Datasheet

0V
V
OUT
t
Switch
Node
V
IN
T
s
2T
s
|V
OUT
|
Inductor
Current
V
OUT
t
Switch
Node
V
IN
T
s
2T
s
|V
OUT
|
Inductor
Current
DCM CCM
3T
s
0V
VIN VOUTSW
LMR70503
SNVS850A –JUNE 2012–REVISED APRIL 2013 www.ti.com
OPERATING DESCRIPTION
The LMR70503 integrates an inverting buck-boost controller and a high-side MOSFET in one tiny 8-bump thin
DSBGA package. A simplified buck-boost converter schematic is shown in Figure 29.
Figure 29. Buck Boost Converter
The LMR70503 controller incorporates a unique peak current mode control method with a minimum switching
frequency limit. The integrated switch is turned off when its current crosses the peak current limit, while it is
turned on when the magnitude of V
OUT
droops below a threshold. When the switch is off, the inductor current
goes through the diode and charges the output capacitor(s). With fixed peak current limit, the switching
frequency decreases with decreased load current. At light load, the switching frequency will decrease to the
audible frequency range, which is not acceptable in many applications. The LMR70503 is designed to operate
with peak current mode control and limit the switching frequency to 500 kHz (typical) minimum, to avoid audible
frequency interference. At light load, when the switching frequency drops to the minimum, the inductor current
limit is reduce instead of frequency to maintain regulation. The LMR70503 also incorporates an internal dummy
load to compensate for the extra charges in the minimum ON-time (T
ON-MIN
) condition. More details on the
LMR70503 operation are described in the later sessions. Typical switching waveforms in discontinuous
conduction mode (DCM) and continuous conduction mode (CCM), including the inductor current, the switch node
voltage and the output voltage ripple (absolute value), are shown in Figure 30.
Figure 30. Typical Waveforms In Buck Boost Converter
Figure 31 illustrates the switching frequency, the peak current limit, the output voltage and the dummy load with
different load current. More details on each operation mode will be described later.
1. No load to very light load: high side switch is turned on for T
ON-MIN
; switching frequency is limited at the
minimum switching frequency; and the dummy load is turned on.
2. Light load: switching frequency is limited at the minimum switching frequency, peak current limit increases
with increased load current; and the dummy load is off.
3. Heavy load: peak current equals the maximum peak current limit; switching frequency increases with
increased load current; and the dummy load is off.
10 Submit Documentation Feedback Copyright © 2012–2013, Texas Instruments Incorporated
Product Folder Links: LMR70503