MP3336 – DUAL-CHANNEL, FLASH LED DRIVER WITH I

MP3336 Rev.1.01

www.MonolithicPower.com

15

5/16/2017

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Switching Frequency Fold-back

When the max switching frequency is set to

2/3/4MHz in switching mode, if the voltage

across the current source is larger than the

value set by the register bits VTH_PAS, the IC

switches to 1MHz to improve efficiency and EMI.

With 1MHz, if the voltage across the current

source is still larger than the value set by the

register bits VTH_PAS, the IC can stretch the

frequency down until it reaches about 100kHz

with FS_SD = 0. If the LED load becomes

lighter and the input voltage increases further,

the IC switches to pass mode from 100kHz.

With FS_SD = 1, the IC switches directly to

pass mode from 1MHz.

Load Balancing

To improve efficiency, the device provides load

balancing to prevent unbalanced forward

voltage drops between the dual LEDs (see

Figure 13). To enable this function, the bit

BAL_ON must be set to 1. Supposing VLED1 >

by step while the LED2 current rises step by

ILED1 < 90%ILED2.

However, the maximum adjustment range of

each LED current is limited to 10%. Because of

the limitation, if there is a large difference

between VLED1 and VLED2, the load balance

function cannot make |VLED1 - VLED2| drop

reduce the voltage difference and improve

efficiency.

operation is the same.

Figure 13: Load Balancing

LED Output Current Reduction

When the MP3336 works in flash mode, output

current reduction may occur due to the

following three conditions:

1. TX is High

During flash mode, if TX goes from low to high,

the flash current drops to the current set by

I_TX in under 5µs and sets the FT_TX bit to 1.

If TX goes low, the current ramps up to the

flash current again (see Figure 14).

Figure 14: Flash Mode with TX Pulse

2. Input DC Current Limit is Triggered

If the input DC current limit set by IL_DC

register is not sufficient, a current reduction

may occur. The actual current in the IFL_ACT

register can be read back during flash.

Input DC Current Limit Protection

The MP3336 uses the input DC current limit

protection to avoid drawing too much current

from the battery. This current limit can be

programmed by the IL_DC register from 1A to

4.2A with 250mA/step when IL_DC < 3A and

with 300mA/step when IL_DC > 3A. If the input

current hits the input current limit threshold, the

LED output current stops and holds until the

input current limit drops below the current limit

threshold. The fault bit FT_IDC is set to 1 if the

input current limit is triggered. The fault bit

remains until it is read back or the input power

is reset. The input current limit protection is

useful in boost mode because it senses the

current of the low-side MOSFET.