RT8243BZQW Datasheet RT8243BZQW | P16-P18

RT8243A/B/C

DS8243A/B/C-07 November 2014www.richtek.com

Application Information

The RT8243A/B/C is a dual,

Mach Response

DRV

mode synchronous buck controller targeted for notebook

system power supply solutions. Richtek's Mach

Response

technology provides fast response to load

steps. The topology circumvents the poor load transient

timing problems of fixed frequency current mode PWMs

while avoiding the problems caused by widely varying

switching frequency in conventional constant on-time and

constant off-time PWM schemes. A special adaptive on-

time control trades off the performance and efficiency over

wide input voltage range. The RT8243A/B/C includes 5V

(LDO5) and 3.3V (LDO3) linear regulators. The LDO5 linear

regulator steps down the battery voltage to supply both

internal circuitry and gate drivers. The synchronous switch

gate drivers are directly powered by LDO5. When V

OUT1

rises above 4.66V, an automatic circuit disconnects the

linear regulator and allows the device to be powered by

OUT1

via the BYP1 pin.

PWM Operation

The Mach Response

DRV

mode controller relies on

the output filter capacitor's Effective Series Resistance

(ESR) to act as a current sense resistor, so that the output

ripple voltage provides the PWM ramp signal. Referring to

the RT8243A/B/C's Function Block Diagram, the

synchronous high side MOSFET will be turned on at the

beginning of each cycle. After the internal one-shot timer

expires, the MOSFET will be turned off. The pulse width

of this one-shot is determined by the converter's input

voltage and the output voltage to keep the frequency fairly

constant over the entire input voltage range. Another one-

shot sets a minimum off-time (400ns typ.). The on-time

one-shot will be triggered if the error comparator is high,

the low side switch current is below the current limit

threshold, and the minimum off-time one-shot has timed

out.

PWM Frequency and On-time Control

For each specific input voltage range, the Mach

Response

control architecture runs with pseudo constant

frequency by feed forwarding the input and output voltage

into the on-time one-shot timer. The high side switch

on-time is inversely proportional to the input voltage as

measured by V

and proportional to the output voltage.

There are two benefits of a constant switching frequency.

First, the frequency can be selected to avoid noise

sensitive regions such as the 455kHz IF band. Second,

the inductor ripple current operating point remains

relatively constant, resulting in easy design methodology

and predictable output voltage ripple. The frequency for

3V SMPS is set higher than the frequency for 5V SMPS.

This is done to prevent audio frequency “beating” between

the two sides, which switch asynchronously for each side.

The TON pin is connected to GND through the external

resistor, R

TON

, to set the switching frequency.

The RT8243A/B/C adaptively changes the operation

frequency according to the input voltage. Higher input

voltage usually comes from an external adapter, so the

RT8243A/B/C operates with higher frequency to have

better performance. Lower input voltage usually comes

from a battery, so the RT8243A/B/C operates with lower

switching frequency for lower switching losses. For a

specific input voltage range, the switching cycle period is

given by :

For 5.5V < V

< 6.5V :

= 61.28p x R

TON

= 44.43p x R

TON

For 6.5V < V

< 12V :

= 51.85p x R

TON

= 44.43p x R

TON

For 12V < V

< 25V :

= 45.75p x R

TON

= 39.2p x R

TON

The on-time guaranteed in the Electrical Characteristics

table is influenced by switching delays in the external

high side power MOSFET. Two external factors that

influence switching frequency accuracy are resistive drops

in the two conduction loops (including inductor and PC

board resistance) and the dead time effect. These effects

are the largest contributors to the change of frequency

with changing load current. The dead time effect increases

RT8243A/B/C

DS8243A/B/C-07 November 2014 www.richtek.com



OUT DROP1 ON IN DROP1 DROP2

f = (V V ) / (t x (V V V ))

where V

DROP1

is the sum of the parasitic voltage drops in

the inductor discharge path, including synchronous

rectifier, inductor, and PC board resistances; V

DROP2

the sum of the resistances in the charging path; and t

is the on-time calculated by the RT8243A/B/C.

Operation Mode Selection

The RT8243A/B supports two operation modes : Diode

Emulation Mode and Ultrasonic Mode. The RT8243C only

supports Ultrasonic Mode. The operation mode can be

set via the ENM pin for RT8243A or SECFB pin for

RT8243B.

the effective on-time by reducing the switching frequency

as one or both dead times. It occurs only in PWM mode

when the inductor current reverses at light or negative

load currents. With reversed inductor current, the

inductor's EMF causes PHASEx to go high earlier than

normal, hence extending the on-time by a period equal to

the low to high dead time. For loads above the critical

conduction point, the actual switching frequency is :

Diode Emulation Mode

In Diode Emulation Mode, the RT8243A/B automatically

reduces switching frequency at light load conditions to

maintain high efficiency. This reduction of frequency is

achieved smoothly. As the output current decreases from

heavy-load condition, the inductor current is also reduced,

and eventually comes to the point that its current valley

touches zero, which is the boundary between continuous

conduction and discontinuous conduction modes. By

emulating the behavior of diodes, the low side MOSFET

allows only partial negative current to flow when the

inductor free wheeling current becomes negative. As the

Part Number RT8243A RT8243B RT8243C

Pin Name ENM SECFB SECFB

Pin-13

Voltage Range

Mode State

4.5V to 5V ASM ASM ASM

2.3V to 3.6V DEM DEM ASM

1.2V to 1.8V ASM ASM ASM

Below 0.8V Shutdown UVP UVP

Table 1. Operation Mode Setting

Slope = (V

-V

OUT

)/L

PEAK

LOAD =

PEAK

Figure 3. Boundary condition of CCM/DEM

IN OUT

LOAD(SKIP) ON

(V V )





where t

is the on-time.

The switching waveforms may appear noisy and

asynchronous when light loading causes diode emulation

operation. This is normal and results in high efficiency.

Trade offs in PFM noise vs. light load efficiency is made

by varying the inductor value. Generally, low inductor values

produce a broader efficiency vs. load curve, while higher

values result in higher full load efficiency (assuming that

the coil resistance remains fixed) and less output voltage

ripple. Penalties for using higher inductor values include

larger physical size and degraded load transient response

(especially at low input voltage levels).

Ultrasonic Mode

The RT8243A/B/C activates a unique type of Diode

Emulation Mode with a minimum switching frequency of

25kHz, called Ultrasonic Mode. This mode eliminates

audio-frequency modulation that would otherwise be

present when a lightly loaded controller automatically

skips pulses. In Ultrasonic Mode, the low side switch gate

driver signal is “OR”ed with an internal oscillator

(>25kHz). Once the internal oscillator is triggered, the

ultrasonic controller pulls LGATEx high and turns on the

load current is further decreased, it takes longer and longer

time to discharge the output capacitor to the level that

requires the next “ON” cycle. The on-time is kept the

same as that in the heavy load condition. In reverse, when

the output current increases from light load to heavy load,

the switching frequency increases to the preset value as

the inductor current reaches the continuous conduction.

The transition load point to the light load operation is shown

in Figure 3. and can be calculated as follows :

RT8243A/B/C

DS8243A/B/C-07 November 2014www.richtek.com

low side MOSFET to induce a negative inductor current.

After the output voltage falls below the reference voltage,

the controller turns off the low side MOSFET (LGATEx

pulled low) and triggers a constant on-time (UGATEx

driven

high). When the on-time has expired, the controller re-

enables the low side MOSFET until the controller detects

that the inductor current dropped below the zero crossing

threshold.

Linear Regulators (LDOx)

The RT8243A/B/C includes 5V (LDO5) and 3.3V (LDO3)

linear regulators. The regulators can supply up to 100mA

for external loads. Bypass LDOx with a minimum 4.7μF

ceramic capacitor. When V

OUT1

is higher than the switch

over threshold (4.66V), an internal 1.5Ω P-MOSFET switch

connects BYP1 to the LDO5 pin while simultaneously

disconnects the internal linear regulator.

Current Limit Setting (ENTRIPx)

The RT8243A/B/C has cycle-by-cycle current limit 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 4).

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 characteristic and

maximum load capability are a function of the sense

resistance, inductor value, and battery and output voltage.

PEAK

LOAD

LIMIT

Figure 4. “Valley” Current Limit

The RT8243A/B/C uses the on resistance of the

synchronous rectifier as the current sense element and

supports temperature compensated MOSFET R

DS(ON)

sensing. The R

ILIM

resistor between the ENTRIPx pin and

GND sets the current limit threshold. The resistor, R

ILIM

is connected to a current source from ENTRIPx

which is

10μA (typ.) at room temperature. The current source has

a 4700ppm/°C temperature slope to compensate the

temperature dependency of the R

DS(ON)

. When the voltage

drop across the sense resistor or low side MOSFET

equals 1/10 the voltage across the R

ILIM

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 R

ILIM

resistor.

Choose a current limit resistor according to the following

equation :

ILIM

= (R

ILIM

x 10μA) / 10 = I

ILIM

x R

DS(ON)

ILIM

= (I

ILIM

x R

DS(ON)

) x 10 / 10μA

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 (SECFB)

The external 14V charge pump is driven by LGATEx. When

LGATEx is low, C1 will be charged by V

OUT1

through D1.

C1 voltage is equal to V

OUT1

minus the diode drop. When

LGATEx becomes high, C1 transfers the charge to C2

through D2 and charges C2 voltage to V

LGATEX

plus C1

voltage. As LGATEx transitions low on the next cycle, C3

is charged to C2 voltage minus a diode drop through D3.

Finally, C3 charges C4 through D4 when LGATEx switches

high. Thus, the total charge pump voltage, V

, is :

= V

OUT1

+ 2 x V

LGATEx

− 4 x V

where V

LGATEx

is the peak voltage of the LGATEx driver

which is equal to LDO5 and V

is the forward voltage

dropped across the Schottky diode.

The SECFB pin in the RT8243B/C is used to monitor the

charge pump via a resistive voltage divider to generate

approximately 14V DC voltage and the clock driver uses

OUT1

as its power supply. In the event where SECFB

drops below its feedback threshold, an ultrasonic pulse

will occur to refresh the charge pump driven by LGATEx.

If there's an overload on the charge pump in which SECFB

can not reach more than its feedback threshold, the

controller will enter Ultrasonic Mode. Special care should

be taken to ensure that enough normal ripple voltage is

present on each cycle to prevent charge pump shutdown.

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RT8243BZQW

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