MAX1577Y Datasheet, PDF (10/11 Pages) Maxim Integrated Products – 1.2A White LED Regulating Charge Pump for Camera Flashes and Movie Lights

1.2A White LED Regulating Charge Pump for
Camera Flashes and Movie Lights
(Figure 2), the ballast resistors must be small enough to
prevent triggering the overvoltage protection:
R_BALLAST < (5.1V - V_F) / I_LED
where I_LED is the current through each LED and V_F is
the forward voltage of the LED at I_LED. Within this limit,
larger values of R_BALLAST improve LED-to-LED match-
ing but also increase the input voltage where the 2x
charge pump makes the transition to the more efficient
1x LDO mode.
When using voltage regulation (Figure 3), select the
ballast resistors based on the full brightness setting as
follows:
With EN1 high and EN2 high:
R_BALLAST = (5.1V - V_F) / I_LED
The LED current at the dimmed settings are calculated
as follows:
with EN1 high and EN2 low:
I_LED = (4.6V - V_F) / R_BALLAST
with EN1 low and EN2 high:
I_LED = (3.4V - V_F) / R_BALLAST
Note that V_F varies as a function of I_LED. Typically, LED
manufacturers provide a plot of forward voltage (V_F) vs.
forward current (I_LED) in the LED data sheet.
PC Board Layout and Routing
The MAX1577Y/MAX1577Z are high-frequency
switched-capacitor regulators. For best circuit perfor-
mance, use a solid ground plane and place C_IN, C_OUT,
and C1 as close to the IC as possible. Connect the
exposed pad to GND directly under the IC and allow suf-
ficient copper area for cooling. Refer to the MAX1577Z
evaluation kit for an example PC board layout.
Chip Information
TRANSISTOR COUNT: 2482
PROCESS: BiCMOS
10 ______________________________________________________________________________________