MAX35104 datasheet(21/80 Pages) MAXIM

21 / 80 page

The controller has an undervoltage comparator that deter-

mines if the target output voltage is at target voltage, con-

sidered power good, or undervoltage. If the output voltage

is below target, the switcher operates in startup limit mode

that is determined by user selectable peak current limit

set by the LT_S[3:0] bits in the Switcher 2 register. This

is essentially a slew rate control on how fast the boost

powers up and can be used to control the current signa-

tures seen by the supply battery. After the output voltage

crosses the undervoltage threshold, the switcher runs in

normal duty mode. There is an additional optional peak

current limit setting for the normal duty mode that is set by

the LT_N[3:0] bits in the Switcher 2 register. Once in nor-

mal duty mode the device waits a programmable switcher

stabilization time before a launch sequence begins. The

stabilization time ensure that the controller has reaches a

stable and repeatable output voltage each time it is pow-

ered. This time is set by the ST[3:0] bits in the Switcher 2

Compensation Component Values

In order to achieve standard operations the boost control-

ler requires that proper loop compensation be applied to

the error-amplifier output (COMP pin). The goal of the

compensator design is to achieve the desired closed-loop

bandwidth and sufficient phase margin at the crossover

frequency of the open-loop gain-transfer function of the

converter. The error amplifier included in the devices is a

transconductance amplifier. Figure 6 shows the compen-

sation network used to apply the necessary loop com-

pensation for the example inductor and output capacitor

values provided, where:

RZ = 22kΩ

CP = 470pF

CZ = 10nF

RSENSE

The external sense resistor value determines the peak

allowable inductor current. For a given limit trim setting,

LT_N[3:0] and LT_S[3:0] in the Switcher 2 register. Adjust

the RSENSE value to adjust the peak allowable current.

Select RSENSE based on the following criteria:

Resistor Value: Select an RSENSE resistor value in which

the largest desired current would result in a 200mV full-

scale current sense voltage. Assuming an LT_x setting of

0h, select RSENSE in accordance to the following equa-

tion and see Table 2 for examples:

RSENSE = 200mV/(Max Current)

Power Dissipation: Select a sense resistor that is rated

for the max expected current and power dissipation (watt-

age). The sense resistor’s value might drift if it is allowed

to heat up excessively.

Kelvin Sense

For best performance, a Kelvin Sense arrangement is

recommended for sense resistor as shown in Figure 7. In

a Kelvin Sense arrangement, the voltage-sensing nodes

across the sense element are placed such that they mea-

sure the true voltage drop across the sense element and

not any additional excess voltage drop that can occur in

the copper PCB traces or the solder mounting of the sense

element. Routing the differential sense lines along the

same path to the device and keeping the path short also

improves the system performance. The analog differential

current-sense traces should be routed close together to

maximize common-mode rejection.

Power Transistor

Use an n-channel MOSFET power transistor with the

MAX35104. To ensure the external n-channel MOSFET

(nFET) is turned on hard, use logic-level or low-threshold

nFETs such that the MAX35104’s internal gate driver’s 2 x

VDD supply voltage is sufficient for proper switching oper-

ation. nFETs provide the highest efficiency because they

do not draw any DC gate-drive current. When selecting

Figure 6. Boost Circuits Components

MAX35104

Gas Flow Meter SoC

www.maximintegrated.com

Maxim Integrated │ 21

0Ω

100mΩ

5µH

B340A-13-F

1µF

MAX35104

FETG

CSIN+

CSIN

COMP

RZ =

22kΩ

CZ =

10nF

CP =

470pF

IRLM10060TRPBF

VDD

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