Richtek RT6211AHGE, RT6211BHGE Schematic [ru]

RT6211A/B

1.5A, 18V, 500kHz, ACOTTM Step-Down Converter

General Description

The RT6211A/B is a high-efficiency, monolithic synchronous step-down DC/DC converter that can deliver up to 1.5A output current from a 4.5V to 18V input supply. The RT6211A/B adopts ACOT architecture to allow the transient response to be improved and keep in constant frequency. Cycle-by-cycle current limit provides protection against shorted outputs and soft-start eliminates input current surge during start-up. Fault conditions also include output under voltage protection, output over current protection, and thermal shutdown.

Features

Integrated 230m /130m MOSFETs

4.5V to 18V Supply Voltage Range

500kHz Switching Frequency

ACOT Control

0.8V 1.5% Voltage Reference

Internal Start-Up into Pre-biased Outputs

Compact Package : SOT-23-6 pin

High/Low Side Over-Current Protection and Hiccup

VOUT Range 0.8V to 6.5V

Applications

Ordering Information

Set-Top Boxes

Portable TVs

RT6211A/B

Package Type



Access Point Routers



DSL Modems

E : SOT-23-6

LCD TVs



Lead Plating System

G : Green (Halogen Free and Pb Free)

Marking Information

UVP Option

RT6211AHGE

H : Hiccup

3J= : Product Code

A : PSM Mode

3J=DNN

DNN : Date Code

B : PWM Mode

Note :

Richtek products are :

RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.

Suitable for use in SnPb or Pb-free soldering processes.

RT6211BHGE

	39= : Product Code
39=DNN	DNN : Date Code

Simplified Application Circuit

					RT6211A/B
					BOOT			CBOOT
VIN					VIN
				CIN					L

LX VOUT

Enable	EN	R1	COUT
			CFF
		FB
		GND
		R2

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RT6211A/B

Pin Configurations

(TOP VIEW)

FB

EN

BOOT

6

5

4

2

3

VIN GND

LX

SOT-23-6

Functional Pin Description

Pin No.

Pin Name

Pin Function

1

VIN

Power Input. Supplies the power switches of the device.

2

GND

System Ground. Provides the ground return path for the control circuitry and

low-side power MOSFET.

3

LX

Switch Node. LX is the switching node that supplies power to the output and

connect the output LC filter from LX to the output load.

4

BOOT

Bootstrap Supply for High-Side Gate Driver. Connect a 100nF or greater

capacitor from LX to BOOT to power the high-side switch.

5

EN

Enable Control Input. Floating this pin or connecting this pin to logic high can

enable the device and connecting this pin to GND can disable the device.

6

FB

Feedback Voltage Input. This pin is used to set the desired output voltage via

an external resistive divider. The feedback voltage is 0.8V typically.

Function Block Diagram

				BOOT	VIN
VIN
	PVCC
Reg
			Minoff
				PVCC
VIBIAS	VREF				UGATE
		OC	Control	Driver		LX
		UV &OV			LGATE
						GND
				GND LX
PVCC
		Ripple		EN		EN
	LX				VIN
		Gen.	+
			--		On-Time	LX
		Comparator
	FB

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RT6211A/B

Detailed Description

The RT6211A/B are high-performance 500kHz 1.5A step-down regulators with internal power switches and synchronous rectifiers. They feature an Advanced Constant On-Time (ACOTTM) control architecture that provides stable operation with ceramic output capacitors without complicated external compensation, among other benefits. The input voltage range is from 4.5V to 18V and the output is adjustable from 0.8V to 6.3V.

The proprietary ACOTTM control scheme improves upon other constant on-time architectures, achieving nearly constant switching frequency over line, load, and output voltage ranges. The RT6211A/B are optimized for ceramic output capacitors. Since there is no internal clock, response to transients is nearly instantaneous and inductor current can ramp quickly to maintain output regulation without large bulk output capacitance.

Constant On-Time (COT) Control

The heart of any COT architecture is the on-time one-shot. Each on-time is a pre-determined “fixed” period that is triggered by a feedback comparator. This robust arrangement has high noise immunity and is ideal for low duty cycle applications. After the on-time one-shot period, there is a minimum off-time period before any further regulation decisions can be considered. This arrangement avoids the need to make any decisions during the noisy time periods just after switching events, when the switching node (LX) rises or falls. Because there is no fixed clock, the high-side switch can turn on almost immediately after load transients and further switching pulses can ramp the inductor current higher to meet load requirements with minimal delays.

Traditional current mode or voltage mode control schemes typically must monitor the feedback voltage, current signals (also for current limit), and internal ramps and compensation signals, to determine when to turn off the high-side switch and turn on the synchronous rectifier. Weighing these small signals in a switching environment is difficult to do just after switching large currents, making those architectures problematic at low duty cycles and in less than ideal board layouts.

Because no switching decisions are made during noisy time periods, COT architectures are preferable in low duty cycle and noisy applications. However, traditional COT control schemes suffer from some disadvantages that preclude their use in many cases. Many applications require a known switching frequency range to avoid interference with other sensitive circuitry. True constant on-time control, where the on-time is actually fixed, exhibits variable switching frequency. In a step-down converter, the duty factor is proportional to the output voltage and inversely proportional to the input voltage. Therefore, if the on-time is fixed, the off-time (and therefore the frequency) must change in response to changes in input or output voltage.

Modern pseudo-fixed frequency COT architectures greatly improve COT by making the one-shot on-time proportional to VOUT and inversely proportional to VIN. In this way, an on-time is chosen as approximately what it would be for an ideal fixed-frequency PWM in similar input/output voltage conditions. The result is a big improvement but the switching frequency still varies considerably over line and load due to losses in the switches and inductor and other parasitic effects.

Another problem with many COT architectures is their dependence on adequate ESR in the output capacitor, making it difficult to use highly-desirable, small, low-cost, but low-ESR ceramic capacitors. Most COT architectures use AC current information from the output capacitor, generated by the inductor current passing through the ESR, to function in a way like a current mode control system. With ceramic capacitors, the inductor current information is too small to keep the control loop stable, like a current mode system with no current information.

ACOTTM Control Architecture

Making the on-time proportional to VOUT and inversely proportional to VIN is not sufficient to achieve good constant-frequency behavior for several reasons. First, voltage drops across the MOSFET switches and inductor cause the effective input voltage to be less than the measured input voltage and the effective output voltage to be greater than the measured output voltage. As the load changes, the switch voltage drops

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RT6211A/B

change causing a switching frequency variation with load current. Also, at light loads if the inductor current goes negative, the switch dead-time between the synchronous rectifier turn-off and the high-side switch turn-on allows the switching node to rise to the input voltage. This increases the effective on-time and causes the switching frequency to drop noticeably.

One way to reduce these effects is to measure the actual switching frequency and compare it to the desired range. This has the added benefit eliminating the need to sense the actual output voltage, potentially saving one pin connection. ACOTTM uses this method, measuring the actual switching frequency (at SW) and modifying the on-time with a feedback loop to keep the average switching frequency in the desired range.

To achieve good stability with low-ESR ceramic capacitors, ACOTTM uses a virtual inductor current ramp generated inside the IC. This internal ramp signal replaces the ESR ramp normally provided by the output capacitor's ESR. The ramp signal and other internal compensations are optimized for low-ESR ceramic output capacitors.

ACOTTM One-shot Operation

The RT6211A/B control algorithm is simple to understand. The feedback voltage, with the virtual inductor current ramp added, is compared to the reference voltage. When the combined signal is less than the reference the on-time one-shot is triggered, as long as the minimum off-time one-shot is clear and the measured inductor current (through the synchronous rectifier) is below the current limit. The on-time one-shot turns on the high-side switch and the inductor current ramps up linearly. After the on-time, the high-side switch is turned off and the synchronous rectifier is turned on and the inductor current ramps down linearly. At the same time, the minimum off-time one-shot is triggered to prevent another immediate on-time during the noisy switching time and allow the feedback voltage and current sense signals to settle. The minimum off-time is kept short (240ns typical) so that rapidly-repeated on-times can raise the inductor current quickly when needed.

Discontinuous Operating Mode (RT6211A Only)

After soft-start, the RT6211B operates in fixed frequency mode to minimize interference and noise problems. The RT6211A uses variable-frequency discontinuous switching at light loads to improve efficiency. During discontinuous switching, the on-time is immediately increased to add “hysteresis” to discourage the IC from switching back to continuous switching unless the load increases substantially.

The IC returns to continuous switching as soon as an on-time is generated before the inductor current reaches zero. The on-time is reduced back to the length needed for 500kHz switching and encouraging the circuit to remain in continuous conduction, preventing repetitive mode transitions between continuous switching and discontinuous switching.

Current Limit

The RT6211A/B current limit is a cycle-by-cycle “valley” type, measuring the inductor current through the synchronous rectifier during the off-time while the inductor current ramps down. The current is determined by measuring the voltage between Source and Drain of the synchronous rectifier, adding temperature compensation for greater accuracy. If the current exceeds the current limit, the on-time one-shot is inhibited until it drops below the current limit level. If the output current exceeds the available inductor current (controlled by the current limit mechanism), the output voltage will drop. If it drops below the output under-voltage protection level (see next section) the IC will stop switching to avoid excessive heat.

The RT6211B also includes a negative current limit to protect the IC against sinking excessive current and possibly damaging the IC. If the voltage across the synchronous rectifier indicates the negative current is too high, the synchronous rectifier turns off until after the next high-side on-time. The RT6211A does not sink current and therefore does not need a negative current limit.

Hiccup Mode

The RT6211AHGE / RT6211BHGE, use hiccup mode UVP. When the protection function is triggered, the IC will shut down for a period of time and then attempt to recover automatically. Hiccup mode allows the circuit to

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RT6211A/B

operate safely with low input current and power dissipation, and then resume normal operation as soon as the overload or short circuit is removed.

Input Under-Voltage Lockout

To protect the chip from operating at insufficient supply voltage, the UVLO is needed. When the input voltage of VIN is lower than the UVLO falling threshold voltage, the device will be lockout.

Shut-down, Start-up and Enable (EN)

The enable input (EN) has a logic-low level. When VEN is below this level the IC enters shutdown mode and supply current drops to less than 6 A. When VEN exceeds its logic-high level the IC is fully operational.

External Bootstrap Capacitor

Connect a 0.1 F low ESR ceramic capacitor between BOOT and SW. This bootstrap capacitor provides the gate driver supply voltage for the high side N-channel MOSFET switch.

Over-Temperature Protection

The RT6211A/B includes an over-temperature protection (OTP) circuitry to prevent overheating due to excessive power dissipation. The OTP will shut down switching operation when the junction temperature exceeds 150 C. Once the junction temperature cools down by approximately 20 C the IC will resume normal operation. For continuous operation, provide adequate cooling so that the junction temperature does not exceed 150 C.

UVP Protection

The RT6211A/B detects under-voltage conditions by monitoring the feedback voltage on FB pin. The function is enabled after approximately 1.7 times the soft-start time. When the feedback voltage is lower than 50% of the target voltage, the UVP comparator will go high to turn off both internal high-side and low-side MOSFETs.

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RT6211A/B

Absolute Maximum Ratings (Note 1)

	Supply Input Voltage ---------------------------------------------------------------------------------	0.3V to 20V
	Switch Voltage, SW-----------------------------------------------------------------------------------	0.8V to (VIN + 0.3V)
	<10ns-----------------------------------------------------------------------------------------------------	5V to 25V
	Switch Node Voltage, LX ----------------------------------------------------------------------------	0.3V to (VIN + 0.3V)
	BOOT Pin Voltage ------------------------------------------------------------------------------------	(VLX – 0.3V) to (VIN + 6.3V)
	Other Pins-----------------------------------------------------------------------------------------------	0.3V to 6V
	Power Dissipation, PD @ TA = 25 C
	SOT-23-6------------------------------------------------------------------------------------------------	0.48W
	Package Thermal Resistance (Note 2)
	SOT-23-6, JA ------------------------------------------------------------------------------------------	208.2 C/W
	Lead Temperature (Soldering, 10 sec.)----------------------------------------------------------	260 C
	Junction Temperature --------------------------------------------------------------------------------	150 C
	Storage Temperature Range -----------------------------------------------------------------------	65 C to 150 C
	ESD Susceptibility (Note 3)
	HBM (Human Body Model) -------------------------------------------------------------------------	2kV

Recommended Operating Conditions		(Note 4)
	Supply Input Voltage ---------------------------------------------------------------------------------	4.5V to 18V
	Ambient Temperature Range-----------------------------------------------------------------------	40 C to 85 C
	Junction Temperature Range ----------------------------------------------------------------------	40 C to 125 C

Electrical Characteristics

(VIN = 12V, TA = 25 C, unless otherwise specified)

Parameter	Symbol		Test Conditions	Min	Typ		Max		Unit
Supply Voltage
VIN Supply Input Operating	VIN			4.5	--		18
Voltage
									V
Under-Voltage Lockout	VUVLO			3.6	3.9		4.2
Threshold
Under-Voltage Lockout	VUVLO			--	340		--		mV
Threshold Hysteresis
Supply Current
Supply Current (Shutdown)	ISHDN	VEN = 0V		--	--		6		µA
Supply Current (Quiescent)	IQ	VEN = 2V, VFB = 0.85V		--	0.8		--		mA
Soft-Start
Soft-Start Time				--	1000		--		µs
Enable Voltage
EN Rising Threshold	VEN_Rising			1.38	1.5		1.62		V
EN Falling Threshold	VEN_Falling			1.16	1.28		1.4
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