SGM3727 Datasheet, PDF (8/13 Pages) SG Micro Corp – High Efficiency 38V Step-Up WLED Driver

SGM3727
OPERATION
High Efficiency,
38V Step-Up WLED Driver
Control Scheme
The SGM3727 uses a minimum off-time, peak current
control scheme to achieve excellent performance under
high duty-cycle operating conditions. This control scheme
initiates a switching cycle only when needed to maintain
output voltage regulation, resulting in very high efficiency
operation.
During each switching cycle, the N-Channel power
MOSFET turns on, increasing the inductor current. The
switching cycle terminates when either the inductor
current reaches the current limit of 1A or when the cycle
lasts longer than the maximum on-time of 6.2μs. Once
the MOSFET turns off, it remains off for at least the
minimum off-time of 420ns, then another switching
begins when the error comparator detects that the output
is falling out of regulation again.
Soft-Start
The SGM3727 limits the in-rush current during start-up
by increasing the current limit in two steps starting from
I_LIM/3 for 256 switch cycles to I_LIM × 2/3 for the next 256
switch cycles and then full current limit. See the Typical
Performance Characteristics for typical start-up behavior.
Shutdown
The SGM3727 features low-current shutdown mode. In
shutdown mode, the control circuitry is disabled, the
LEDs are disconnected and the quiescent supply current
drops to less than 1μA. To disable the SGM3727, simply
drive EN/SET to a logic low for more than 3ms. To enable
the ICs, drive EN/SET to a logic high or connect it to the
input supply.
Over-Voltage Protection
The SGM3727 includes internal over-voltage protection
circuitry that monitors the OVP pin voltage. Over-voltage
protection is critical when one of the LEDs in the LED
string fails as an open circuit. When this happens, the
feedback voltage drops to zero, and the control switches
at maximum on time causing the output voltage to keep
rising until it exceeds the maximum voltage rating of the
power MOSFET. The SGM3727's over-voltage protection
detects this condition and switching ceases if the voltage
at the OVP pin reaches 1.24V and stays for about 10µs.
To set the maximum output voltage, connect a resistor
divider from the output node to GND, with center tap at
OVP, and select the two resistors with the following
equation:
Single Wire Pulse Dimming Control
The SGM3727 implements a pulse dimming method to
control the brightness of white LEDs. Users can easily
configure the LED current from 1.25mA to 30mA by a
serial pulse. The dimming of white LEDs' current can be
achieved by applying a pulse signal to the EN/SET pin.
There are totally 32 current steps could be set by users.
The detail operation of brightness dimming is shown in
Figure 1.
R_OV2 = R_OV1 ×

VOVP
1.24V



1_{}
where V_OVP is the over-voltage detection threshold, R_OV1
is the resistor between OVP pin and GND, and R_OV2 is
the resistor from the output to the OVP pin. As a first
estimate, the OVP threshold can often be set to 3.8V
times the number of LEDs in the string. The C_OUT has an
effect on the final V_OVP. Considering the OVP comparator
has some delay, the final measured V_OVP could be higher
than the set V_OVP if C_OUT is small.
30μs < tREADY 0.5μs < tIH
0.5μs < tIL < 500μs
3ms < t_SHDN
EN
0
1
2
3
45
29
31 0
1
Shutdown
I_LEDx
100% _{31/32 30/32 29/32 28/32 3/32}
2/32
100%
1/32
31/32
Figure 1. Brightness Control by Pulse Dimming
Shutdown
SG Micro Corp
8
www.sg-micro.com