Datasheet

2.8 3.2 3.6 4.0 4.4 4.8 5.2
0
50
100
150
200
250
MAX LOADING (mA)
VIN (V)
VOUT = -5V
VOUT = -3.3V
VOUT = -2.5V
VOUT = -1.5V
VOUT = -0.9V
-50 -40 -30 -20 -10 0
0
1
2
3
4
5
6
7
8
9
10
DUMMY LOAD CURRENT (mA)
FB VOLTAGE (mV)
VIN=5.5V -40°C
VIN=5.5V 25°C
VIN=5.5V 125°C
VIN=2.8V -40°C
VIN=2.8V 25°C
VIN=2.8V 125°C
LMR70503
SNVS850A –JUNE 2012–REVISED APRIL 2013 www.ti.com
Figure 32. Dummy Load Current vs. FB Voltage
Constant Peak Current Operation
If the load current increases in the minimum switching frequency mode, the peak current limit will reach the
maximum peak current limit (I
PEAK-MAX
). After this point, the LMR70503 behaves as a constant peak current
converter with frequency modulation. The transition load level between the constant frequency mode and the
constant peak current mode varies with V
IN
, V
OUT
and L.
The I
PEAK-MAX
is trimmed to 320 mA in the LMR70503. Due to propagation delays in the comparator and gate
drive, the measured peak inductor current will be higher than the trimmed value. The additional offset on the
maximum peak current is proportional to the inductor current rising slope: V
IN
/ L, approximately. For a typical
inductor, the inductance will reduce at hot temperature. Therefore, I
PEAK-MAX
is the highest with 5.5 V input
voltage at hot temperature.
In the constant peak current operation mode, the switching frequency will increase with the increased load
current, until the high side switch off time equals the minimum off-time (T
OFF-MIN
) limit. If the load keeps
increasing when the switch operates with T
OFF-MIN
, V
OUT
will drop out of regulation due to loading limits of buck-
boost type of converters. The maximum loading capability is higher with higher V
IN
, larger L, lower V
OUT
, and less
losses in the converter. Figure 33 shows the measured maximum load current measured with the typical BOM
shown in Table 1. To increase the maximum loading capability with given V
IN
and V
OUT
, one can choose a higher
inductance value and a diode with lower forward voltage drop V
F
.
Figure 33. LMR70503 Loading Capability vs. V
IN
, L = 6.8 µH
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