AAT2848 Datasheet, PDF (14/19 Pages) Advanced Analogic Technologies – High-Current Charge Pump For Backlight Display and Flash Applications

ChargePump^TM
PRODUCT DATASHEET
AAT2848
High-Current Charge Pump For Backlight Display and Flash Applications
The AAT2848’s flash charge pump conversion efficiency
is defined as the power delivered to the flash white LED
load divided by the input power:
η=
P_LEDs
P_IN
=
V_LED1 · I_LED1 + V_LED2 · I_LED2
V_IN · I_IN
V_LEDx = White LED Forward Voltage (V_F)
I_LEDx = White LED Bias Current (I_D)
X = Number of Flash White LEDs
The expression to define the estimated ideal flash effi-
ciency (η) for the AAT2848 in 1x mode is as follows:
η=
P_LEDs
P_IN
=
V_LED1 · I_LED1 + V_LED2 · I_LED2
V_IN · I_IN
η
=
X
·
V_LEDX · I_LEDX
V_IN · I_IN
;
X
=
1
or
2
and
I_IN
=
X
·
I_LEDX
η
=
V_LEDX
V_IN
The AAT2848’s flash charge pump is a fractional charge
pump which will boost the input supply voltage in the
event where V_IN is less then the required output voltage
across the flash white LED load.
With an ideal 1.5x charge pump, the input current is
1.5x of the output current. The expression to define the
estimated ideal flash efficiency (η) for the AAT2848 in
1.5x mode is as follows:
η=
P_LEDs
P_IN
=
V_LED1 · I_LED1 + V_LED2 · I_LED2
V_IN · I_IN
η
=
X
·
V_LEDX · I_LEDX
V_IN · I_IN
;
X
=
1
or
2
and
I_IN
=
1.5(X
·
I_LEDX)
η
=
V_LEDX
1.5V_IN
The same calculations apply for the AAT2848 in 2x mode
where for an ideal 2x charge pump, the input current is
2x of the output current. The expression for the esti-
mated ideal flash efficiency (η) for the AAT2848 in 2x
mode is as follows:
η=
P_LEDs
P_IN
=
V_LED1 · I_LED1 + V_LED2 · I_LED2
V_IN · I_IN
η
=
X
·
V_LEDX · I_LEDX
V_IN · I_IN
;
X
=
1
or
2
and
I_IN
=
2(X
·
I_LEDX)
η=
V_LEDX
2V_IN
Capacitor Selection
Careful selection of the four most critical external capac-
itors C_IN, C1, C2, and C_OUT are important because they
will affect turn on time, output ripple and transient per-
formance (the flash timeout and fade functionality
capacitors are not as critical to the normal operation of
the AAT2848). Optimum performance will be obtained
when low ESR (<100mΩ) ceramic capacitors are used.
In general, low ESR is defined as a resistance that is less
than 100mΩ.
X7R and X5R type ceramic capacitors are highly recom-
mended over all other types of capacitors for use with the
AAT2848. For the charge pump section, a 1μF or greater
capacitor is required for the fly (C1 and C2) capacitors.
The required input capacitor (C_IN) is 4.7μF or greater and
the required output capacitor (C_OUT) is 2.2μF or greater.
Ceramic capacitors offer many advantages over their
tantalum and aluminum electrolytic counterparts. A
ceramic capacitor typically has very low ESR, is lowest
cost, has a smaller printed circuit board (PCB) footprint,
and is non-polarized. Low ESR ceramic capacitors maxi-
mize charge pump transient response.
Before choosing a particular capacitor, verify the capaci-
tor’s performance with the characteristics illustrated in
the component’s data sheet. Performance verification
will help avoid undesirable component related perfor-
mance deficiencies. Suggested typical ceramic capacitor
components for AAT2848 are listed in Table 3.
PCB Layout
To achieve adequate electrical and thermal performance,
careful attention must be given to the PCB layout. In the
worst-case operating condition, the chip must dissipate
considerable power at full load. Adequate heat-sinking
must be achieved to ensure intended operation.
Figures 6 and 7 illustrate an example PCB layout (evalu-
ation board). See Figures 4 and 5 for the accompanying
schematic of the evaluation board. The bottom of the
package features an exposed metal pad. The exposed
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