MAX1577Y Datasheet, PDF (9/11 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
Overvoltage Protection
Output overvoltage protection (OVP) protects the IC
and other components in case the output is open cir-
cuit. The MAX1577Z always limits the output voltage to
5.3V. With the MAX1577Y, the output OVP voltage
varies slightly depending on the state of the EN_ inputs
(see Table 1).
Output-Voltage Regulation Mode
When the voltage across the sense resistor (V_OUT -
V_CS) is less than 24mV, the MAX1577Y/MAX1577Z reg-
ulate the output voltage. For applications requiring a
fixed output voltage instead of current regulation, sim-
ply connect CS directly to OUT as shown in Figure 3.
The MAX1577Z regulates to a fixed 5.1V. The
adjustable regulation voltage (3.4V, 4.6V, or 5.1V) of
the MAX1577Y depends on the state of the EN_ inputs
(see Table 1).
Thermal Protection
Thermal protection prevents damage to the IC during
overload conditions. When the die temperature
exceeds +160°C, the MAX1577Y/MAX1577Z turn off
until the die cools by 20°C. Once the die has cooled,
the MAX1577Y/MAX1577Z turn on and initiate soft-start.
During a continuous overload, this produces a pulsed
output. Continous operation at high-output-current set-
tings may induce thermal protection.
Capacitor Selection
Ceramic capacitors are recommended due to their low
equivalent series resistance (ESR), small size, and low
cost. Select capacitors that maintain their capacitance
over temperature and DC bias. Typically X5R, X7R, or
better ceramic capacitors perform well.
A 4.7µF input capacitor and 10µF output capacitor are
recommended for most applications. Larger values can
be used to reduce ripple.
For the transfer capacitor (C1), a 1µF capacitor is suit-
able for output currents up to 600mA. For output cur-
rents between 600mA and 1200mA, use a 2.2µF
capacitor.
Input Ripple
In 1x LDO mode there is no charge-pump switching,
and thus very little input ripple. The input ripple in 2x
charge-pump mode depends on the input source
impedance. Typical waveforms showing the ripple in 1x
and 2x mode are in the Typical Operating
Characteristics. In sensitive applications, increase the
input capacitance to reduce input ripple.
Applications Information
Setting the Output Current
For applications requiring a regulated output current,
see the circuit of Figure 2. The output regulation current
is set by R_CS as follows:
R_CS = (V_OUT - V_CS) / I_OUT
where I_OUT is the output current (up to 1200mA) and
(V_OUT - V_CS) is the current-sense threshold (see Table 1).
For camera-flash applications with a short pulse dura-
tion, a 0.25W resistor is recommended.
Setting the Output Voltage
To regulate the output voltage instead of the current,
connect CS directly to the output as shown in Figure 3.
In this configuration the MAX1577Z regulates the output
to 5.1V. The output voltage of the MAX1577Y is set to
5.1V, 4.6V, or 3.4V using the EN1 and EN2 inputs as
shown in Table 1.
Driving Camera Strobe/Flash
In applications that do not require dimming, such as a
camera strobe/flash, connect EN1 and EN2 together
and drive as a single on/off control input. Pulse this
input high to generate a strobe output.
PWM Dimming
To vary the brightness from 20% to 100% using pulse-
width modulation (PWM), hold EN2 high and apply the
PWM signal (up to 5kHz) to EN1. Holding EN2 high
keeps the part enabled and prevents repeated soft-
starts. The duty cycle of the PWM signal applied to EN1
controls the brightness, where 0% duty cycle corre-
sponds to 20% brightness and 100% duty cycle corre-
sponds to 100% brightness. A PWM signal with a
logic-high voltage between 1.6V and 5.5V can be safe-
ly used regardless of the input voltage. The voltage of
the PWM signal does not affect the brightness level.
For highest movie-mode efficiency, it is best to choose
R_CS so 0% PWM duty cycle provides the desired
movie-mode current. Then, the flash current level is
adjusted by the PWM duty cycle. When the movie-
mode current is more than 33% of the flash current, it is
more efficient to apply the PWM dimming signal to EN2
while holding EN1 high.
Ballast Resistors for Multiple LEDs
When using multiple parallel LEDs, add a ballast resis-
tor in series with each LED to help balance the current
between the LEDs. When using current regulation
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