RT8205A datasheet(21/28 Pages) RICHTEK

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RT8205A/B/C

DS8205A/B/C-06 July 2012

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driver waveform to become the complement of the high-

side gate-driver waveform. This in turn causes the inductor

current to reverse at light loads as the PWM loop to

maintain a duty ratio of VOUT/VIN. The benefit of forced-

CCM mode is to keep the switching frequency fairly

constant, but it comes at a cost : The no-load battery

current can be 10mA to 40mA, depending on the external

MOSFETs.

Reference and linear Regulators (REF, VREGx)

The 2V reference (REF) is accurate within ±1% over

temperature, making REF useful as a precision system

reference. Bypass REF to GND with a 0.22uF (min)

capacitor. REF can supply up to 50uA for external loads.

Loading REF reduces the VOUTx output voltage slightly

because of the reference load-regulation error.

VREG5 regulator supplies total of 70mA for internal and

external loads, including MOSFET gate driver and PWM

controller. VREG3 regulator supplies up to 70mA for

external loads. Bypass VREG5 and VREG3 with a 4.7uF

(min) capacitor; use an additional 1uF per 5mA of internal

and external load.

When the 5V main output voltage is above the VREG5

switchover threshold, an internal 1.5

ΩN-Channel MOSFET

switch connects VOUT1 to VREG5 while simultaneously

shutting down the VREG5 linear regulator. Similarly, when

the 3.3V main output voltage is above the VREG3

switchover threshold, an internal 1.5

ΩN-Channel MOSFET

switch connects VOUT2 to VREG3 while simultaneously

shutting down the VREG3 linear regulator. It can decrease

the power dissipation from the same battery, because the

converted efficiency of SMPS is better than the converted

efficiency of linear regulator.

Current-Limit Setting (ENTRIPx)

The RT8205A/B/C has cycle-by-cycle current limiting

control. The current-limit circuit employs a unique “valley”

current sensing algorithm. If the magnitude of the current-

sense signal at PHASEx is above the current-limit

threshold, the PWM is not allowed to initiate a new cycle

(Figure 2). The actual peak current is greater than the

current-limit threshold by an amount equal to the inductor

ripple current. Therefore, the exact current-limit

Figure 2. “valley” Current-Limit

The RT8205A/B/C uses the on-resistance of the

synchronous rectifier as the current-sense element. Use

the worse-case maximum value for RDS(ON) from the

MOSFET datasheet, and add a margin of 0.5%/

°C for the

rise in RDS(ON) with temperature.

The RILIMresistor between the ENTRIPx pin and GND sets

the over current threshold. The resistor RILIM is connected

to a 10uA current source from ENTRIPx. When the voltage

drop across the sense resistor or low-side MOSFET

equals 1/10 the voltage across the RILIM resistor, positive

current limit will be activated. The high-side MOSFET will

not be turned on until the voltage drop across the MOSFET

falls below 1/10 the voltage across the RILIM resistor.

Choose a current limit resistor by following equation :

VILIM = (RILIM x 10uA) / 10 = IILIM x RDS(ON)

RILIM = (IILIM x RDS(ON)) x 10 / 10uA

Carefully observe the PC board layout guidelines to ensure

that noise and DC errors do not corrupt the current-sense

signal at PHASEx and GND. Mount or place the IC close

to the low-side MOSFET.

Charge Pump (LG1_CP or SECFB)

The external 14V charge pump is driven by LGATE1 (Figure

3 and Figure 4). When LGATE1 is low, the C1 will be

charged by D1 from VOUT1. C1 voltage is equal to VOUT1

minus a diode drop. When LGATE1 transitions to high,

the charges from C1 will transfer to C2 through D2 and

charge it to VLGATE1 plus VC1. As LGATE1 transitions low

on the next cycle, C2 will charge C3 to its voltage minus

a diode drop through D3. Finally, C3 charges C4 through

IL, peak

ILIM

ILoad

characteristic and maximum load capability are a function

of the sense resistance, inductor value, and battery and

output voltage.

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