MAX1967EUB+ Datasheet MAX1966ESA+, MAX1967EUB+, MAX1966ESA+T | P7-P9

MAX1966/MAX1967

Low-Cost Voltage-Mode PWM

Step-Down Controllers

_______________________________________________________________________________________ 7

Detailed Description

The MAX1966/MAX1967 are BiCMOS switch-mode

power-supply controllers designed to implement sim-

ple, buck-topology regulators in cost-sensitive applica-

tions. The main power-switching circuit consists of two

N-channel MOSFETs (or a dual MOSFET), an inductor,

and input and output filter capacitors. An all N-channel

synchronous-rectified design provides high efficiency

at reduced cost. Gate drive for the N-channel high-side

switch is provided by a flying capacitor boost circuit

that uses a 0.1µF capacitor connected to BST.

Major circuit blocks of the MAX1966/MAX1967 are

shown in Figures 1 and 2:

• Control Logic

• Gate Driver Outputs

• Current-Limit Comparator

• Clock Generator

• Ramp Generator

• Error Amplifier

• Error Comparator

• Soft-Start

• 5V Linear Regulator (MAX1967)

• 800mV Reference

• Thermal Shutdown

In the MAX1996, most blocks are powered from V

. In

the MAX1967, an internal 5V linear regulator steps

down the input voltage to supply both the IC and the

gate drivers. The synchronous-rectified gate driver is

directly powered from 5V V

, while the high-side-switch

gate driver is indirectly powered from V

plus an exter-

nal diode-capacitor boost circuit.

Resistorless Current Limit

The MAX1966/MAX1967 use the R

DS(ON)

of the low-

side N-channel MOSFET to sense the current. This

eliminates the need for an external sense resistor usual-

ly placed in series with the output. The voltage mea-

sured across the low-side R

DS(ON)

is compared to a

fixed -305mV reference (Figures 1 and 2). The peak

inductor current limit is given by the equation below:

MOSFET Gate Drivers

The DH and DL drivers are optimized for driving

MOSFETs with low gate charge. An adaptive dead-time

circuit monitors the DL output and prevents the high-

side FET from turning on until the low-side MOSFET is

fully off. There must be a low-resistance, low-inductance

connection from the DL driver to the MOSFET gate for

the adaptive dead-time circuit to work properly.

Otherwise, the sense circuitry in the MAX1966/

MAX1967 detects the MOSFET gate as off while there

is charge left on the gate. Use very short, wide traces

measuring no less than 50mils to 100mils wide if the

MOSFET is 1in away from the MAX1966/MAX1967. The

same type of adaptive dead-time circuit monitors the

DH off edge. The same recommendations apply for the

gate connection of the high-side MOSFET.

The internal pulldown transistor that drives DL low is

robust, with a 1.1Ω typical on-resistance. This helps

prevent DL from being pulled up due to capacitive cou-

pling from the drain to the gate of the low-side synchro-

nous-rectifier MOSFET during the fast rise time of the

inductor node. The gate drivers are capable of driving

up to 1A. Use MOSFETs with combined total gate

charge of less than 200nC and a maximum V

of 3.5V.

Internal Soft-Start

The MAX1966/MAX1967 feature an internally set soft-

start function that limits inrush current. It accomplishes

this by ramping the internal reference input to the con-

troller transconductance amplifier from 0 to the 0.8V

reference voltage. The ramp time is 1024 oscillator

cycles that begins when initial power is applied. At the

nominal 100kHz switching rate, the soft-start ramp is

approximately 10ms. The soft-start does not function if

the MAX1966/MAX1967 are shut down by pulling

COMP/EN low.

High-Side Gate-Drive Supply (BST)

Gate-drive voltage for the high-side N-channel switch is

generated by a flying-capacitor boost circuit (Figures 3

and 4). The flying capacitor is connected between BST

and LX.

On startup, the synchronous rectifier (low-side MOS-

FET) forces LX to ground and charges the boost

capacitor to 5V. On the second half-cycle, the

MAX1966/MAX1967 turn on the high-side MOSFET by

closing an internal switch between BST and DH. This

provides the necessary gate-to-source voltage to drive

the high-side FET gate above its source at the input

voltage.

Internal 5V Linear Regulator

(MAX1967)

All MAX1967 functions are internally powered from an

on-chip, low-dropout 5V regulator. The MAX1967 has a

maximum regulator input voltage (V

VIN

) of 28V. The

VCC pin must be connected to VL through a 10Ω resis-

tor and VL must be bypassed with a 2.2µF capacitor to

GND. For operation at V

VIN

< 5V, connect VL to VIN

ImVR

PEAK DS ON

= 305 /

()

MAX1966/MAX1967

Low-Cost Voltage-Mode PWM

Step-Down Controllers

8 _______________________________________________________________________________________

MAX1966

OUT

2.7V TO

5.5V INPUT

VIN

COMP/EN

GND

BST

Figure 3. MAX1966 Typical Application

MAX1967

5V TO

28V INPUT

VIN

COMP/EN

GND

BST

VCC

OUT

10Ω

Figure 4. MAX1967 Typical Application

TEMPERATURE

SHUTDOWN

CONTROL

LOGIC

RAMP

GENERATOR

SOFT-START

100kHZ

CLOCK

GENERATOR

GND

CURRENT-LIMIT

COMPARATOR

ERROR

COMPARATOR

ERROR

AMPLIFIER

-305mV

BST

VIN

COMP/EN

800mV

REF

MAX1966

Figure 1. MAX1966 Functional Diagram

TEMPERATURE

SHUTDOWN

CONTROL

LOGIC

5V LINEAR

REGULATOR

RAMP

GENERATOR

SOFT-START

100kHZ

CLOCK

GENERATOR

GND

CURRENT-LIMIT

COMPARATOR

ERROR

COMPARATOR

ERROR

AMPLIFIER

-305mV

BST

VIN

COMP/EN

800mV

REF

INTERNAL

CHIP

SUPPLY

MAX1967

Figure 2. MAX1967 Functional Diagram

SEE TABLE 1 FOR COMPONENT VALUES. SEE TABLE 1 FOR COMPONENT VALUES.

MAX1966/MAX1967

Low-Cost Voltage-Mode PWM

Step-Down Controllers

_______________________________________________________________________________________ 9

and keep a 0.1µF capacitor between VL and GND

close to the chip. The V

-to-V

dropout voltage is typi-

cally 70mV at 25mA current, so when V

VIN

is less than

5V, V

is typically V

VIN

- 70mV.

The internal linear regulator can source a minimum of

25mA to supply the IC and power the low-side and

high-side FET drivers.

Duty-Factor Limitations for Low

OUT

VIN

Ratios

The MAX1966/MAX1967s’ output voltage is adjustable

down to 0.8V. However, the minimum duty factor may

limit the ability to supply low-voltage outputs from high-

voltage inputs. With high-input voltages, the required

duty factor is approximately:

where R

DS(ON)

x I

LOAD

is the voltage drop across the

synchronous rectifier. The MAX1966/MAX1967s’ mini-

mum duty factor is 10%, so the maximum input voltage

VIN(DFMAX)

) that can supply a given output voltage is:

If the circuit cannot attain the required duty factor dic-

tated by the input and output voltages, the output volt-

age still remains in regulation. However, there may be

intermittent or continuous half-frequency operation as

the controller attempts to lower the average duty factor

by deleting pulses. This can increase output voltage

ripple and inductor current ripple, which increases

noise and reduces efficiency. Furthermore, circuit sta-

bility is not guaranteed.

Applications Information

Design Procedure

Component selection is primarily dictated by the follow-

ing criteria:

1) Input Voltage Range: The maximum value

VIN(MAX)

) must accommodate the worst-case

high-input voltage. The minimum value (V

VIN(MIN)

)

must account for the lowest input voltage after

drops due to connectors, fuses, and switches are

considered. In general, lower input voltages pro-

vide the best efficiency.

2) Maximum Load Current: There are two current val-

ues to consider. Peak load current (I

LOAD(MAX)

)

determines the instantaneous component stresses

and filtering requirements and is key in determining

output capacitor requirements. I

LOAD(MAX)

also

determines the required inductor saturation rating

and the design of the current-limit circuit. Con-

tinuous load current (I

LOAD

) determines the thermal

stresses, input capacitor, and MOSFETs, as well as

the RMS ratings of other heat-contributing compo-

nents such as the inductor.

3) Inductor Value: This choice provides tradeoffs

between size, transient response, and efficiency.

Higher inductance value results in lower inductor

ripple current, lower peak current, lower switching

losses, and, therefore, higher efficiency at the cost

of slower transient response and larger size. Lower

inductance values result in large ripple currents,

smaller size, and poorer efficiency, while also pro-

viding faster transient response. Except for low-cur-

rent applications, most circuits exhibit a good

balance between efficiency and economics with a

minimum inductor value that causes the circuit to

operate at the edge of continuous conduction

(where the inductor current just touches zero with

every cycle at maximum load). Inductor values lower

than this grant no further size-reduction benefit.

Table 1 shows representative values for some typical

applications up to 5A. With proper component selec-

tion, outputs of 20A or more are practical with the

MAX1966/MAX1967. The components listed in Table 1

were selected assuming a minimum cost design goal.

The MAX1966/MAX1967 can effectively operate with a

wide range of components.

Setting the Output Voltage

An output voltage between 0.8V and (0.9V x V

VIN

) can

be configured by connecting F

pin to a resistive

divider between the output and GND (Figures 3 and 4).

Select resistor R2 in the 1kΩ to 10kΩ range. R1 is then

given by:

where V

= 0.8V.

Inductor Selection

Determine an appropriate inductor value with the fol-

lowing equation:

where LIR is the ratio of inductor ripple current to aver-

age continuous current at a minimum duty cycle.

Choosing LIR between 20% to 50% results in a good

V f LIR I

OUT

IN OUT

VIN OSC LOAD MAX

=×

−

()

×××

()

OUT

1=−













VVRI

VIN DFMAX OUT DS ON LOAD() ()

≤+ ×

()

VR I V

OUT DS ON LOAD VIN

+×

()

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