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Buck-Boost µModule Regulator's External Current Sense Resistor For Paralleling ICs, Current Sharing and Overcurrent Protection

Kevin B. Scott - Senior Strategic Marketing Engineer Mar 17th 2015

Linear Technology's µModule 4-switch buck-boost regulators provide an integrated, high-efficiency solution for applications where the input voltage can be either above or below the desired output voltage. These fixed frequency, current mode devices require only a few external components to provide a minimal design time power solution. The newest ICs in this product family are the LTM8055 and LTM8056. The LTM8055 (Figure 1) is a 5V to 36VIN device that can provide 12V@8.5A from a 24V supply. The LTM8056 has a 5V to 58V input voltage range and provides 12V@5.4A from a 24V supply. Both devices integrate the inductor, reducing exteral component count to a sense resistor, a resistor divider to set the output voltage, a resistor to set the switching frequency, and bulk input and output capacitance.

12VOUT from 5VIN to 36VIN Buck-Boost RegulatorFigure 1. LTM8055 with IOUT = 3A to 7.5A Depending on VIN

Output Current Limit

The LTM8055/LTM8056 feature an external output current sense resistor (Figure 2) that delivers a precise average current for applications such as driving LEDs or charging a battery or battery stack. It also facilates accurate current sharing when multiple devices are used to increase available output current. Most applications should use the output sense resistor as a general guideline. 

Set The LTM8055 Output Current Limit with an External Sense ResistorFigure 2. External output current sense resistor

VOUT and IOUT internally connect to a differential amplifier that limits the current when the voltage VOUT-IOUT reaches 58mV. The output current monitor pin (IOUTMON) produces a voltage that is proportional to the voltage between VOUT and IOUT. Current limiting occurs when IOUTMON equals 1.2V. When the device is used as a battery charger, the light load indicator (LL) pin can be used for C/10 indication or C/10 termination because the LL pin pulls low when the voltage across the output sense resistor falls to about 1/10th of the full scale programmed current limit.

Current Sharing

Two or more LTM8055s may be combined to provide increased output current by configuring them as a master and a slave, as shown below in Figure 3. Each LTM8055 is equipped with an output current monitor and current sense adjust (CTL) pin. The IOUTMON pin’s 0V to 1.2V signal represents the current passing through the output sense resistor. Additionally, a voltage less than 1.2V applied to the current sense adjust (CTL) pin further limits the sense resistor current.

Two or More LTM8055s May Be Connected in a Master/Slave Configuration for Increased Output CurrentFigure 3. Master-Slave Current Sharing Configuration

By applying the voltage of the master’s IOUTMON pin to the slave’s CTL pin, the two units will source the same current to the load, assuming the output current sense resistors are the same value. The master-slave power supply is very simple to design; see page 12 of the LTM8055 data sheet or page 13 of the LTM8056 data sheet. Refer to Table 1 in the data sheet for specific component values.

Input Current Limit

The LTM8055/LTM8056 also feature an average input current limit option (Figure 4). Placing a sense resistor between VIN and IIN results in similar current limiting at the input, which is ideal for a current limited input power supply. The input current limit is given by IIN(LIM) = 50mV / RSENSE.

Set the LTM8055 Input Current Limit with an External Sense ResistorFigure 4. Using the Input Current Limit

Output Current Limit Provides Overcurrent Protection

The  LTM8055/LTM8056 have two current sense resistors (excluding the optional input current sense resistor), an internal sense resistor at the bottom of the 4-switch configuration and the external average output current sense. Our previous family of buck-boost converters (LTM4605/7/9) do not have the external current sense resistor option.

LTM8055 Block DiagramFigure 5. LTM8055/LTM8056 Block Diagram

When the input voltage is lower than the output voltage, the converter operates in boost mode. In this mode, switch S4 is the power switch, S3 is the steering switch, S1 is always on and S2 is always off. When S4 closes, the current ramps up in the inductor, traveling through S4, through the lowside sense resistor (RSENSE) where it is monitored and to ground. This results in peak current cycle-by-cycle current limiting. When the power switch S4 turns off, switch S3 turns on and the inductor current ramps down and current is delivered to the load.

Now observe the case when the regulator is in buck mode. In buck mode (shown in Figure 6), switch S1 is the power switch, S3 is always on, S4 is always off and S2 is the steering switch. When S1 turns on, the inductor current flows through S1, through the inductor and to the output. Note that with the sense resistor on the lowside, current does NOT flow through it as the inductor current ramps up; there is no peak-current limit or cycle-by-cycle current limiting.

BuckBoost_BuckIFlowSwONFigure 6. LTM8055 Buck Mode Current Flow with Switch S1 Closed

When switch S1 opens, current flows up through the sense resistor, through switch S2 and the inductor and to the output (Figure 7); but this is on the downward ramp of the inductor current. Without a sense resistor to monitor the peak current, the peak current is determined solely by the switching frequency, the circuit components used and the input and output voltages desired.

BuckBoost_BuckIFlowSwOFFFigure 7. LTM8055 Buck Mode Current Flow with Switch S1 Open

As the current ramps up,  the equation to determine the peak current (assuming the switches are lossless) is given by:

IPEAK = IOUT + I/2*IRAMP,

where IRAMP is given by

BuckPeakCurrentEqn

This assumes the t(OFF) time is long enough to allows the current to ramp back down to its intial starting point (which may or may not be the case), and it assumes the current does not saturate due to a large VIN-VOUT differential voltage across the inductor. t(ON) is determined by the input-output voltage differential and the switching frequency; and with large differentials, potentially damaging large peak currents can flow in the circuit.

The LTM8055/LTM8556's output current limit prevents overcurrent conditions by limiting the maximum average output current; which is why most applications should use the added protection of the external sense resistor. Linear Technology strives to provide not only devices that meet your power budget needs, but also devices that provide value-added features and that are robust enough to carry the Linear Technology name. In addition to µModule Buck-Boost Regulators we offer a wide array of monolithic and controller-based buck-boost solutions.