Datasheet

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5 Operation Description
The LMR70503 controller incorporates a unique peak current mode control method with a minimum
switching frequency limit. 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
reduced instead of frequency to maintain regulation. The LMR70503 also incorporates an internal dummy
load to maintain output regulation in the minimum ON-time (T
ON-MIN
) condition. The maximum load the
LMR70503 can provide is limited by the maximum peak inductor current and the minimum off time
(T
OFF_MIN
) of the high side switch. The maximum load varies with input voltage, output voltage and the
inductor value. For more details on the LMR70503 operation, see LMR70503 SIMPLE SWITCHER Buck-
Boost Converter For Negative Output Voltage in DSBGA (SNVS850).
6 Enable and Disable
The LMR70503 features an enable (EN) pin and associated comparator to allow the user to easily
sequence the LMR70503 from an external voltage rail, or to manually set the input UVLO threshold.
Enable threshold levels are referred to the LMR70503 ground, instead of the lowest potential: the negative
output voltage. It is important to ensure that a valid input voltage (2.8 V ≤ V
IN
≤ 5.5 V) is present on the VIN
pin before the EN input is asserted. Also, the voltage on the EN pin must always be less than V
IN
. This
applies to both static and dynamic operation, and during start up and shut down sequences. The demo
board includes a resistor divider (R6 and R7) to pull EN pin up to half of V
IN
. The LMR70503 will be
enabled when V
IN
is higher than the UVLO level (typically 2.55 V with 0.13 V hysteresis band). Use the EN
post to control the EN pin externally.
7 Inductor and Diode
The inductor and diode selected on the demo board are optimized for very small total solution size. Larger
size inductor and diode can improve overall efficiency if size constraints allow. Bigger footprints are
provided for customer convenience.
8 Soft Start and Soft Off
The soft start action is inherent with the maximum peak current limit and the minimum off time. During
start up, the inductor current rises to the maximum peak current limit, then the high-side switch is turned
off for T
OFF-MIN
to let the inductor current charge the output capacitor(s). Then the high-side turns on to
repeat the cycle. After the output voltage is charged to the regulation level, the LMR70503 will operate in
steady state. The soft start time will be longer with more output capacitance, lower supply voltage V
IN
, and
more loading during start up. When shutdown, the LMR70503 incorporates an output voltage discharge
feature to bring the output voltage to zero volts, regardless of the load current.
9 Quick Setup Procedures
Step 1: Connect a power supply to VIN terminals
Step 2: Connect a load to VOUT terminals
Step 3: EN terminal can be left floating for normal operation. It can be connected to external signal for
sequencing.
Step 4: Set V
IN
= 5V, with 0A load applied, check V
OUT
with a voltmeter. Nominal -5 V
Step 5: Apply a 50mA load and check V
OUT
. Nominal -5 V
3
SNVU155A–June 2012–Revised April 2013 AN-2264 LMR70503 Demo Board
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