LTC3336ACBZ-1-R7 datasheet(15/24 Pages) AD

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LTC3336

Rev. A

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during this load condition. The capacitor can be sized

using Equation 2 for an acceptable ripple VOUT+ – VOUT–.

COUT = tLOAD •

ILOAD – 0.95 • IPEAK

VOUT+ – VOUT–

⎛

⎝⎜

⎞

⎠⎟

(2)

In Equation 2, tLOAD is the duration of the excessive tran-

sient load current, IPEAK is the programmed IPEAK setting

and VOUT+ – VOUT– is the desired VOUT ripple.

At zero or light load conditions VOUT should not exceed

the typical 5% overvoltage threshold. Otherwise, it loses

some power in the 1MΩ overvoltage pull-down resistor.

A standard surface mount ceramic capacitor can be used

for COUT. These capacitors can be obtained from manu-

facturers such as Vishay, Murata, TDK, and Taiyo Yuden.

Table 3. Minimum Required Buck Output Capacitor Size for a

Sleep Time of 50µs at Max Load

VOUT

(V)

IPEAK

(mA)

COUT

(µF)

1.2

6.8

1.2

100

1.2

300

220

2.5

3.3

2.5

100

2.5

300

100

5.0

2.2

5.0

4.7

5.0

100

5.0

300

Inductor Selection

The buck is optimized to work with inductor values calcu-

lated with Equation 3 or larger. The calculated values rep-

resent a suitable trade-off between size and efficiency for

typical applications. A larger inductor will benefit higher

input voltage applications by reducing the switching fre-

quency of the P-channel MOSFET switch, thereby improv-

ing efficiency by reducing gate charge loss. Choose an

inductor with a saturation current rating, ISAT, greater

than the programmed IPEAK setting. The DC Resistance

(DCR) of the inductor can have an impact on efficiency

as it is a source of loss. Trade-offs between price, size,

and DCR should be evaluated. Equation 3 can be used to

calculate the minimum inductor value that works well with

the LTC3336 depending on VIN, VOUT, IPEAK and maxi-

mum switching frequency ƒ. A good target for switching

frequency is 2MHz.

Examples of inductor series to choose from: LPS4018

(0.56μH to 1000μH), MSS7348 (3.3μH to 1000μH), and

MSS1210 (10μH to 10,000μH).

ƒ • 10% •IPEAK •

VIN − VOUT

VOUT

⎛

⎝⎜

⎞

⎠⎟

(3)

Explanation of symbols used in Equation 3 and how they

are derived:

= L •

∆I

∆I =IPEAK −IVALLEY = 10% •IPEAK

VL = Voltage Across Inductor

= Switching Frequency

= TON + TOFF

TON = L •

∆I

VIN − VOUT

duration of

P-Channel MOSFET On

TOFF = L •

∆I

VOUT

duration of

P-Channel MOSFET Off

Dynamic Output Voltage Change

The selection bits OUT[3:0] can be driven from the out-

puts of a microcontroller and dynamically changed while

the buck is running.

A start-up default voltage, VDEFAULT, can be set with

one (or more) pins tied high. Refer to Table 4 and the

APPLICATIONS INFORMATION

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