Data Sheet

LT7826

analog.com

Rev. 0

13 of 19

APPLICATIONS INFORMATION

The Typical Application is an LT7826 voltage divider circuit. The converter can convert VHIGH voltage to VLOW voltage

with a 2:1 step-down ratio and supply 8A load current in the steady state operation. In pre-balance or overcurrent

conditions, the converter automatically limits the maximum power switch current to 485mA for thermal protection.

Pre-balance Mode with Reduced Loading Capacity

To avoid surge current through the circuit, after power up, LT7826 would enter a pre-balance mode if VVLOW is lower

than VVHIGH/2 - 0.265V or higher than VVHIGH/2 + 0.265V. In pre-balance mode, the maximum current through the internal

power switch is limited to 485mA, and LT7826 would switch at a frequency of 250kHz, or half of the default frequency

of full-power switching. The flying capacitor and output capacitor would be gradually charged up by pre-balance

mode operation. As VVLOW gets close to VVHIGH/2, the charging current would further reduce due to the lower overdrive

voltage over the power switch inside LT7826. When VVLOW is within the window of VVHIGH/2 ± 265mV, LT7826 would exist

in pre-balance mode and start full-power switching.

Because of the reduced current capability during pre-balance mode, the loading current should be less than 30mA

in pre-balance mode to ensure that VVLOW can get close enough to VVHIGH/2 to activate full-power switching.

Effective Open-Loop Output Resistance And Load Regulation

LT7826 does not regulate the output voltage through feedback closed loop system. The VLOW voltage is very close

to half the VHIGH voltage in steady-state operation. As the load current increases, the output voltage decreases. The

output resistance is very low, depending on the switching frequency and the capacitance of CFLY and CLOW. In many

applications, multi-layer ceramic capacitors (MLCC) are selected as flying capacitors. The voltage coefficients of

MLCC capacitors strongly depend on the type and size of capacitors. Normally, larger-size X7R MLCC capacitors are

better than X5R in terms of voltage coefficient. The MLCCs still drop 20% to 30% capacitance with high DC bias

voltage. Capacitance derating needs to be considered when estimating the output resistance of the switched

capacitor circuits.

Input/Output Capacitor and Flying Capacitor Selection

In switched capacitor applications, large AC currents flow through the flying capacitors and input/output capacitors.

Low ESR ceramic capacitors are highly recommended for these applications. Ensure the maximum RMS capacitor

current is within the spec, or higher-rated capacitors are preferred. Note that capacitor manufacturers’ ripple current

ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose

capacitors rated at a higher temperature than required. Several capacitors may be paralleled to meet size or height

requirements in the design. The LTspice® simulation tool can be used to quantify the root mean square (RMS) current.

Generally, the higher the capacitance of the flying capacitor, the lower the flying capacitor voltage ripple, and the

higher power efficiency the switched capacitor converter can achieve. In a voltage divider application, flying

capacitor voltage ripple can be estimated with the following equation:

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Selecting the flying capacitor so that the ripple voltage is around 100mV at the full load condition is a good start.

The input capacitor’s RMS current is approximately half of the load current. The input capacitor must be selected to

accommodate the maximum load conditions.

The output capacitor would largely impact the output voltage ripple at the switching frequency. The higher the

capacitance at the output port, the smaller voltage ripples the output would contain.