LM3248 Datasheet, PDF (15/31 Pages) Texas Instruments – LM3248 2.7 MHz, 2.5A Adjustable Boost-Buck DC/DC Converter

LM3248
www.ti.com
SNVSA01A – JULY 2013 – REVISED SEPTEMBER 2013
V_BOOST = VCON * 2.5 + 0 mV [V_BOOST > V_BATT] (3G or 4G mode)
or
V_BOOST = VCON * 2.5 + 125 mV [V_BOOST > V_BATT] (2G mode)
Once in Boost mode, the Boost voltage automatically adjusts according to the following equations:
V_BOOST = VCON * 2.5 + 450 mV [V_BOOST > V_BATT] (3G or 4G mode)
or
V_BOOST = VCON * 2.5 + 950 mV [V_BOOST > V_BATT] (2G mode)
In Boost mode, internal circuitry will automatically engage the large bypass switch to prevent the Boost output
from ever falling below the battery voltage, V_BATT, by more than a few hundred millivolts (200 mV typ.) when
Boost-Bypass conditions are not met. This functionality is described below.
When operating in Boost mode, the BOOST_GATE signal should be toggled between HIGH and LOW logic
levels at a rate synchronous with the Transmission Time Interval (TTI) frame, per the Startup Timing
requirements given in Figure 2. This provides sufficient setup time for the Boost bias circuits to activate before
the new (possibly higher) Buck output level is updated, ensuring a smooth transition between levels.
Note that V_BOOST programmed output levels are determined by the VCON command voltage; not by the Buck
DC/DC converter output voltage, V_{CC_PA}. This is necessary to minimize Boost output voltage tracking delay
times.
Boost active due to VCON * 2.5
crossing VBATT-VBYP threshold
(VBOOST §9&21* 2.5 + Voverhead)
Voverhead §0.45V (3G/4G)
§0.95V (2G)
Boost transitions to
Bypass due to VBOOST
target (VCON * 2.5)
dropping below
VBATT-VBYP threshold.
VBATT
VBOOST (in Bypass)
VBATT ± 0mV (3G/4G)
VBATT ± 125mV (2G)
VCON * 2.5
VBOOSTBYP §5BOOST * Input Current
VBYP §(RBOOST + RBUCK) *
LoadCurrent
Figure 28. Boost Operation showing VCON-V_BATT Relationship
Boost-Bypass Function
The LM3248 provides a bypass function with very low dropout resistance when the Boost stage is not needed to
support the output voltage requirements. The Boost circuit automatically enters Boost-Bypass mode when
BOOST_GATE is high and the VCON voltage is approximately less than VCON(max), as shown below:
VCON(max) = ^{[VBATT í 0 mV]} (3G or 4G mode)
2.5
VCON(max) = ^{[VBATT í 125 mV]} (2G mode)
2.5
In Boost-Bypass mode, the low resistance (75 mΩ typ.) bypass circuit effectively connects the PVIN_BOOST and
OUT_BOOST pins in parallel with the boost inductor and boost output power transistor. The Boost-Bypass circuit
is active at all times when BOOST_GATE=LOW. When Boost-Bypass is active, the minimum total dropout
resistance of the path between the PVIN_BOOST and OUT_BOOST pins is 60 mΩ (typ.). The Boost-Bypass
mode of operation is valid for 2.7V < V_BATT < 5.5V.
Copyright © 2013, Texas Instruments Incorporated
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