MAX1513 データシートの表示（PDF） - Maxim Integrated

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MAX1513

TFT-LCD Power-Supply Controllers

Maxim Integrated 

TFT-LCD Power-Supply Controllers

L

VIN

RS3

CS

RS4

CS+

CS-

GATE

MAX1513

GND

MAX1514

FB

VMAIN

Figure 10. Lossless Current Sensing with VSENSE < 80mV

( ) VSENSE = 2.6A × 56mΩ 1 + 0.005 × 40°C = 175mV

Because VSENSE would be greater than 100mV, the cir-

cuit in Figure 9 should be used and the scale factor is:

SF = 100mV = 0.571

175mV

Calculate RS1 and RS2:

RS1

=

489Ω

0.571

=

856Ω

RS2

=

856 × 0.571

1 - 0.571

= 1139Ω

The closest 1% standard values for RS1 and RS2 are

866Ω and 1.13kΩ, respectively.

If the 2.2µH inductor used in the typical operating cir-

cuit (Figures 1 and 2) has a typical DCR of 10mΩ and a

maximum DCR of 14mΩ, the RC time constant of the

sense network is:

τ = 2.2µH = 220µs

10mΩ

Select CS = 0.1µF, RS is:

RS

=

220µs

0.1µF

=

2200Ω

Assuming ∆T is 40°C and TC is 0.5%, the worst-case

high sense voltage over temperature is:

( ) VSENSE = 2.6A × 14mΩ 1 + 0.005 × 40°C = 44mV

Because VSENSE would be much less than 80mV, the

circuit in Figure 10 can be used to improve the current-

limit accuracy. Calculate RS3 and RS4:

RS3 =

15V - 4.5V

× 2600Ω = 2614Ω

15V - 4.5V - 0.1V + 0.044V

RS4 = 2614Ω - 2600Ω = 14Ω

The closest 1% standard values for RS3 and RS4 are

2.61kΩ and 14.0Ω, respectively.

Output-Capacitor Selection

The output capacitor and its equivalent series resistance

(ESR) affect the circuit’s stability, output voltage ripple,

and transient response. The Output-Capacitor Stability

Requirement section discusses the output capacitance

requirement based on the loop stability. This section

deals with how to determine the output capacitance

according to the ripple voltage and load-transient

requirements.

The total output voltage ripple has two components: the

ohmic ripple due to the capacitor’s equivalent series

resistance (ESR), and the capacitive ripple caused by

the charging and discharging of the output capacitance:

VRIPPLE = VRIPPLE(ESR) + VRIPPLE(C)

VRIPPLE(ESR) ≈ IPEAK × RESR

VRIPPLE(C) ≈

IMAIN

COUT

×

⎛

⎝⎜

VMAIN -

VMAIN ×

VIN

fOSC

⎞

⎠⎟

where VMAIN is the output voltage of the step-up regu-

lator, IMAIN is the output current, COUT is the output

capacitance, RESR is the ESR of the output capacitor,

fOSC is the switching frequency, and IPEAK is the peak

inductor current (see the Inductor Selection section).

20 ______________________________________________________________________________________

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