LTC7825

11

Rev. A

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APPLICATIONS INFORMATION

The Typical Application on the first page of this data sheet

is a LTC7825 voltage divider circuit. The converter can

convert VHIGH voltage to VLOW voltage with a 2:1 step-

down ratio and supply 12A load current in steady-state

operation. In overcurrent or start-up conditions, the con-

verter automatically limits the VHIGH supply peak current

to 200mA for thermal protection.

VOLTAGE DIVIDER PRE-BALANCE BEFORE SWITCHING

In voltage divider applications, the LTC7825 can achieve

soft start-up to minimize the inrush current at all initial

conditions. The voltages on the flying capacitors and

VLOW capacitors are pre-charged to half of the VHIGH volt-

age internally. The pre-charge current during start-up is

around 200mA. To further minimize the thermal stress,

the TIMER pin can be used to control the pre-charge tim-

ing. The pre-charge is only enabled at 0.5 < VTIMER < 1.0V.

WINDOW COMPARATOR PROGRAMMING

In steady-state operation, VLOW voltage should be always

close to VHIGH/2. A floating window comparator monitors

the voltage on the VLOW pin and compares it with VHIGH/2.

The window hysteresis voltage can be programmed and

is equal to one fifth of the voltage at the HYS_PRGM

pin. There is a precision 10µA current flowing out of the

HYS_PRGM pin. A single resistor from the HYS_PRGM

pin to GND sets the HYS_PRGM pin voltage, which equals

the resistor value multiplied by 10µA current (e.g., the

voltage is 1V with a 100k resistor from the HYS_PRGM

pin to GND). With a 100k resistor on the HYS_PRGM pin,

the VHIGH/2 voltage has to be within (VLOW ± 200mV)

window during the normal operation, otherwise a fault is

triggered and the LTC7825 stops switching.

The window hysteresis voltage can be linearly pro-

grammed from 100mV to 480mV with different resistor

values on the HYS_PRGM pin. If the HYS_PRGM pin is

tied to SGND/INTVCC, a default 240mV/400mV hysteresis

window is applied internally. The hysteresis window volt-

age must be programmed large enough to tolerate the

VLOW pin voltage ripple and voltage drop at maximum

load conditions.

EFFECTIVE OPEN-LOOP OUTPUT RESISTANCE AND

LOAD REGULATION

The LTC7825 does not regulate the output voltage through

a feedback closed loop system. The VLOW voltage is very

close to half the VHIGH voltage in steady-state operation.

The output resistance is very low because RDS(ON) resis-

tance from VHIGH to VLOW is just 20mΩ to 30mΩ depend-

ing on the switching frequency and the capacitance of CFLY

and CLOW. If the output disconnect MOSFET is used, the

output resistance from VLOW to VOUT is around 2mΩ.

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

considered when estimating the output resistance of the

switched capacitor circuits.

INTVCC REGULATORS AND EXTVCC

The LTC7825 features a PMOS LDO that supplies power

to INTVCC from the VCC supply. INTVCC powers the gate

drivers and most of the LTC7825’s internal circuitry. The

linear regulator regulates the voltage at the INTVCC pin to

4.5V. EXTVCC connects to INTVCC through another PMOS

LDO and can supply the needed power when EXTVCC volt-

age is higher than 5V and VCC is higher than 6V. Both

LDOs can supply the driver current and must be bypassed

to ground with a minimum of 4.7µF ceramic capacitor

or low ESR electrolytic capacitor. Good bypassing is

needed to supply the high transient currents required by

the MOSFET gate drivers.

In high input voltage applications, the LDO loss may cause

the IC die temperature to rise. A low voltage supply on

the EXTVCC pin may be used to reduce the IC temperature

rise. When the voltage on the EXTVCC pin is higher than

5V, the linear regulator from EXTVCC is enabled. The junc-

tion temperature can be estimated by using the equations

given in Note 2 of the Electrical Characteristics. Using the