility and
variability of the module design in terms of selectable output
voltages as well as wide range of output current capability.
There several types of suitable external pass NPN transistors
which could be used. The choice of the particular type
depends mostly on the expected power dissipation of the
pass transistor. The following parts provide good solution and
have been bench tested with the 33730:
BCP68T1 (SOT-223)
NJD2873T4 (DPAK)
MJB44H11 (D²PAK)
Available from ON Semiconductor.
NOTE: The 33730 linear regulators have been designed to
use low ESR ceramic output capacitors - see Figure 8 and
Figure 9 for the recommended values.
STANDBY REGULATORS
The 33730 integrates two standby linear regulators, the
V_KAM and the optional standby regulator V_DD3 (see Figure 9)
for the optional standby circuit).The output voltage levels of
both standby linear regulators are programmable and
supervised by the Reset control circuits (RSTKAM, and/or
RST3). Both the V_KAM and V_DD3 outputs are capable of
delivering I_{VKAM_LIM} and I_{VDD3_LIM} of load current. See
Table 6 for the V_KAM and V_DD3 standby output voltage
selection details.
The V_KAM standby regulator will keep functioning even
below V_{UVLO_f} but the specified drop out voltage may not be
maintained.
NOTE: The 33730 standby regulators have been designed
to use low ESR ceramic output capacitors - see Figure 8 and
Figure 9 for recommended values.
33730
16
Analog Integrated Circuit Device Data
Freescale Semiconductor